Calendar month seasonality of HbA1c and its phase relationship with climatological temperature in 633,838 clinical measurements

Objective

HbA1c reflects glucose exposure over the preceding 8-12 weeks. We characterised the lagged seasonal association between monthly climatological temperature normals and HbA1c in a 12-year hospital cohort and examined whether a similar pattern was present in patient-level records. Research design and methods: We analysed 633,838 HbA1c measurements (387,678 patients; 633,737 with complete covariates) from a Chinese tertiary hospital (2014-2025). Cross-correlation was assessed for lags of 0-6 months using monthly HbA1c means and fixed monthly climatological temperature normals. Patient-level distributed-lag regression with cluster-robust standard errors and generalised estimating equations was used as a complementary analysis.