Why does activated partial thromboplastin time prolongation occur in severe fever with thrombocytopenia syndrome?

Abstract

Severe fever with thrombocytopenia syndrome (SFTS) is caused by infection with SFTS virus and this mortality rate is 16.2% to 30%. An 85-year-old male patient presented to the emergency department of the hospital with primary complaints of fever and consciousness disturbance. Haemophagocytic syndrome and prolonged activated partial thromboplastin time (APTT) without associated prolonged prothrombin time were observed, suggesting SFTS, which was eventually diagnosed. APTT-only prolongation has been reported previously with SFTS, but the mechanism is unknown. The absence of coagulation factors was determined by a cross-mixing study. In addition, examination of intrinsic coagulation factors showed reduced factor XI activity. These results suggest that factor XI is causally related to APTT-only prolongation in SFTS.

Background

Severe fever with thrombocytopenia syndrome (SFTS) is caused by infection with SFTS virus, which is a novel bunyavirus, identified in 2011, for which the infection vectors are ticks in the Ixodidae family.¹

SFTS has been reported in China, South Korea and Japan. Five hundred and seventeen cases (including 70 deaths) of SFTS in Japan have been reported between March 2013 and May 2020, with most cases occurring in western Japan. The median age of patients was 75 years. The seroprevalence is reported to be 0.14% in healthy individuals aged 50 years and older living in the endemic area of Japan. Haemaphysalis longicornis is a tick in the Ixodidae family and a main vector of SFTS.

H. longicornis is found in China, Japan, the Russian Far East, South Korea, Australia and New Zealand and has also been observed in the USA. Human-to-human transmission has also been reported.^2–4

The mortality rate has been reported to be 16.2%–30%, and no treatment method has been established.^{5 6} The symptoms of SFTS are fever, thrombocytopenia, leucopenia, haemorrhagic tendency, gastrointestinal symptoms and encephalopathy. Complication of SFTS by haemophagocytic syndrome increases the mortality rate to 75%.⁷

There have been previous reports of haematology tests showing prolonged activated partial thromboplastin time (APTT) without prolonged prothrombin time.⁸ The causes of prolonged APTT in SFTS are unknown.

The present report pertains to a patient with SFTS whose life was saved by supportive therapy. The cause of APTT-only prolongation was identified using samples collected from the patient.

Case presentation

An 85-year-old male patient presented to the emergency department of the hospital with primary complaints of fever and consciousness disturbance. He had a 4-day history of fever and abnormal gait. The patient also had Parkinson’s disease and hypertension and was treated with arotinolol and benidipine. He had no history of smoking and was a social drinker.

The findings at admission were as follows: consciousness level: Glasgow Coma Scale (GCS) 14 (E4V4M6); body temperature: 39.3°C; heart rate: 58 beats/min; blood pressure: 130/66 mm Hg; respiratory rate: 29 breaths/min and SpO₂: 100% (nasal cannula, 3 L/min oxygen). No gastrointestinal symptoms such as vomiting and diarrhoea were seen, but physical examination showed oral mucosal haemorrhage and nuchal rigidity. In terms of dermatological findings, neither purpura nor tick bite marks were found.

The laboratory results showed several abnormal values (table 1). Haematology tests showed decreased counts for peripheral leucocytes (1000/μL (normal range: 4000–8500)) and platelets (3×10⁴/μL (normal range: 13–35×10⁴/μL)). Coagulation tests showed that the prothrombin time international normalised ratio was 1.09 (normal range: 0.8–1.3), APTT was 50.5 s (normal range: 25–40 s), and D-dimer concentration was 18.98 µg/mL (normal range: 0.00–1.00 µg/mL). APTT-only prolongation was found, but factor VIII activity was 90% (normal range: 60–150), and the results were negative for factor VIII inhibitor. C-reactive protein and procalcitonin levels were mildly elevated at 0.50 mg/dL (normal range: 0.00–0.03 mg/dL) and 0.23 ng/dL (normal range: 0.00–0.05 ng/dL), respectively. The other findings included the following: aspartate aminotransferase level, 290 U/L (normal range: 8–35 U/L); alanine aminotransferase level, 96 U/L (normal range: 5–43 U/L); blood urea nitrogen level, 38.2 mg/dL (normal range: 8–20 mg/dL) and serum creatinine level, 1.23 mg/dL (normal range: 0.00–1.10 mg/dL). The ferritin and soluble interleukin-2 receptor levels were elevated at 14 974 ng/dL (normal range: 16–194 ng/dL) and 1190 U/mL (normal range: 157–474 U/mL), respectively. The blood glucose concentration was 140 mg/dL (normal range: 70–109 mg/dL). Qualitative urinalysis results were 3+ and 2+ for protein and occult blood, respectively. The cerebrospinal fluid findings were as follows: cell count, 6/μL; cerebrospinal fluid glucose level, 66 mg/dL and protein level, 40 mg/dL. Bone marrow aspiration showed a haemophagocytic picture, and haemophagocytic syndrome was diagnosed. Brain MRI was performed. Apart from acute microinfarcts in the right parietal and temporal lobe, no signs suggesting encephalitis, such as signal changes in the temporal lobes, were found.

Table 1.

Laboratory results of the patient

CBC		Coagulation
WBC (×103/μL)	1.0	PT-INR	1.09
RBC (×104/μL)	470	APTT (s)	50.5
Plt (×104/μL)	3.0	D-dimer (μg/mL)	18.98
Blood chemistry		factor VIII activity (%)	90
TP (g/dL)	5.6	Urinalysis
ALB (g/dL)	2.8	pH	5.5
AST (U/L)	290	Protein	3+
ALT (U/L)	96	Occult blood	2+
ALP (U/L)	151	Ketones	–
LDH (U/L)	802	Sugar	–
BUN (mg/dL)	38.2	Urobilinogen	–
Cre (mg/dL)	1.23	Cerebrospinal fluid findings
Na (mEq/L)	136	Cell count (/μL)	6
K (mEq/L)	4.2	Glu (mg/dL)	66
Cl (mEq/L)	105	Protein (mg/dL)	40
CRP (mg/dL)	0.50
Glu (mg/dL)	140
Ferritin (ng/dL)	14 974
sIL2R (U/mL)	1190

ALB, albumin; ALP, alkaline phosphatase; ALT, aminotransferase; APTT, activated partial thromboplastin time; AST, aspartate aminotransferase; BUN, blood urea nitrogen; Cl, chlorine; Cre, creatinine; CRP, C reactive protein; Glu, glucose; K, potassium; LDH, lactate dehydrogenase; Na, sodium; Plt, platelets; PT-INR, prothrombin time international normalized ratio; RBC, red blood cell; sIL2R, soluble interleukin-2 receptor; TP, total protein; WBC, white blood cell.

Investigations

SFTS, characterised by consciousness disturbance, haemophagocytic syndrome, liver disease, kidney disease and APTT-only prolongation, was considered, and on day 3 of the disease, samples were sent to a public health centre for PCR tests. On day 8 of the disease, the PCR results were positive, and SFTS was diagnosed.

Detailed tests were then performed for the aetiology of the APTT-only prolongation. A cross-mixing study was conducted with stored plasma collected on day 7 of the disease. A cross-mixing study is used to determine whether APTT prolongation is caused by inhibitors or by the absence of coagulation factors. Blood plasma from the patient (patient’s plasma) and from healthy people (normal plasma) is mixed together in varying ratios, and APTT is measured for each mixture. A response curve is prepared by plotting APTT on the X-axis and the plasma mixture ratio on the Y-axis, and the cause of the prolongation is assessed visually. If the response curve is convex at the top, the APTT prolongation is judged to be due to inhibitors, whereas if it convex at the bottom, it is judged to be due to coagulation factor absence. This study is used as a diagnostic tool for haemophilia and antiphospholipid antibody syndrome.

The results of the cross-mixing study in our patient are shown in figure 1. The profile indicated a convexity at the bottom; hence, the APTT prolongation was judged to be due to the absence of coagulation factors.

Figure 1.

The results of the cross-mixing study are shown. The study was performed using patient’s plasma and normal plasma. The ratio of normal plasma to patient’s plasma is shown on the X-axis and APTT(s) is shown on the Y-axis. The blue curve shows APTT(s) measured immediately after mixing the normal plasma and patient’s plasma, whereas the red curve shows APTT(s) of the mixed plasma after incubation at 37°C for 2 hours. Due to the quantity of normal plasma available being insufficient, the value for 100% normal plasma after incubation was not measured. APTT, activated partial thromboplastin time.

In addition, using the same sample, factor IX, XI and XII activities were measured, and the results are shown in table 2. Factor XI activity was low at 52% (normal range: 75%–145%), but none of the other extrinsic coagulation factor activities were low.

Table 2.

The results of coagulation factor activity

Variable	Reference range	Results
Coagulation factor IX activity (%)	70–130	110%
Coagulation factor XI activity (%)	75–145	52%
Coagulation factor XII activity (%)	50–150	67%

Treatment

After treating haemophagocytic syndrome with 500 mg/day methylprednisolone for 3 days, administration of 1.0 mg/kg/day prednisone was initiated, and the dosage was gradually decreased until the final dose on day 26 of the disease. Then, as supportive therapy, 20 Japanese units (10 units containing approximately 2.0×10¹¹ platelets) of platelet transfusion were administered, and 75 µg of granulocyte colony-stimulating factor was administered intravenously to treat leucopenia. It was not initially possible to rule out bacterial meningitis, Rickettsia infection or herpes simplex encephalitis; hence, 3 g/day meropenem, 200 mg/day minocycline and 500 mg/day acyclovir were administered by drip infusion, and this treatment was continued until PCR results confirmed SFTS.

Outcome and follow-up

The lowest consciousness level, GCS 8, was reached on day 3 of the disease, after which it gradually improved, reaching GCS 15 on day 12 of the disease, and not subsequently decreasing. The lowest platelet count, 1×10⁴/µL, was reached on day 3 of the disease, on which day 10 Japanese units of platelet transfusion were administered, and the count transiently increased to 3×10⁴/µL. However, the count decreased to 2×10⁴/µL on day 6 of the disease; hence, 10 Japanese units of platelet transfusion were administered again, after which the count increased to 18×10⁴/µL.

The fever subsided on day 5 of the disease, and the body temperature subsequently remained stable at approximately 36°C.

Because of favourable progression, the patient was discharged from the hospital on day 29 of the disease without any neurological sequelae.

Discussion

SFTS is an infection caused by SFTS virus, a newly identified bunyavirus, with a high mortality rate.¹ It is transmitted by ticks, but it has been reported in patients with no tick bite marks and is difficult to diagnose if no history of contact with ticks can be ascertained by interviewing the patient.⁹ According to the revised version of the guidelines for examination and treatment of patients with SFTS released by the Japanese Disease Control and Infection Center in 2019, APTT-only prolongation often occurs in SFTS. In our patient, APTT-only prolongation was the primary symptom that led to SFTS diagnosis.

Kim et al¹⁰ have proposed APTT prolongation measurement as a method to discriminate SFTS from Rickettsia, which is another tick-transmitted infection. The study by Kim et al showed that if no significant difference in PT prolongation is found between SFTS and Rickettsia, and APTT is 35 s or longer, there is a high probability of SFTS virus infection, suggesting APTT prolongation to be characteristic of SFTS infection. There have also been reports of APTT prolongation in severe SFTS cases.

Jia et al¹¹ have reported that when patients with SFTS were divided into those who died and those who survived, the former showed significant APTT prolongation. APTT prolongation by 51.9 s or more was found to predict death with 84.85% sensitivity and 81.65% specificity. Age >66 years and blood urea nitrogen level >7.32 mmol/L were also found to be death-predicting factors.

In addition, Song et al¹² have reported a weak correlation, using the Pearson correlation coefficient, in patients with SFTS between serum viral load and APTT prolongation. Song et al also divided 70 patients with SFTS into three groups, (1) the mild group, with recovery without complications, (2) the severe group, with at least one complication, and (3) the fatal group, and both APTT and PT were found to be significantly prolonged in the severe and fatal groups in comparison with the mild group.

The above reports show that patients with severe SFTS have prolonged APTT, suggesting that in terms of a clinical analysis of SFTS, APTT prolongation may be a starting point for diagnosis. Despite these reports, it is still not understood why APTT prolongation occurs in SFTS.

Dengue virus similarly induces APTT prolongation. Hsieh et al¹³ divided 71 dengue fever patients into those who survived and those who died and found APTT to be significantly prolonged in those who died, with APTT prolongation being an independent predictive factor for hospitalised death. As a potential mechanism underlying APTT prolongation, non-structural protein-1 has been reported to inhibit prothrombin activity, thus prolonging APTT.¹⁴ However, as with SFTS, the mechanisms underlying APTT prolongation in dengue fever is unknown, and further research is needed.

No reports have been published describing the cause of APTT prolongation in SFTS, and it is hoped that this report will be helpful for the elucidation of this research area, leading to an understanding of the pathology of SFTS. One limitation of this report is that the cross-mixing study and the coagulation factor measurements used samples collected on day 7 of the disease, and hence, the measurements were only reference values. However, it has been suggested that the absence of factor XI may be a cause of APTT prolongation in SFTS. It is hoped that coagulation factor tests can be performed in patients with SFTS when APTT prolongation actually occurs, which should help elucidate the cause of APTT-only prolongation.

Learning points.

• In cases involving fever, thrombocytopenia and activated partial thromboplastin time (APTT) prolongation, severe fever with thrombocytopenia syndrome (SFTS) should be considered a possible diagnosis.

• Absence of factor XI is a possible cause of APTT-only prolongation in SFTS.

• SFTS was diagnosed promptly, without coagulation factor measurements, with samples collected at the initial stage of the disease. In the future, further measurements of coagulation factors should facilitate elucidation of the cause of APTT prolongation.