Skip to main content
NCBI home page
Wolters Kluwer - PMC COVID-19 Collection logoLink to Wolters Kluwer - PMC COVID-19 Collection
. 2022 Feb 16;50(6):e557–e568. doi: 10.1097/CCM.0000000000005465

Aberrant Fibrin Clot Structure Visualized Ex Vivo in Critically Ill Patients With Severe Acute Respiratory Syndrome Coronavirus 2 Infection

Lisa S Brubaker 1,2,✉, Arun Saini 2,3, Trung C Nguyen 2,3, Marina Martinez-Vargas 2,4, Fong W Lam 2,3, Qizhi Yao 1,2, Michele M Loor 1, Todd K Rosengart 1, Miguel A Cruz 2,4,✉
PMCID: PMC9112654  NIHMSID: NIHMS1789436  PMID: 35170535

OBJECTIVES:

Disseminated fibrin-rich microthrombi have been reported in patients who died from COVID-19. Our objective is to determine whether the fibrin clot structure and function differ between critically ill patients with or without COVID-19 and to correlate the structure with clinical coagulation biomarkers.

DESIGN:

A cross-sectional observational study. Platelet poor plasma was used to analyze fibrin clot structure; the functional implications were determined by quantifying clot turbidity and porosity.

SETTING:

ICU at an academic medical center and an academic laboratory.

PATIENTS:

Patients admitted from July 1 to August 1, 2020, to the ICU with severe acute respiratory syndrome coronavirus 2 infection confirmed by reverse transcription-polymerase chain reaction or patients admitted to the ICU with sepsis.

INTERVENTIONS:

None.

MEASUREMENTS AND MAIN RESULTS:

Blood was collected from 36 patients including 26 ICU patients with COVID-19 and 10 ICU patients with sepsis but without COVID-19 at a median of 11 days after ICU admission (interquartile range, 3–16). The cohorts were similar in age, gender, body mass index, comorbidities, Sequential Organ Failure Assessment (SOFA) score, and mortality. More patients with COVID-19 (100% vs 70%; p = 0.003) required anticoagulation. Ex vivo fibrin clots formed from patients with COVID-19 appeared to be denser and to have smaller pores than those from patients with sepsis but without COVID-19 (percent area of fluorescent fibrin 48.1% [SD, 16%] vs 24.9% [SD, 18.8%]; p = 0.049). The turbidity and flow-through assays corroborated these data; fibrin clots had a higher maximum turbidity in patients with COVID-19 compared with patients without COVID-19 (0.168 vs 0.089 OD units; p = 0.003), and it took longer for buffer to flow through these clots (216 vs 103 min; p = 0.003). In patients with COVID-19, d-dimer levels were positively correlated with percent area of fluorescent fibrin (ρ = 0.714, p = 0.047). Denser clots (assessed by turbidity and thromboelastography) and higher SOFA scores were independently associated with delayed clot lysis.

CONCLUSIONS:

We found aberrant fibrin clot structure and function in critically ill patients with COVID-19. These findings may contribute to the poor outcomes observed in COVID-19 patients with widespread fibrin deposition.

Keywords: COVID-19, fibrin clot structure, fibrin polymerization, fibrinogen, hypercoagulability, severe acute respiratory syndrome coronavirus 2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for the COVID-19 pandemic and causes severe-to-critical illness in 19% of infected patients (1). Hypercoagulability is a common phenotype in these critically ill patients despite the use of prophylactic- or treatment-dose anticoagulation (2–5). Further, markers of fibrin polymerization (e.g., elevated d-dimer levels and procoagulant profiles on viscoelastic assays) have been associated with adverse outcomes in severe cases of COVID-19 (6–11). Numerous autopsy studies have reported disseminated fibrin-rich microthrombi in patients who died from COVID-19, which is again suggestive of an association between widespread fibrin deposition and poor outcomes in COVID-19 (12–15).

The pathophysiology of the augmented fibrin deposition in COVID-19 is multifactorial (16). Although many of the etiologic factors unique to the viral infection are outside of the scope of this report (17–19), there is clear evidence that patients with COVID-19 have enhanced plasma fibrin formation potential compared with critically ill patients without COVID-19 (20). Despite this evidence, changes in the fibrin clot structure in COVID-19 have not been described. Considering the proclivity for fibrin formation, we hypothesize that fibrin clots will be denser and less porous in critically ill patients with COVID-19 compared with critically ill patients without COVID-19.

Our specific objectives were: 1) to compare the fibrin clot properties in critically ill patients with or without COVID-19 and 2) to determine correlation between fibrin clot properties in experimental (plasma-based) assays and the whole-blood coagulation assays clinically used to assess hypercoagulability. The outcomes of this study have the potential to impact clinical care for patients with COVID-19 as several interventions with the potential to favorably alter fibrin clot properties have been reported (21, 22), and it is well described that the fibrin clot structure affects fibrinolysis (23–25).

MATERIALS AND METHODS

This study was approved by the Baylor College of Medicine (BCM) Institutional Review Board (IRB) protocols H-48730 and H-47607 (approved by board 4 of the BCM IRB and the Michael E DeBakey Department of Veterans Affairs Medical Center Subcommittee on Research Safety). This study is reported following Strengthening the Reporting of Observational Studies in Epidemiology reporting guidelines for observational studies (26).

Study Participants and Clinical Data

Inclusion criteria for this study were critically ill patients in the ICU at Baylor St. Luke’s Medical Center between July 1 and August 1, 2020; exclusion criteria were patients in the ICU who did not have a positive reverse transcriptase-polymerase chain reaction test for SARS-CoV-2 and who did not have sepsis as defined by Sepsis-3.0 criteria (27). During this period, the first available residual venous blood sample that was drawn into sodium citrate tubes as part of routine clinical care was collected for patients meeting criteria. Platelet poor plasma (PPP) was isolated by centrifugation at 3,000 rpm for 15 minutes at room temperature and frozen at −80°C until it was used for assays. To obtain healthy human blood, informed consent was obtained from healthy donors without acute illness, and PPP was processed as above. Researchers were blinded to patient status (i.e., COVID-19 diagnosis) during the collection, processing, and experimentation stages. Additional methods are in the data supplement.

Imaging the Fibrin Clot Structure

PPP was thawed at 37°C. The fibrin clot structures were obtained as described previously (26) with the exception that PPP was supplemented with 2% human fibrinogen conjugated to Alexa Fluor 488 (Thermo Scientific‚ Waltham‚ MA), and clots were formed on a 0.15-mm chambered cover glass. The clots were imaged using laser scanning confocal microscopy at a magnification of 60×. Image analysis, specifically percent area occupied by fluorescent fibrin, was performed using Fiji (Bethesda‚ MD).

Flow-Through Assays

Plasma clots were formed by mixing PPP 1:5 with buffer containing 1× tris-buffered saline, 1% bovine serum albumin, 1 U of human thrombin (Sigma‚ St. Louis‚ MO), and 2.4 mM of calcium in the base of a 3-mL syringe with a stopcock to control flow-through. The syringes were incubated at 37°C for 2 hours to allow clot formation; 1 mL of buffer (1× tris-buffered saline) was gently pipetted over the formed clots. The stopcock was opened, and the duration of time required for all of the buffer to flow through the clot was recorded.

Buffer mixed with fluorescent submicron beads (Bangs Laboratories, Fishers, IN) was also used for flow-through experiments. An equal volume of each sized bead (0.2, 0.5, and 0.8 um) was added to a microcentrifuge tube, and this combined volume was diluted 1:10 with buffer containing 1× tris-buffered saline. The concentration and proportion of each sized bead within the mixture was determined by flow cytometry using the Amnis ImageStreamx MkII (Luminex, Austin, TX); 1,000 events were counted, and the 0.2-, 0.5-, and 0.8-um populations were discriminated by gating via side scatter/green fluorescence. Clots were formed in the syringes as described above, and 200 uL of the bead/buffer mixture was gently added over the clots. The flow-through was collected in microcentrifuge tubes. After all 200 uL of the bead/buffer mixture was collected, flow cytometry was again performed to determine how the concentration and proportion of 0.2-, 0.5-, and 0.8-um beads within the mixture changed. The sieving coefficient was calculated by determining the concentration of 0.2-um beads in the mixture postflow-through/the concentration of 0.2 um beads in the mixture preflow-through; this calculation was repeated for the 0.5- and 0.8-um beads.

Fibrin Polymerization Assays

Fibrin polymerization and fibrinolysis were evaluated by turbidity as described (26). Briefly, plasma was mixed 1:2.5 with buffer containing 1× tris-buffered saline, 1% bovine serum albumin, and 2.4 mM of calcium in a 96-well plate. Immediately prior to inserting the plate into the spectrophotometer, an equal volume of buffer containing 1U of thrombin was added to each well. For assays that concurrently measured fibrinolysis, the buffer was supplemented with 150 ng/mL of human tissue plasminogen activator (Sigma Aldrich). The progression of fibrin clot formation and fibrinolysis was evaluated by tracking turbidity using a spectrophotometer set to λ 405 nm. Variables calculated from turbidimetric curves include maximum absorbance (optical density units), slope of fibrin polymerization, 50% clot lysis time, and slope of fibrinolysis as described (28).

Statistical Analysis

To test whether the presence of COVID-19 results in a different fibrin structure (i.e., percent area occupied by fluorescent fibrin) in critically ill subjects, we calculated a sample size of four subjects in the control group and eight subjects in the COVID-19 group. This was determined by using an estimated difference of 10% between the group means with an sd of 6%, alpha of 0.05, power of 80%, and enrollment ratio of 2:1 (COVID-19 vs non-COVID-19 with sepsis). Additional samples were collected as available due to the anticipated heterogeneity between clinical variables between the groups.

Comparison of categorical variables between groups was performed using the Pearson chi-square or Fisher exact test as appropriate. Continuous variables were expressed as mean with sd for normally distributed variables or as median with interquartile range (IQR) for variables that were not normally distributed. Comparisons between groups were conducted through the independent samples t test/Mann-Whitney U test. Associations between clinical, laboratory, and plasma-based assay parameters were analyzed using Spearman rank correlations. Agreement between correlation coefficients for the plasma-based assays between cohorts was assessed using Cohen kappa. p values were two-sided, and statistical significance was determined by a p value less than 0.05. All analyses were performed using IBM SPSS Statistics Version 26.0 (IBM Corp, Armonk, NY).

RESULTS

We obtained blood from all 42 patients meeting criteria during the study period including six healthy donors, 10 critically ill septic patients without COVID-19, and 26 critically ill patients with COVID-19. The non-COVID-19 sepsis and COVID-19 cohorts were similar in age, gender, body mass index, and preadmission antiplatelet and anticoagulant use as summarized in Supplementary Table 1 (http://links.lww.com/CCM/H29). The COVID-19 cohort had a higher proportion of Black and Hispanic/Latino patients compared with the non-COVID-19 with sepsis cohort (80.8% vs 40.0%; p = 0.039). More patients with COVID-19 were diabetic, whereas more septic patients without COVID-19 had chronic obstructive pulmonary disease or chronic lung disease.

Clinical data and key outcomes are also presented in Supplementary Table 1 (http://links.lww.com/CCM/H29). There was no difference in the overall occurrence of thrombotic events between the non-COVID-19 with sepsis and COVID-19 cohorts (20% vs 30.8%; p = 0.765). Notably, more patients with COVID-19 received intermediate/therapeutic dose anticoagulation during hospitalization. There were no differences in need for invasive organ support (i.e., renal replacement therapy, extracorporeal membrane oxygenation, and mechanical ventilation), length of stay, or mortality between cohorts.

Plasma was collected as available from patients at a single time point during their ICU stay, and clinical characteristics at this time point were compared between the cohorts (Table 1). Most patients (60% of patients without COVID-19 but with sepsis and 73.1% of patients with COVID-19; p = 0.454) had samples collected late in their ICU course (greater than 72 hr after ICU admission). The median day of ICU admission that the plasma was collected was 9 (IQR, 2–19) in the non-COVID-19 with sepsis cohort compared with 13 (IQR, 3–16) in the COVID-19 cohort (p = 0.715). On the date of plasma collection, more septic patients without COVID-19 had normal chest radiographs, but the percent of patients requiring invasive respiratory support did not differ between the cohorts. The median Sequential Organ Failure Assessment (SOFA [29]) score was similar between the cohorts. Regarding outcomes occurring at or before the date of plasma collection (Table 1), we also found no difference in the occurrence of thrombotic events or need for invasive organ support between the cohorts. At the time of plasma collection, more patients with COVID-19 were receiving intermediate or therapeutically dosed anticoagulation compared with septic patients without COVID-19 (96.2% vs 50%; p = 0.003).

TABLE 1.

Clinical Characteristics of Critically Ill Patients Without COVID-19 and With Sepsis or With COVID-19 on the Date Plasma Was Collected for Experimentation

Time Point: Date of Plasma Collection Non-COVID-19 With Sepsis (n = 10) COVID-19 (n = 26) p
ICU day that plasma was collected (day 0 = day of ICU admission), median (IQR) 9 (2–19) 13 (3–16) 0.715
Sepsis diagnosis (Sepsis-3 criteria) (%)
 Confirmed 100.00 100.00 1
Chest radiograph findings (%)
 Normal 40.00 0.00 < 0.001
 Unilateral opacity 0.00 0.00
 Bilateral opacities 40.00 96.20
 Interstitial infiltrates 20.00 3.80
Respiratory support (%)
 Invasive support 60.00 73.10 0.454
Sequential Organ Failure Assessment Score, median (IQR) 8.5 (3.3–15.5) 6.5 (3–11.3) 0.689
WBC count (×103/uL), median (IQR) 10.5 (7.7–15.1) 10.0 (7.1–15.8) 1
Hemoglobin (g/dL), median (IQR) 8.3 (7.3–9.6) 8.8 (8.3–10.9) 0.155
Platelet count (×103/uL), median (IQR) 121 (57–226) 250 (184–281) 0.023
Prothrombin time (s), median (IQR) 18.5 (16.0–28.8) 15.1 (14.1–16.7) 0.005
Partial thromboplastin time (s), median (IQR) 55.4 (41.8–75.6) 45.8 (39–59.3) 0.224
d-dimer (µg/mL), median (IQR)a 7.09 6.0 (1.8–12.1) na
Fibrinogen (mg/dL), median (IQR)b 176 (135–342) 597 (465–797) 0.003
Heparinase-corrected thromboelastography
 R value (min), median (IQR)c 6.25 (sd = 1.48) 6.5 (5.6–9.6) NA
 Fibrinogen activity angle (º), median (IQR)c 66.7 (sd = 8.7) 75.9 (69.8–78.2) NA
 Maximum amplitude (mm), median (IQR)c 57.1 (sd = 6.9) 75.6 (72.4–82.2) NA
 Clot lysis 30 min after maximum clot strength (%), median (IQR)d 0.05 (sd = 0.07) 0.2 (0–0.5) NA
Thrombotic event on or before date of plasma collection (%)
 None 80.00 76.90 0.791
 Ischemic stroke 0.00 0.00
 Myocardial infarction 10.00 7.70
 Acute limb ischemia 0.00 7.70
 Deep vein thrombosis 0.00 3.80
 Pulmonary embolism 10.00 3.80
Need for renal replacement therapy on or before date of plasma collection (%) 30.00 34.60 1
Need for extracorporeal membrane oxygenation on or before date of plasma collection (%) 10.00 15.40 1
Anticoagulation (%)
 None 50.00 3.80 0.001
 Prophylactic 0.00 0.00
 Intermediate 0.00 42.30
 Therapeutic 50.00 53.80
Anticoagulation agent (%)
 None 50.00 3.80 0.009
 Low-molecular-weight heparin 0.00 3.80
 Heparin 40.00 84.60
 Other 10.00 7.70
Open in a new tab

IQR = interquartile range, NA = too few subjects in non-COVID-19 with sepsis cohort to do statistical comparison.

a

n = 1 non-COVID-19 with sepsis, 26 COVID-19.

b

n = 5 non-COVID-19 with sepsis, 16 COVID-19.

c

n = 2 non-COVID-19 with sepsis (mean and sd reported), 20 COVID-19.

d

n = 2 non-COVID-19 with sepsis (mean and sd reported), 7 COVID-19.

Reference ranges: WBC count: 4.3–11.3 × 103/µL, hemoglobin: 12–17.5 g/dL, platelet count: 150–450 × 103/µL, prothrombin time: 11.9–14.2 s, partial thromboplastin time: 22.5–36 s, d-dimer: < 0.5 µg/mL, fibrinogen: 225–434 mg/dL, heparinase-corrected thromboelastography results: R value: 4–7 min, fibrinogen activity angle: 61°–73°, maximum amplitude: 55–65 mm, clot lysis 30 min after maximum clot strength: 0–5%.

Sepsis diagnosis based on Sepsis-3 criteria. 
Boldface font indicates p < 0.05.

Hematologic parameters and markers of coagulation/fibrinolysis on the date of plasma collection were consistent with prior reports for patients with COVID-19 (Table 1) (6–8). The platelet count was significantly greater in patients with COVID-19 compared with septic patients without COVID-19 (250 [IQR, 184–281] vs 121 [IQR, 57–226] ×103/µL; p = 0.023). d-dimer levels were markedly elevated in the COVID-19 cohort (6.0 µg/mL [IQR, 1.8–12.1 µg/mL], reference value less than 0.5 µg/mL), and fibrinogen was also elevated in the COVID-19 cohort relative to the reference range and in comparison with the non-COVID-19 with sepsis cohort (597 mg/dL [IQR, 465–797 mg/dL] vs 176 mg/dL [IQR, 135–342 mg/dL]; p = 0.003). Of all patients with thromboelastography (TEG) data, all patients with COVID-19 (20/20) demonstrated hypercoagulable TEG parameters (i.e., heparinase-corrected maximum amplitude [MA] greater than 67 mm) despite receipt of intermediate or therapeutically dosed anticoagulation, whereas no septic patients without COVID-19 (0/2) demonstrated hypercoagulable TEG parameters.

Ex Vivo Plasma Clots From Patients With COVID-19 Have Increased Fibrin Network Density

Figure 1A shows representative images of the fibrin structure formed in plasma from healthy donors, septic patients without COVID-19, and patients with COVID-19. The 2D images give the impression that ex vivo clots from patients with COVID-19 have smaller pores and are denser than clots from septic patients without COVID-19. Corresponding to the representative images, the average percent area occupied by fluorescent fibrin was significantly greater in patients with COVID-19 compared with septic patients without COVID-19 (48.1% [sd, 16%] vs 24.9% [sd, 18.8%]; p = 0.049) (Fig. 1B).

Figure 1.

Figure 1.

Open in a new tab

Aberrant fibrin clot structure in critically ill patients with COVID-19. A, Confocal microscopy images of fibrin clots at a magnification of ×60 formed in plasma from healthy donors (HDs), critically ill patients without COVID-19 (−C19−), or critically ill patients with COVID-19 (+C19+). Representative images from three different subjects in each cohort are displayed. Scale bars, 50 um. B, Quantification of confocal images of fibrin clots was performed using Fiji by ImageJ. Percent area of fluorescent fibrin was determined for healthy donors (×) to determine a reference value. Percent area was determined and compared between −C19− (open circle) and +C19+ (open diamond) patients. Lines show the mean ± 1 sd; symbols indicate values for individual subjects performed in triplicate. *p = 0.049.

Ex Vivo Plasma Clots From Patients With COVID-19 Have a Higher Resistance to Flow and Reduced Sieving Coefficient

To assess the functionality of clot porosity, we measured the resistance to flow and the sieving quality of the clots. Figure 2A shows that the median time required for buffer to flow through clots from patients with COVID-19 was significantly greater compared with septic patients without COVID-19 (216 min [IQR, 141–441 min] vs 82 min [IQR, 57–126 min]; p < 0.001). In a similar experiment, fluorescent submicron beads (0.2, 0.5, and 0.8 um) were added to the buffer prior to flowing the buffer through the clots. The proportion and concentration of each sized bead within the mixture was determined by flow cytometry at baseline and after clot flow-through. Figure 2B shows that the mixture became enriched for 0.2-um beads, whereas the 0.5- and 0.8-um beads were depleted after flow-through. This pattern was more pronounced in the COVID-19 cohort compared with the non-COVID-19 with sepsis cohort (p < 0.001). The bead concentration was used to determine the sieving coefficient that ranges from 0 (no transport) to 1 (unrestricted transport). The sieving coefficient was lower for each sized bead in the patients with COVID-19 compared with septic patients without COVID-19 (Fig. 2C), but these results did not reach statistical significance.

Figure 2.

Figure 2.

Open in a new tab

Reduced functional porosity in fibrin clots from critically ill patients with COVID-19. A, Box and whisker plot representing how long it took buffer to flow through fibrin clots. Flow-through was performed in clots from five healthy donors, 10 critically ill patients without COVID-19 (−C19−), and 10 critically ill patients with COVID-19 (+C19+) in duplicate. *p < 0.001. B, Similar experiment to (A) but buffer contained a mixture of equal volumes of 0.2-, 0.5-, and 0.8-um fluorescent beads. The first column shows the submicron bead composition in the buffer at baseline (preflow-through) and the subsequent columns show the bead composition of the buffer after it flowed through the clots formed from healthy human donors, critically ill patients without COVID-19 (−C19−), or critically ill patients with COVID-19 (+C19+). Bead composition within the buffer was determined by FACS analysis, values represent the mean percent composition based on three clots/cohort performed in triplicate, and the composition was compared between −C19− and +C19+ cohorts using Pearson χ2, *p = <0.001. C, The same experiment as B but bead concentration pre-/postflow-through was used to determine the sieving coefficient. Sieving coefficient = bead concentration postflow-through/bead concentration preflow-through. Bars represent the mean sieving coefficient based on three clots per cohort performed in triplicate and error bars represent 1 sd. FACS = fluorescence activated cell sorting.

Patients With COVID-19 Have Enhanced Ex Vivo Plasma Fibrin Formation Potential

Next, we used a turbidimetric assay to measure the kinetics of fibrin polymerization and fibrinolysis (30). The maximum absorbance change associated with fibrin polymerization was significantly greater in patients with COVID-19 compared with septic patients without COVID-19 (Fig. 3, A and B). This represents increased fibrin formation potential as there were no changes in the kinetics of fibrin polymerization associated with COVID-19 status.

Figure 3.

Figure 3.

Open in a new tab

Increased fibrin formation potential in critically ill patients with COVID-19. A, Values for the combined plasma clot turbidity and lysis assay. Fibrin polymerization was performed in triplicate using plasma from six healthy donors 9 critically ill patients without COVID-19 (−C19−), and 24 critically ill patients with COVID-19 (+C19+). Fibrinolysis was performed in triplicate concurrently for two of the healthy donors, 8 of the −C19− patients, and 21 of the +C19+ patients. B, Representative curves constructed from the values in A using Microsoft Excel. IQR = interquartile range, ODU = optical density units.

Increased Fibrin Formation Potential Is Associated With Clinical Markers of Hypercoagulability

We performed Spearman correlation analyses to explore the relationship between the ex vivo plasma-based fibrin clot assays. The heat maps for each cohort are presented in Figure 4, A and B, and the numerical correlation coefficients and p values are presented in Supplementary Tables 2 and 3 (http://links.lww.com/CCM/H29). Whereas there was a strong positive correlation between percent area and maximum absorbance in the non-COVID-19 with sepsis cohort (ρ = 1, p = 0.01), these variables were not correlated in the COVID-19 cohort (ρ = −0.657, p = 0.156). There was no linear relationship between either percent area or maximum absorbance with flow-through duration in either cohort.

Figure 4.

Figure 4.

Open in a new tab

Increased fibrin formation potential is associated with clinical markers of hypercoagulability in correlation analysis. Spearman rank correlations were performed for clinical metrics, conventional coagulation biomarkers, and the plasma-based assays incritically ill patients without COVID-19 (A) and critically ill patients with COVID-19 (B). Positive correlations are shaded in blue, whereas negative correlations are shaded in red, as defined in the legend. *demonstrates the correlation coefficient had a p value < 0.05. BMI = body mass index, CL = clot lysis, FP = fibrin polymerization, HTEG = heparinase-corrected thromboelastography, MA = maximum amplitude, Pao2:Fio2 = ratio of Pao2 to Fio2, PT = prothrombin time, PTT = partial thromboplastin time, SOFA = Sequential Organ Failure Assessment score.

Finally, we correlated the plasma-based assay results with clinical parameters and biomarkers of coagulation. d-dimer correlated positively with the SOFA score (ρ = 0.435, p = 0.026) and negatively with the Pao2:Fio2 ratio (ρ = −0.486, p = 0.012) in subjects with COVID-19. d-dimer also correlated strongly and positively with the percent area (fluorescent fibrin) (ρ = 0.714, p = 0.047) in this cohort.

The TEG alpha angle represents the speed of fibrin polymerization and is clinically augmented with fibrinogen. Interestingly, there was no association between the alpha angle and fibrinogen levels (ρ = −0.049, p = 0.873), nor between the alpha angle and the rate of fibrin polymerization (ρ = 0.204, p = 0.403). The TEG MA represents the highest vertical amplitude of the TEG tracing (or clot strength) and can be augmented with supplemental platelets (and to a lesser extent, fibrinogen). Consistently, there was a positive and significant correlation between platelets and TEG MA (ρ = 0.507, p = 0.023). There was no relationship between fibrinogen and TEG MA (ρ = 0.325, p = 0.279). Fibrinogen and TEG MA each correlated positively with the rate of fibrin polymerization in the turbidity assay (ρ = 0.613, p = 0.02 and ρ = 0.527, p = 0.02). In the COVID-19 cohort, there was a positive correlation between both the TEG MA and maximum absorbance with the time required for 50% clot lysis (ρ = 0.482, p = 0.05 and ρ = 0.808, p < 0.001). There was also an association between illness severity (evidence of higher SOFA) and a reduced slope for clot lysis in COVID-19.

DISCUSSION

Significant fibrin deposition has been observed in patients with COVID-19 and is thought to contribute to the morbidity and mortality of this disease (12–15). This cross-sectional study focuses on elucidating the ex vivo fibrin clot structure in critically ill patients with or without COVID-19 and correlating these findings with clinical parameters. Plasma clots from critically ill subjects with COVID-19 were denser, had greater resistance to flow, and reduced functional porosity compared with a similar cohort of patients with sepsis but without COVID-19. There was a strong correlation between the density of the fibrin clots with d-dimer levels. Thus, the aberrant clot structure and associated functional changes reported herein may explain why elevated d-dimer levels are pathologic in critically ill patients with COVID-19 (8, 11, 28, 31, 32).

Changes in the architecture of the fibrin clot, including density and pore size, are associated with a high risk for thrombosis in certain diseases (33–35). Further, a clot’s proclivity to fibrinolysis is affected by the fibrin network density (21, 25, 36, 37). In this study, the fibrin network density was greater in the COVID-19 cohort (as assessed by both our plasma-based turbidity assay and whole-blood TEG), and there was prolongation of the clot lysis time in patients with COVID-19 with higher SOFA scores. d-dimer levels were also associated with the percent area of fluorescent fibrin formed in the ex vivo clots—again suggesting a relationship between clot structure and fibrinolysis in COVID-19. One can argue that the altered density and porosity in the fibrin clots negatively influences fibrinolysis and possibly disease outcomes in patients with COVID-19. In fact, impaired fibrinolysis has been demonstrated in COVID-19 (20, 28). Previous reports demonstrated that the level of fibrinogen only has minor effects on fibrin structure in vitro and that other plasma proteins (e.g., vitronectin and von Willebrand factor) influence fibrin clot structure (38–40). These reports may support our finding that the level of fibrinogen was not directly associated with clot density in our ex vivo assays or TEG. More studies are necessary to investigate which other blood components contribute to the alteration of fibrin polymerization and fibrinolysis in COVID-19.

One limitation of our study is the small sample size. Our study was appropriately powered to detect a statistically significant difference in the ex vivo fibrin clot structure between critically ill patients with COVID-19 versus those with sepsis but without COVID-19. Our observation that the structure and function of ex vivo fibrin clots are altered in COVID-19 is novel. We acknowledge that the limited sample size required for this observation makes it difficult to ascertain the clinical implications with certainty. This is compounded by the significant difference in anticoagulation use between the cohorts—despite relative similarity in other key clinical variables. Nevertheless, we report some noteworthy clinical associations. Specifically, we observed that d-dimer levels positively correlated with the SOFA score and negatively correlated with the Pao2:Fio2 ratio in patients with COVID-19. This pattern suggests that fibrin polymerization and subsequent degradation are pathologic in COVID-19 and agrees with several previous reports (9, 11, 41).

The occurrence of thrombotic events in the patients with COVID-19 was lower than we anticipated and precluded a logistic regression analysis for this variable. Regarding the low frequency of thrombotic events in this study, we previously reported a high frequency of clinically significant thromboses in patients with hypercoagulable TEG parameters despite use of the recommended standard-dose thromboprophylaxis (6). Based on those findings, we developed and implemented a TEG-based intermediate-intensity anticoagulation protocol for patients with COVID-19 admitted to our ICU (42) (Supplementary Fig. 1, http://links.lww.com/CCM/H29). Thus, because all the patients with COVID-19 in this study had hypercoagulable TEG parameters, they were accordingly given intermediate or therapeutically dosed anticoagulation; most of these subjects (65.4%) received this therapy upon admission. Whether the low frequency of thrombotic events observed in our COVID-19 cohort was due to this protocol remains to be defined, and several randomized controlled trials are currently addressing the benefits of therapeutic-intensity anticoagulation (43–46). This protocol may limit the generalizability of our results.

A final limitation is that our study used PPP under static conditions to assess ex vivo fibrin clot structure and clot formation dynamics. Factors within whole blood have an impact on clot formation (47). Hemodynamic flow also impacts fibrin deposition and fiber diameter during clot formation (48, 49). These variables may influence why there was not a stronger correlation between the turbidity assay of this study and the patients’ TEG results as TEG uses whole blood and low shear stress (50).

CONCLUSIONS

This is the first report of aberrant fibrin clot structure and function in critically ill patients with COVID-19. The augmented clot density may contribute to the poor outcomes of COVID-19 patients with observed widespread fibrin deposition.

ACKNOWLEDGMENTS

We thank the blood bank at Baylor St. Luke’s Medical Center for assistance in procurement of patient samples.

Supplementary Material

ccm-50-e557-s001.docx (4.9MB, docx)
Open in a new tab

Footnotes

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website (http://journals.lww.com/ccmjournal).

Drs. Brubaker, Saini, Nguyen, Martinez-Vargas, Lam, and Cruz helped in conception/design. All authors helped in data acquisition, data interpretation, critical revisions, and final approval. All individuals claiming authorship meet all of the following three conditions: 1) authors made substantial contributions to conception and design, and/or acquisition of data, and/or analysis and interpretation of data, 2) authors participated in drafting the article or revising it critically for important intellectual content, and 3) authors gave final approval of the version to be published.

Drs. Brubaker and Martinez-Vargas are supported by grant no. T32 HL139425 from the National Institutes of Health (NIH) and National Heart, Lung, and Blood Institute. Drs. Nguyen and Cruz are supported in part by NIH National Institute of General Medical Sciences (NIGMS) R01 GM112806. Dr. Cruz is also supported in part by NIH-National Institute of Neurological Disorders and Stroke R01 NS094280 and by the Merit Review Award I01 BX002551 from the U.S. Department of Veterans Affairs Biomedical Laboratory Research and Development Service. Dr. Cruz also reported being the founder of A2 Therapeutics and owning patent no. 7879793. The research does not involve any products or commercial interest related to A2 therapeutics or the patent held by Dr. Cruz. Dr. Lam is supported by NIH-NIGMS K08 GM123261. Dr. Brubaker’s institution received funding from the National Heart, Lung, and Blood Institute and A2 Therapeutics. Drs. Nguyen, Lam, and Cruz received support for article research from the NIH. Dr. Nguyen’s institution received funding from the NIH. Dr. Lam received funding from NIH-NIGMS (K08 GM123261); he disclosed that he is listed as an inventor on U.S. Patents 10617751 (Edible Vaccines) and 10695401 (Recombinant Vimentin for Inflammation). Dr. Cruz’s institution received funding from the National Institute of Neurological Disorders and Stroke (R01 NS094280) and the Merit Review Award I01 BX002551 from the U.S. Department of Veteran’s Affairs Biomedical Laboratory Research and Development Service; he disclosed that he is the founder of A2 Therapeutics and owns patent no. 7879793. The remaining authors have disclosed that they do not have any potential conflicts of interest.

The funders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication. The contents of this article do not represent the views of the Department of Veterans Affairs or the US government.

REFERENCES

  • 1.Wu Z, McGoogan JM: Characteristics of and important lessons from the coronavirus disease 2019 (COVID-19) outbreak in China: Summary of a report of 72 314 cases from the Chinese Center for Disease Control and Prevention. JAMA 2020; 323:1239–1242 [DOI] [PubMed] [Google Scholar]
  • 2.Cui S, Chen S, Li X, et al. : Prevalence of venous thromboembolism in patients with severe novel coronavirus pneumonia. J Thromb Haemost 2020; 18:1421–1424 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 3.Klok FA, Kruip MJHA, van der Meer NJM, et al. : Confirmation of the high cumulative incidence of thrombotic complications in critically ill ICU patients with COVID-19: An updated analysis. Thromb Res 2020; 191:148–150 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 4.Llitjos JF, Leclerc M, Chochois C, et al. : High incidence of venous thromboembolic events in anticoagulated severe COVID-19 patients. J Thromb Haemost 2020; 18:1743–1746 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 5.Thomas W, Varley J, Johnston A, et al. : Thrombotic complications of patients admitted to intensive care with COVID-19 at a teaching hospital in the United Kingdom. Thromb Res 2020; 191:76–77 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 6.Mortus JR, Manek SE, Brubaker LS, et al. : Thromboelastographic results and hypercoagulability syndrome in patients with coronavirus disease 2019 who are critically ill. JAMA Netw Open 2020; 3:e2011192. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 7.Pavoni V, Gianesello L, Pazzi M, et al. : Evaluation of coagulation function by rotation thromboelastometry in critically ill patients with severe COVID-19 pneumonia. J Thromb Thrombolysis 2020; 50:281–286 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 8.Tang N, Li D, Wang X, et al. : Abnormal coagulation parameters are associated with poor prognosis in patients with novel coronavirus pneumonia. J Thromb Haemost 2020; 18:844–847 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 9.Yao Y, Cao J, Wang Q, et al. : D-dimer as a biomarker for disease severity and mortality in COVID-19 patients: A case control study. J Intensive Care 2020; 8:49. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 10.Zhang L, Yan X, Fan Q, et al. : D-dimer levels on admission to predict in-hospital mortality in patients with Covid-19. J Thromb Haemost 2020; 18:1324–1329 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 11.Short SAP, Gupta S, Brenner SK, et al. ; STOP-COVID Investigators: D-dimer and death in critically ill patients with coronavirus disease 2019. Crit Care Med 2021; 49:e500–e511 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 12.Ackermann M, Verleden SE, Kuehnel M, et al. : Pulmonary vascular endothelialitis, thrombosis, and angiogenesis in Covid-19. N Engl J Med 2020; 383:120–128 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 13.Sauter JL, Baine MK, Butnor KJ, et al. : Insights into pathogenesis of fatal COVID-19 pneumonia from histopathology with immunohistochemical and viral RNA studies. Histopathology 2020; 77:915–925 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 14.Rapkiewicz AV, Mai X, Carsons SE, et al. : Megakaryocytes and platelet-fibrin thrombi characterize multi-organ thrombosis at autopsy in COVID-19: A case series. Lancet 2020; 24:100434. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 15.Fox SE, Akmatbekov A, Harbert JL, et al. : Pulmonary and cardiac pathology in African American patients with COVID-19: An autopsy series from New Orleans. Lancet Respir Med 2020; 8:681–686 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.Piazza G, Morrow DA: Diagnosis, management, and pathophysiology of arterial and venous thrombosis in COVID-19. JAMA 2020; 324:2548–2549 [DOI] [PubMed] [Google Scholar]
  • 17.Mackman N, Antoniak S, Wolberg AS, et al. : Coagulation abnormalities and thrombosis in patients infected with SARS-CoV-2 and other pandemic viruses. Arterioscler Thromb Vasc Biol 2020; 40:2033–2044 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 18.Mucha SR, Dugar S, McCrae K, et al. : Coagulopathy in COVID-19. Clev Clin J Med 2020; 14:14. [DOI] [PubMed] [Google Scholar]
  • 19.Iba T, Levy JH, Levi M, et al. : Coagulopathy of coronavirus disease 2019. Crit Care Med 2020; 48:1358–1364 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 20.Bouck E, Denorme F, Holle L, et al. : COVID-19 and sepsis are associated with different abnormalities in plasma procoagulant and fibrinolytic activity. Arterioscler Thromb Vasc Biol 2021; 41:401–414 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 21.Undas A, Ariëns RA: Fibrin clot structure and function: A role in the pathophysiology of arterial and venous thromboembolic diseases. Arterioscler Thromb Vasc Biol 2011; 31:e88–e99 [DOI] [PubMed] [Google Scholar]
  • 22.Valladolid C, Martinez-Vargas M, Sekhar N, et al. : Modulating the rate of fibrin formation and clot structure attenuates microvascular thrombosis in systemic inflammation. Blood Adv 2020; 4:1340–1349 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 23.Gabriel DA, Muga K, Boothroyd EM: The effect of fibrin structure on fibrinolysis. J Biol Chem 1992; 267:24259–24263 [PubMed] [Google Scholar]
  • 24.Carr ME, Jr, Alving BM: Effect of fibrin structure on plasmin-mediated dissolution of plasma clots. Blood Coagul Fibrinolysis 1995; 6:567–573 [DOI] [PubMed] [Google Scholar]
  • 25.Gould TJ, Vu Trang T, Stafford AR, et al. : Cell-free DNA modulates clot structure and impairs fibrinolysis in sepsis. Arterioscler Thromb Vasc Biol 2015; 35:2544–2553 [DOI] [PubMed] [Google Scholar]
  • 26.von Elm E, Altman D, Egger M, et al. : Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Statement: Guidelines for Reporting Observational Studies, 2019. Available at: https://www.equator-network.org/reporting-guidelines/strobe/. Accessed February 11, 2021 [DOI] [PMC free article] [PubMed]
  • 27.Singer M, Deutschman CS, Seymour CW, et al. : The third international consensus definitions for sepsis and septic shock (Sepsis-3). JAMA 2016; 315:801–810 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 28.Wright FL, Vogler TO, Moore EE, et al. : Fibrinolysis shutdown correlates to thromboembolic events in severe COVID-19 infection. J Am Coll Surg 2020; 231:193–203.e1 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 29.Vincent JL, Moreno R, Takala J, et al. : The SOFA (Sepsis-related Organ Failure Assessment) score to describe organ dysfunction/failure. On behalf of the Working Group on Sepsis-Related Problems of the European Society of Intensive Care Medicine. Intensive Care Med 1996; 22:707–710 [DOI] [PubMed] [Google Scholar]
  • 30.Pieters M, Philippou H, Undas A, et al. ; Subcommittee on Factor XIII and Fibrinogen, and the Subcommittee on Fibrinolysis: An international study on the feasibility of a standardized combined plasma clot turbidity and lysis assay: Communication from the SSC of the ISTH. J Thromb Haemost 2018; 16:1007–1012 [DOI] [PubMed] [Google Scholar]
  • 31.He X, Yao F, Chen J, et al. : The poor prognosis and influencing factors of high D-dimer levels for COVID-19 patients. Sci Rep 2021; 11:1830. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 32.Léonard-Lorant I, Delabranche X, Séverac F, et al. : Acute pulmonary embolism in patients with COVID-19 at CT angiography and relationship to d-dimer levels. Radiology 2020; 296:E189–E191 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 33.Ząbczyk MA-OX, Natorska J, Undas A: Fibrin clot properties in atherosclerotic vascular disease: From pathophysiology to clinical outcomes. J Clin Med 2021; 10:2999. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 34.Undas A, Zawilska K, Ciesla-Dul M, et al. : Altered fibrin clot structure/function in patients with idiopathic venous thromboembolism and in their relatives. Blood 2009; 114:4272–4278 [DOI] [PubMed] [Google Scholar]
  • 35.Mills JD, Ariëns RA, Mansfield MW, et al. : Altered fibrin clot structure in the healthy relatives of patients with premature coronary artery disease. Circulation 2002; 106:1938–1942 [DOI] [PubMed] [Google Scholar]
  • 36.Collet JP, Park D, Lesty C, et al. : Influence of fibrin network conformation and fibrin fiber diameter on fibrinolysis speed: Dynamic and structural approaches by confocal microscopy. Arterioscler Thromb Vasc Biol 2000; 20:1354–1361 [DOI] [PubMed] [Google Scholar]
  • 37.Cilia La Corte AL, Philippou H, Ariens RA: Role of fibrin structure in thrombosis and vascular disease. Adv Protein Chem Struct Biol 2011; 83:75–127 [DOI] [PubMed] [Google Scholar]
  • 38.Miszta A, Pelkmans L, Lindhout T, et al. : Thrombin-dependent incorporation of von Willebrand factor into a fibrin network. J Biol Chem 2014; 289:35979–35986 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 39.Ryan EA, Mockros LF, Weisel JW, et al. : Structural origins of fibrin clot rheology. Biophys J 1999; 77:2813–2826 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 40.Wu YP, Bloemendal HJ, Voest EE, et al. : Fibrin-incorporated vitronectin is involved in platelet adhesion and thrombus formation through homotypic interactions with platelet-associated vitronectin. Blood 2004; 104:1034–1041 [DOI] [PubMed] [Google Scholar]
  • 41.Zhang L, Yan X, Fan Q, et al. : D-dimer levels on admission to predict in-hospital mortality in patients with Covid-19. J Thromb Haemost 2020; 19:19. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 42.Brubaker L, Mortus J, Cruz M, et al. : Thromboelastography might be more applicable to guide anticoagulant therapy than fibrinolytic therapy in critically ill patients with COVID-19. J Am Coll Surg 2021; 232:227–229 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 43.Webb S, McArthur C, Bonten M, et al. : Randomized, Embedded, Multifactorial Adaptive Platform Trial for Community-Acquired Pneumonia (REMAP-CAP), 2021. ClinicalTrials.gov. Available at: https://clinicaltrials.gov/ct2/show/NCT02735707. Accessed February 12, 2021 [DOI] [PMC free article] [PubMed]
  • 44.Hochman J: Anti-thrombotics for Adults Hospitalized With COVID-19 (ACTIV-4), 2021. ClinicalTrials.gov. Available at: https://clinicaltrials.gov/ct2/show/NCT04505774. Accessed February 12, 2021
  • 45.Lawler P, Goligher E, Zarychanski R: Antithrombotic Therapy to Ameliorate Complications of COVID-19 (ATTACC) (ATTACC), 2021. ClinicalTrials.gov. Available at: https://clinicaltrials.gov/ct2/show/NCT04372589. Accessed February 12, 2021
  • 46.INSPIRATION: Effect of intermediate-dose vs standard-dose prophylactic anticoagulation on thrombotic events, extracorporeal membrane oxygenation treatment, or mortality among patients with COVID-19 admitted to the intensive care unit: The INSPIRATION randomized clinical trial. JAMA 2021; 325:1620–1630 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 47.Hranjec T, Estreicher M, Rogers B, et al. : Integral use of thromboelastography with platelet mapping to guide appropriate treatment, avoid complications, and improve survival of patients with coronavirus disease 2019-related coagulopathy. Crit Care Explor 2020; 2:e0287. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 48.Neeves KB, Illing DA, Diamond SL: Thrombin flux and wall shear rate regulate fibrin fiber deposition state during polymerization under flow. Biophys J 2010; 98:1344–1352 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 49.Machlus KR, Cardenas JC, Church FC, et al. : Causal relationship between hyperfibrinogenemia, thrombosis, and resistance to thrombolysis in mice. Blood 2011; 117:4953–4963 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 50.Bontekoe IJ, van der Meer PF, de Korte D: Thromboelastography as a tool to evaluate blood of healthy volunteers and blood component quality: A review. Vox Sang 2019; 114:643–657 [DOI] [PubMed] [Google Scholar]

Articles from Critical Care Medicine are provided here courtesy of Wolters Kluwer Health

RESOURCES