Abstract
BACKGROUND: Severe fever with thrombocytopenia syndrome (SFTS), an emerging infectious disease, has a high case fatality rate (CFR) in severe patients. Thrombocytopenia is one of the features of SFTS, and a platelet count lower than 50×10(9)/L is a risk factor for mortality in patients with SFTS. However, there have been no studies on the value of platelet recovery in the prognosis of SFTS patients. METHODS: From January 2009 to December 2020, laboratory-confirmed severe SFTS patients with platelet counts lower than 50×10(9)/L were enrolled and divided into a survival group and a death group based on 90-day prognosis. Descriptive analysis of baseline data compared characteristics between the survival and death groups. Multivariate Cox proportional hazards regression models identified independent mortality risk factors for SFTS patients. A nomogram-presented prediction model was constructed via multivariate Cox regression. The performance of nomogram was measured by the receiver operating characteristic (ROC) curve, calibration diagram, and decision curve analysis (DCA). RESULTS: 144 SFTS patients with platelet counts< 50×10(9)/L during the disease were included. After three months of follow-up, 109 patients survived and 35 patients died. The cut-off values for predicting fatal outcomes were 40×10(9)/L for platelet levels on day three (PLT Day3) and 50×10(9)/L for platelet levels on day five (PLT Day5), respectively. Statistical analysis showed a significant difference (p<0.001) in platelet recovery to these levels within 3 or 5 days. Kaplan-Meier analysis showed that patients with unrecovered PLT on day 5 had a higher cumulative incidence of mortality than those with recovered PLT on day 5. Multivariate Cox regression found age ≥65, failure of platelet count to reach 50×10(9)/L in 5 days, prolonged APTT, and elevated LDH as independent mortality risk factors (p<0.05). Subgroup analysis showed a significant association between whether the platelet count recovers to 50×10(9)/L within 5 days (PLT-Day5) and mortality in all subgroups. CONCLUSIONS: Whether the platelet count recovers to 50×10(9)/L within 5 days (PLT-Day5), aspartate aminotransferase (APTT), lactate dehydrogenase (LDH) and age are independent predictors of mortality in severe SFTS patients with platelet counts below 50×10(9)/L during the course of the disease. Patients whose platelet count recovers from the lowest value to 50×10(9)/L within five days have a better prognosis.