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, single-centre 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 suggested a visual L-shaped pattern between serum calcium levels and in-hospital mortality, but without statistically significant evidence of non-linearity. 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 may serve as a useful marker for risk stratification in SFTS.