Association between serum calcium and in-hospital mortality in patients with severe fever with thrombocytopenia syndrome: A retrospective cohort study

Objective Severe fever with thrombocytopenia syndrome (SFTS) is a viral infectious disease characterized by high mortality. Calcium dysregulation is implicated in viral replication and host immune response. We aimed to evaluate the association between serum calcium levels and in-hospital mortality in patients with SFTS. Methods In this retrospective cohort study, we enrolled 809 laboratory-confirmed SFTS patients admitted between April 2017 and June 2024. Hypocalcemia was defined as corrected calcium < 8.12 mg/dL. Multivariate logistic regression, restricted cubic spline (RCS), and longitudinal analyses were employed to assess associations between serum calcium and mortality. Results The in-hospital mortality rate was 11.87% (96/809). Hypocalcemia was observed in 72.68% of patients (588/809) with SFTS upon admission. Multivariate logistic regression analysis revealed that lower serum calcium was an independent predictor of in-hospital mortality, alongside older age, neurological symptoms, higher viral loads, lower albumin levels, elevated creatinine, and prolonged activated partial thromboplastin time (APTT). Restricted cubic spline (RCS) analysis revealed a significant L-shaped association between serum calcium levels and in-hospital mortality, with a critical inflection point at 7.56 mg/dL. Dynamic analysis showed that serum calcium levels increased and normalized by day 6 in survivors but peaked and declined thereafter in non-survivors. Hypocalcemia was independently associated with hypoalbuminemia, hypoglobulinemia, hyponatremia, and monocytopenia. Furthermore, serum calcium levels were inversely correlated with viral load, creatine kinase, APTT, IL-6, and IL-10 (all p < 0.05). Conclusions Hypocalcemia is exceedingly common in SFTS patients and serves as a robust, independent prognostic marker for in-hospital mortality. Its association with high viral replication and host inflammatory response underscores its clinical significance. Serial monitoring of serum calcium could enhance risk stratification and guide management decisions in SFTS.