Abstract
Background
D-lactic acid is a specific marker produced almost exclusively by bacterial species; thus, the appearance of this marker in synovial fluid may indicate periprosthetic joint infection (PJI). Recently, studies have investigated the accuracy of enzyme-linked laboratory tests that detect D-lactic acid in synovial fluid to diagnose PJI. However, to our knowledge, no studies have determined the usefulness of rapid strip tests that detect D-lactic acid in synovial fluid in the diagnosis of PJI.
Questions/purposes
(1) What is the best cutoff value for the rapid D-lactic acid strip test for diagnosing PJI? (2) What are the diagnostic accuracies (sensitivity, specificity, positive predictive value [PPV], and negative predictive value [NPV]) of the rapid D-lactic acid strip test and two different rapid leukocyte esterase (LE) strip tests?
Methods
This prospective study enrolled 157 patients who underwent revision THA or TKA from May 2021 to February 2022 at a single orthopaedic center. Seventy percent (110 of 157) were eligible for analysis; 10% of these patients (15 of 157) were excluded based on the exclusion criteria (causes of revisions and additional comorbidities that may interfere with the results), and 20% (32 of 157) of the synovial fluid samples could not be tested (dry taps and blood-contaminated samples that could not be centrifuged). We performed the following off-label diagnostic tests on synovial fluid samples collected from all patients: the D-lactic acid strip test (QuantiQuickTM, BioAssay System), two different LE strip tests (10 EA from ARKRAY and BM 10 from BioMaxima). Differently colored strips were marked with symbols (from [-] to [++++] for D-lactic acid and from [-] to [+++] for LE tests) according to the manufacturers’ instructions. For the LE tests, results were different for (++), which corresponds to a minimal value of 250 leu/mL for 10 EA and 125 leu/mL for BM 10 tests. The diagnostic standard for the presence or absence of PJI in this study was the International Consensus Meeting (ICM) 2018 criteria; based on these criteria (without the application of an LE test as a minor criterion), all patients were assessed and divided into two groups. Patients who did not meet the criteria for PJI and underwent revision for aseptic loosening, implant malposition, instability, or implant damage were included in the aseptic revision total joint arthroplasty group (68 patients). Patients with a fistula penetrating the joint, those with two positive culture results of the same pathogen, or those with ≥ 6 points according to ICM 2018 minor criteria were enrolled in the PJI group (42 patients). To ascertain the best cutoff value for the rapid D-lactic acid and both LE strip tests for diagnosing PJI, we used collected results, generated a receiver operating characteristic curve, and calculated the Youden index. To determine the accuracies of the diagnostic tests, we calculated their sensitivities, specificities, PPVs, and NPVs against the diagnostic standard (the ICM 2018 criteria).
Results
The best cutoff value for D-lactic acid was 22.5 mg/L, which corresponded to a reading of (+) on the test strip. For D-lactic acid, in the diagnosis of PJI, the sensitivity was 83% (95% confidence interval [CI] 68% to 92%) and specificity was 100% (95% CI 93% to 100%). For both LE strip tests, the best cutoff value was the same as that proposed in the ICM 2018 criteria. For LE (10 EA), the sensitivity was 81% (95% CI 66% to 91%) and specificity was 99% (95% CI 91% to 100%); for LE (BM 10), sensitivity was 81% (95% CI 65% to 91%) and specificity was 97% (95% CI 89% to 100%).
Conclusion
A rapid off-label D-lactic acid strip test is valuable for diagnosing PJI. The results of this study indicate very good accuracy with comparable sensitivity and specificity for both LE strip tests. The usefulness of the test in a group of patients with chronic inflammatory diseases and the reproducibility of the reading by different researchers were not analyzed in this study and require further investigations. Before a rapid D-lactic strip test is routinely used for diagnosing PJI, multicenter studies on a larger group of patients should be conducted.
Level of Evidence Level II, diagnostic study.
Introduction
The increasing number of total joint arthroplasties (TJAs) worldwide has resulted in the growth in the number of septic and aseptic complications [13]. Periprosthetic joint infections (PJIs) are considered one of the most devastating complications and require an accurate diagnosis and identification of the pathogen. Although several markers are used routinely for diagnosing infection and were included in the newest definitions of PJI [8, 17, 18, 23, 25], the development of different, novel blood and synovial fluid biomarkers with higher accuracy is necessary [4, 9, 21, 28]. The current line of research has focused on improving sensitivity and specificity in parallel with simplifying and increasing the test’s availability [10, 20]. Recently, several rapid cassette and strip tests dedicated to determining leukocyte esterase (LE), alpha-defensin, and calprotectin in synovial fluid have been applied to the diagnosis of PJI [5, 15, 26]. The other marker with great potential is lactic acid, which is synthesized from pyruvic acid in two isomeric forms. Although L-lactic acid is produced by human cells in anaerobic conditions, the D isomer of this molecule is mostly a specific product of the bacterial metabolism of carbohydrates because of the action of D-lactate dehydrogenase [2, 19]. The main reasons that D-lactic acid may be detected in humans include cellular production by the methylglyoxal pathway, diet supplementation, or the presence of bacteria (for example, from colonic microbiota or bacteria causing the infection) [2]. Therefore, an increasing concentration of this marker, especially in sterile body fluids (for example, synovial fluid), may indirectly indicate that there is an infection.
Although there are many laboratory tests with high sensitivity and specificity for diagnosing PJI, there is still no available rapid point-of-care test with high accuracy. The need to perform rapid diagnostics in the outpatient clinic or operating room intraoperatively prompted us to conduct research on a rapid test that detects D-lactic acid based on the promising results of laboratory tests [11, 28]. The utility of D-lactic acid in diagnosing a native joint infection and PJIs was recently investigated with laboratory ELISA tests with good results [7, 11, 16, 28]. However, the effectiveness of commercially available, rapid, semiquantitative strip tests that can diagnose PJI has not, to our knowledge, been examined.
We therefore asked: (1) What is the best cutoff value for the rapid D-lactic acid strip test for diagnosing PJI? (2) What are the diagnostic accuracies (sensitivity, specificity, positive predictive value [PPV], and negative predictive value [NPV]) of the rapid D-lactic acid strip test and two different rapid LE strip tests?
Patients and Methods
Study Design and Setting
Participants
A total of 157 patients who underwent revision THA or TKA in a single orthopaedic academic center between May 2021 and February 2022 were assessed in terms of the inclusion and exclusion criteria. The inclusion criteria were revision hip and knee arthroplasties performed for aseptic implant loosening or implant malposition, instability, or pain, with a maximal value of 1 point in the preoperative minor criteria of the International Consensus Meeting (ICM) 2018 [18], and patients with chronic PJI confirmed according to the ICM 2018 criteria. The exclusion criteria were revision arthroplasties performed for periprosthetic fracture, prosthesis dislocation, or second stage of the PJI treatment; inconclusive PJI diagnosis; early PJI (occurring within 4 weeks of arthroplasty or hematogenous infection with signs of infection at the prosthesis site for less than 4 weeks); chronic inflammatory diseases (such as rheumatoid arthritis, ankylosing spondylitis, or psoriatic arthritis); the current treatment of malignancy; or suspected systemic infection. Patients who did not agree to participate in the study and those with missing necessary data were not considered in the preliminary analysis. Seventy percent (110 of 157) were eligible for further analysis; 10% of the patients (15 of 157) were excluded because they had causes of revision and additional comorbidities that might have interfered with the results. Additionally, 20% (32 of 157) had synovial fluid samples that could not be tested (dry taps and blood-contaminated samples that could not be centrifuged). These patients were divided into two experimental groups: PJI and aseptic revision TJA (Fig. 1).
Fig. 1.
This flow diagram shows the patients who were included in our study; PJI = periprosthetic joint infection.
Patients’ Baseline Data
Thirty-four percent of the included patients (37 of 110) were men and 66% (73 of 110) were women. The hip was operated on in 26% of the patients (29 of 110) and the knee was involved in 74% (81 of 110). The median age of the patients was 69 years (interquartile range [IQR] 62 to 75 years), and the median BMI was 30 kg/m2 (IQR 28 to 33 kg/m2). The median blood C-reactive protein concentration in the revision TJA group was 2.4 mg/L (IQR 1.1 to 4.1 mg/L), and that in the PJI group was 42 mg/L (IQR 19 to 68 mg/L). The median erythrocyte sedimentation rate was 10 mm/h (IQR 7 to 15 mm/h) in the revision TJA group and 55 mm/h (IQR 40 to 80 mm/h) in the PJI group (Table 1). In the revision TJA group, aseptic implant loosening was the main cause of revision (in 47% of infections [32 of 68]). Additionally, 24% (16 of 68) of revisions were performed for instability, 19% (13 of 68) for implant malposition, and 10% (seven of 68) for other causes (Table 2). PJI was diagnosed according to the ICM 2018 criteria. A sinus tract in the joint was observed in 13 patients, two positive culture results were identified in nine, and PJI was confirmed according to the minor criteria in 20 (Table 3). The most frequent pathogen was methicillin-sensitive Staphylococcus aureus, cultured in 24% (10 of 42) of infections. In two infections, two pathogens caused the infection and in three infections, no bacteria were identified (Table 4).
Table 1.
Demographic and clinical data
| Parameter | arTJA (n = 68) | PJI (n = 42) | p value |
| Sex, female, % (n) | 79 (54) | 45 (19) | < 0.001a |
| Knee, % (n) | 79 (54) | 64 (27) | 0.12a |
| Age in years, median (IQR) | 69 (62-75) | 69 (64-74) | > 0.99b |
| Weight in kg, mean (range) | 80 (54-115) | 89 (62-135) | 0.002c |
| BMI in kg/m2, median (IQR) | 29 (27-31) | 30 (28-34) | 0.28b |
| Blood CRP level in mg/L, median (IQR) | 2.4 (1.1-4.1) | 42 (19-68) | < 0.001b |
| ESR in mm/h, median (IQR) | 10 (7-15) | 55 (40-80) | < 0.001b |
| D-lactic acid level, % (n) (-) (< 22.5 mg/L) (+) (≥ 22.5 mg/L) (++) (≥ 45 mg/L) (+++) (≥ 90 mg/L) (++++) (≥ 180 mg/L) |
100 (68) 0 (0) 0 (0) 0 (0) 0 (0) |
17 (7) 33 (14) 19 (8) 24 (10) 7 (3) |
|
| Leukocyte esterase (10 EA), % (n) (++, +++) (≥ 250 leu/μL) |
1 (1) |
81 (34) |
|
| Leukocyte esterase (BM 10), % (n) (++, +++) (≥ 125 leu/μL) |
3 (2) |
81 (34) |
Fisher exact test (2 x 2).
Mann-Whitney U test.
t test. arTJA = aseptic revision total joint arthroplasty; PJI = periprosthetic joint infection; CRP = C-reactive protein; ESR = erythrocyte sedimentation rate. (-) = negative result of the strip test; (+; ++; +++; ++++) = positive results of the strip test depending on the color intensity.
Table 2.
Diagnoses in the aseptic revision group
| Diagnosis | Hip (n = 14) | Knee (n = 54) |
| Aseptic implant loosening, % (n) | 86 (12) | 37 (20) |
| Instability, % (n) | 0 (0) | 30 (16) |
| Implant malposition, % (n) | 0 (0) | 24 (13) |
| Other, % (n) Implant failure Pain Arthrofibrosis |
14 (2) 0 (0) 0 (0) |
0 (0) 7 (4) 2 (1) |
Table 3.
Preoperative criteria resulting in an infection diagnosis among the PJI group
| Criterion | Hip (n = 15) | Knee (n = 27) |
| Fistula or prosthesis visualized through the skin, % (n) | 20 (3) | 37 (10) |
| Two positive culture results, % (n) | 20 (3) | 22 (6) |
| Minor criteria (≥ 6 points according to the ICM 2018 criteria), % (n) | 60 (9) | 45 (11) |
Data in the table are the number of patients with each diagnostic criterion. PJI = periprosthetic joint infection; ICM = International Consensus Meeting.
Table 4.
Microbiological culture results for patients in the PJI group
| Pathogen | Hip (n = 15) | Knee (n = 27) |
| MSSA | 20 (3) | 26 (7) |
| MRCNS | 20 (3) | 15 (4) |
| MSCNS | 13 (2) | 7 (2) |
| S. agalactiae | 7 (1) | 7 (2) |
| MSSE | 7 (1) | 4 (1) |
| E. faecalis | 7 (1) | 4 (1) |
| MRSA | 7 (1) | 0 |
| K. pneumoniae | 0 | 4 (1) |
| E. faecium | 0 | 4 (1) |
| E. cloace | 0 | 4 (1) |
| P. oralis | 7 (1) | 0 |
| C. acnes | 7 (1) | 0 |
| S. mitis | 0 | 4 (1) |
| Streptococcus G | 7 (1) | 0 |
| S. haemoliticus | 0 | 4 (1) |
| Two pathogens | 0 | 7 (2) |
| False negative | 0 | 11 (3) |
Data are presented as % (n); PJI = periprosthetic joint infection; MSSA = methicillin-sensitive Staphylococcus aureus; MRCNS = methicillin-resistant coagulase-negative Staphylococcus; MSCNS = methicillin-sensitive coagulase-negative Staphylococcus; MSSE = methicillin-sensitive Staphylococcus epidermidis; MRSA = methicillin-resistant Staphylococcus aureus.
Test Methods
All synovial fluid samples were collected before or during surgery with a syringe and needle (18 G). Synovial fluid samples were immediately transferred to our hospital’s laboratory, where the rapid D-lactic test (QuantiQuickTM, BioAssay Systems) was performed, with different cutoffs: (+) ≥ 22.5 mg/L, (++) ≥ 45 mg/L, (+++) ≥ 90 mg/L, and (++++) ≥ 180 mg/L. LE tests used the following thresholds: (+) ≥ 75 leu/μL, (++) ≥ 250 leu/μL, and (+++) ≥ 500 leu/μL for 10 EA strips (ARKRAY) and (+) ≥ 70 leu/μL, (++) ≥ 125 leu/μL, and (+++) ≥ 500 leu/μL for BM 10 strips (BioMaxima). The D-lactic acid test and both LE tests were used off-label on synovial fluid samples but according to the manufacturers' instructions. In the case of contaminated blood samples, 10 minutes of centrifugation at 3000 rpm was performed. The D-lactic acid test was performed per the manufacturer’s recommendations using reagent tubes that were delivered with a strip set. To achieve two-time dilution (reagent to synovial fluid ratio 1:1), 400 μL of synovial fluid was added to the tube with the reagent. Afterward, a reaction pad on the strip was dipped into the tube for 5 seconds, the drops were removed, and after 5 minutes, the result was read. The reaction was based on the dehydrogenase catalyzed oxygenation of D-lactic acid in which the formed nicotinamide adenine dinucleotide reduces a chromogenic reagent on the strip (Fig. 2A). The LE tests were read in accordance with the manufacturer’s recommendations: 90 seconds for 10 EA and 120 seconds for BM 10 strips (Fig. 2B and 2C).
Fig. 2.
These photographs show representative strip tests with positive results for PJI (Participant 10 with PJI caused by E. faecalis): (A) D-lactic acid test (QuantiQuickTM, BioAssay Systems), (B) LE test (Aution Sticks 10 EA), and (C) LE test (BM 10, BioMaxima). A color image accompanies the online version of this article.
The tests were visually examined by two orthopaedic surgeons (DG, PW) and one microbiologist (AG), and the results were entered into an electronic dataset for statistical analysis. If there was not a unanimous decision about the strip test result, the final decision was made based on the opinions of two of the three researchers (DG, AG, PW). Three different verdicts in this study were not obtained.
Diagnostic Standard
The diagnostic standard against which the two tests (D-lactic and LE) were compared was the presence or absence of PJI as determined by the ICM 2018 criteria (LE tests were not used in the diagnostic process). Based on these criteria, all patients were assessed and divided into two groups; 68 patients were included in the aseptic revision TJA group (14 revision THAs and 54 revision TKAs) and 42 were included in the PJI group (15 revision THAs and 27 revision TKAs).
Primary and Secondary Study Outcomes
Our primary study outcome was to determine the best cutoff value for the rapid D-lactic acid test strip for diagnosing PJI. To obtain this, we used data from the D-lactic acid strip tests, generated a receiver operating characteristic curve, and calculated the Youden index.
Our secondary outcome was to determine the accuracies of the D-lactic acid test and to compare them with the accuracies of two different LE diagnostic strip tests in the same synovial fluid samples; we calculated their sensitivities, specificities, PPVs, and NPVs against the diagnostic standard (the ICM 2018 criteria).
Ethical Approval
This study was approved by the bioethics committee of our institution (number 35/2021) and was performed in accordance with the ethical standards of the 1964 Declaration of Helsinki.
Statistical Analysis
For the statistical analysis, Microsoft Excel 2019 (Microsoft Corp) and Statistica 13.1 (Tibco Software Inc) were used. Continuous variables including demographic and clinical data are presented as means with ranges and medians with interquartile ranges (IQRs). The Shapiro-Wilk test was performed to determine the data’s normality. Statistical differences were calculated with the Fisher exact test , t-test, and the Mann-Whitney U test. The best cutoff value for the D-lactic acid and LE strip tests was determined according to the Youden index, which captures the performance of dichotomous diagnostic tests and estimates the probability of an informed decision (stated as sensitivity + specificity – 1). Receiver operating characteristic curves were generated, and the areas under the curve were calculated to determine the optimal cutoffs and compare the tests’ diagnostic values. Sensitivity, specificity, PPV, and NPV were calculated regarding the calculated cutoff values.
Results
Best Cutoff Value for the D-lactic Acid and LE Tests
The best cutoff value for the D-lactic acid test was 22.5 mg/L, which corresponded to a reading of (≥ +) on the test strip. Similarly, for both LE tests, the best cutoff value was (≥ ++) for positive results (Fig. 3A-C).
Fig. 3.
These receiver operating characteristic curves represent the strip tests of (A) D-lactic acid, (B) 10 EA LE, and (C) BM 10 LE in the diagnosis of PJI. A color image accompanies the online version of this article.
Diagnostic Accuracy of the D-lactic Acid Test and LE Tests
The sensitivity, specificity, PPV, and NPV for D-lactic acid were 83% (95% confidence interval [CI] 68% to 92%), 100% (95% CI 93% to 100%), 100% (95% CI 88% to 100%), and 91% (95% CI 81% to 96%), respectively. For LE (10 EA), with the minimal value of 250 leu/μL (≥ ++) for a positive result, the sensitivity was 81% (95% CI 66% to 91%), specificity was 99% (95% CI 91% to 100%), PPV was 97% (95% CI 83% to 100%), and NPV was 89% (95% CI 80% to 95%). For LE (BM 10), with the minimal value of 125 leu/μL (≥++) for a positive result, the sensitivity, specificity, PPV, and NPV were 81% (95% CI 65% to 91%), 97% (95% CI 89% to 100%), 94% (95% CI 80% to 99%), and 89% (95% CI 79% to 95%), respectively (Table 5).
Table 5.
Diagnostic value of investigated rapid strip tests in patients with confirmed PJI compared with those undergoing aseptic revision total joint arthroplasty
| Cutoff values for positive result | Sensitivity, % (95% CI) | Specificity, % (95% CI) | PPV, % (95% CI) | NPV, % (95% CI) |
| D-lactic acid | ||||
| ≥ + (22.5 mg/L) | 83 (68-92) | 100 (93-100) | 100 (88-100) | 91 (81-96) |
| ≥ ++ (45 mg/L) | 50 (34-66) | 100 (93-100) | 100 (81-100) | 76 (66-84) |
| ≥ +++ (90 mg/L) | 31 (18-47) | 100 (93-100) | 100 (72-100) | 70 (60-79) |
| ≥ ++++ (180 mg/L) | 7 (2-21) | 100 (93-100) | 100 (31-100) | 64 (54-69) |
| LE (10 EA) | 81 (66-91) | 99 (91-100) | 97 (83-100) | 89 (80-95) |
| LE (BM 10) | 81 (65-91) | 97 (89-100) | 94 (80-99) | 89 (79-95) |
For both LE tests, the (++) cutoff value was applied (≥ 250 leu/μL for 10 EA and ≥ 125 leu/μL for BM 10). PJI = periprosthetic joint infection; PPV = positive predictive value; NPV = negative predictive value; LE = leukocyte esterase.
Discussion
Diagnosing PJI is a complex process in which different radiologic, laboratory, microbiological, and histopathologic tests are usually required [1, 3, 6, 12, 22]. The newest ICM 2018 and European Bone and Joint Infection Society definitions propose the use of various synovial fluid markers, including alpha-defensin and LE tests, to differentiate aseptic and septic complications [17, 18]. D-lactic acid is a pathogen-specific marker that is produced directly by bacterial species. In contrast, other biomolecules such as C-reactive protein, LE, and alpha-defensins are produced by human cells and may indirectly indicate the inflammatory and infection response. Therefore, the presence of D-lactic acid in synovial fluid may play an important role in the diagnosis of bacterial PJI, and it should not cause false-positive results in patients with other comorbidities, especially chronic inflammatory diseases (such as rheumatoid arthritis, ankylosing spondylitis, and psoriatic arthritis). Single studies assessing the usefulness of laboratory tests that detect D-lactic acid in synovial fluid for diagnosing PJI were conducted and had promising results [11, 28]. However, the efficacy of the rapid strip test for that purpose was not investigated. Only a few rapid tests play a role in the diagnosis of PJI, and their sensitivity and specificity do not exceed those performed in the laboratory. Moreover, the development, validation, and implementation of the new strip test in clinical practice may improve point-of-care diagnostics. Thus, we conducted the first study that we know of to assess the rapid D-lactic acid strip test for diagnosing PJI and compared its accuracy with that of LE strip tests from two manufacturers and different leukocyte ranges corresponding to positive (++) results.
Limitations
Several limitations of this study must be considered. The first is the relatively small groups of included patients. However, there were rigorous selection criteria to avoid misdiagnosis, incorrect reading caused by blood-contaminated samples that could not be centrifuged, and dry tap joints. Additionally, small groups of patients prevented the performance of valuable statistical analyses based on demographic data such as gender, age, joint type, and BMI, which should be considered before analyzing these results. The second limitation was the manual method for assessing the strip color. Unfortunately, before evaluating the test results, the investigators were not fully blinded to the samples. Moreover, different researchers may interpret the color of the strip differently, which might impact the obtained result; thus, future calculations of the intraobserver reliability are needed. Another limitation of the test is the condition necessary for its performance, which was established by the manufacturer. For this study and to fulfill the assumption of the rapid test, we decided to standardize and simplify the protocol for using the strip. Thus, we performed the tests without examining the pH level, although the enzymes in the strips should have a pH of approximately 7 to work properly. We are aware that optimizing the pH level of samples may benefit the test’s accuracy; however, it is not possible to perform the test quickly in the operating room and during ambulatory visits. Some of the patients underwent antibiotic therapy before samples were collected and tested. Because of a lack of reliable evidence, we cannot make conclusions about the impact of prior antibiotic administration on the results of this study. However, all patients included in this study did not receive antibiotics for a minimum of 4 weeks preoperatively. Lastly, because of a short postoperative follow-up period, we cannot verify the relevance of a preoperative diagnosis. However, the risk of misdiagnosis was very low, according to Parvizi et al. [18] (according to the ICM 2018 criteria, sensitivity and specificity were very high: 97.7% and 99.5%, respectively).
Best Cutoff Value for the D-lactic Acid Test
The best cutoff value for the rapid D-lactic acid strip test for diagnosing PJI was (≥ +), which corresponds to a concentration of ≥ 22.5 mg/L. In similar studies calculating the best threshold for D-lactic acid in the diagnosis of PJI, Yermak et al. [28] and Karbysheva et al. [11] reported thresholds of 1.263 mmol/L and 1.3 mmol/L, respectively. After converting units from mmol/L to mg/L, their results corresponded to the most intensive color of the strip (++++) applied in our study. However, we do not recommend using the (++++) threshold for a positive result when diagnosing PJI because it has a lower sensitivity (7%) than (≥ +) did (83%). Future studies should ask, what is the cause of these differences, including the test type (rapid strip versus laboratory) and accuracy of the test, method of reading (manual versus automatic), and the application of the diagnostic standard (such as ICM 2018 criteria, Musculoskeletal Infection Society criteria, institutional criteria, or European Bone and Joint Infection Society criteria for PJI).
Diagnostic Accuracy for the D-lactic Acid Test and LE Tests
The D-lactic acid rapid strip test with (≥ +) gives a high diagnostic accuracy comparable to that of LE strip tests with ( ≥ ++) thresholds. Yermak et al. [28] assessed the efficacy of D-lactic acid in 148 patients using ELISA tests. With the calculated 1.263 mmol/L cutoff value, sensitivity was 86.4% and specificity was 80.8%. Six infections in their study were classified as false-negative regarding D-lactic acid, and PJI was recognized according to the working European Bone and Joint Infection Society definition of PJI. The authors of that study emphasized that the diagnostic test’s performance strongly depends on the applied infection definition. Karbysheva et al. [11] conducted a similar study on a larger group of 224 patients and calculated a threshold of 1.3 mmol/L for positive results. In accordance with the Musculoskeletal Infection Society criteria for PJI, they reported a sensitivity of 94.3% and specificity of 78.4%. Finally, Li et al. [16] summarized the results of five studies and concluded that D-lactic acid has potential value in the diagnosis of PJI, with 82% sensitivity and 76% specificity. They suggested the concentration of D-lactic acid likely reflects the virulence of bacteria and microbial load. We agree that the sensitivity value, which depends on the stated true-positive result, might be impacted by the difference in bacteria species’ capacity to produce D-lactic acid or the temporary concentration at the time of sample collection. However, we found 100% specificity, higher than values reported by other studies. In patients without infection, D-lactate is present in the human body in a nanomolar concentration [2] that is undetectable by the strips, and the results should be negative. We believe the differences in specificities are caused by the application of different diagnostic standards rather than technical issues and types of tests. However, these speculations require further studies for confirmation.
In this study, we demonstrated the similar diagnostic value of D-lactic acid and LE strip tests. Both tests can be routinely applied for screening for PJI preoperatively and in intraoperative diagnosis. Similar to LE tests, with D-lactic acid tests, it is not possible to read the result when synovial fluid is contaminated by blood, which is noted in 23.6% to 29.2% of samples [27]. In these cases, centrifugation can be a solution, but it delays the time to achieve a result. In our study, 22.7% of synovial fluid aspirations were blood-contaminated, and after centrifugation, 94% of these were clear and testable. Other studies have emphasized two technical issues that are important for achieving reliable results with LE strip tests [14, 29]. First, centrifugation lightens the color of the strip, and in specific cases, can downgrade the sensitivity and specificity, which vary from 92.5% to 100% and from 82.1% to 100%, respectively [14]. We agree with Li et al. [14]; centrifugation should be done only when necessary. Second, Zheng et al. [29] observed that the results differ depending on the reading time and whether centrifugation is performed. The test should be interpreted within 5 minutes before centrifugation and within 10 minutes after centrifugation. In our study, we did not analyze the impact of these issues (centrifugation and the optimal reading time) on the results, which should be investigated in future research.
Lastly, we did not find a difference between the D-lactic acid strip test and the fast alpha-defensin lateral flow test in terms of sensitivity and specificity. Alpha-defensin is one of the most valuable markers for diagnosing PJI and can be determined with different methods. Recent studies emphasized that lateral flow tests are less accurate than those performed in the laboratory, such as ELISA [1, 24]. Ahmad et al. [1] revealed the SynovasureTM (Zimmer Biomet) test has lower pooled sensitivity and specificity values (78% and 97%, respectively) than a laboratory alpha-defensin immunoassay (89% and 97%, respectively). Similarly, Suen et al. [24] confirmed these results and reported a sensitivity of 77.4% and specificity of 95.3% for the lateral flow test and 91.3% and 96.5%, respectively, for ELISA. This conclusion should be verified in a study comparing these two tests using the same synovial fluid samples collected from patients with PJI and those without.
Conclusion
Our study suggests an off-label rapid D-lactic acid strip test is a reliable tool to diagnose PJI, with sensitivity and specificity comparable to that of LE strip tests. The main advantage of this test is that it detects a molecule produced directly by bacteria, in contrast to the other laboratory tests, which are focused on human inflammatory markers. Because of its wide availability, this inexpensive test can be a valuable support for clinicians, especially in point-of-care diagnosis. However, this is the first study we know of that assessed D-lactic acid for that purpose; thus, validation and multicenter investigations are needed in a larger group of patients, especially those with chronic inflammatory diseases.
Acknowledgment
We thank the staff of the central laboratory, operating rooms, and departments of our institution who participated in this study.
Footnotes
Each author certifies that there are no funding or commercial associations (consultancies, stock ownership, equity interest, patent/licensing arrangements, etc.) that might pose a conflict of interest in connection with the submitted article related to the author or any immediate family members.
All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.
Ethical approval for this study was obtained from the Centre of Postgraduate Medical Education, Otwock, Poland (number 35/2021).
Contributor Information
Aleksandra Grajek, Email: olagrajek@tlen.pl.
Piotr Walczak, Email: dr_walczak@wp.pl.
Jacek Kowalczewski, Email: jackow@o2.pl.
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