The predictive effect of platelet recovery on the prognosis of Severe Fever with Thrombocytopenia Syndrome

Bing Tian^1,2Yuan Gao^1,3Qiyue Sheng⁴Xuelan Mao^1,5Jinyong Wang¹Shujun Zhang⁶Jingxia Peng⁷Lijia Li¹Yumeng Hou¹Jingyi Chen⁸Zhiqian Wang⁹Yu Di^10*Bo Zhou^11*Bao Deng^1*

• ¹Department of Infectious Diseases, The First Affiliated Hospital of China Medical University, Shenyang, Liaoning Province, China
• ²Zhongda Hospital, Southeast University, Nanjing, Jiangsu Province, China
• ³National Clinical Research Center for Laboratory Medicine, Department of Laboratory Medicine, the First Hospital of China Medical University, Shenyang City, China
• ⁴Department of Infectious Disease, Jinhua Municipal Central Hospital, jinhua, China
• ⁵Department of Gastroenterology, Guiqian International General Hospital, guiyang, China
• ⁶Department of Infectious Disease, Kuandian Country Hospital, kuandian, China
• ⁷Department of Infectious Disease, Fengcheng Center Hospital, fengcheng, China
• ⁸Clinical College of Ophthalmology, School of Medicine, Nankai University, Tianjin, China
• ⁹School of Public Health, China Medical University, Shenyang, Liaoning Province, China
• ¹⁰Department of Ophthalmology, ShengJing Hospital of China Medical University, Shenyang, Liaoning Province, China
• ¹¹Department of Clinical Epidemiology and Evidence-Based Medicine, The First Hospital of China Medical University, Shenyang City, China

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 50×10⁹/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 50×10⁹/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 < 50×10⁹/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 40×10⁹/L for platelet levels on day three (PLT Day3) and 50×10⁹/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 50×10⁹/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 50×10⁹/L within 5 days (PLT-Day5) and mortality in all subgroups. Conclusions: PLT-Day5, APTT, LDH and age are independent predictors of mortality in severe SFTS patients with platelet counts below 50×10⁹/L during the course of the disease. Faster platelet recovery means better prognosis.