Season of sampling and the interpretation of serum homocysteine results: a within-person analysis of 268,877 health-screening examinations

Abstract

OBJECTIVES: Serum total homocysteine (tHcy) is interpreted against fixed reference intervals and a decision limit. Whether the month of blood collection is a material source of variation, and whether it reflects the seasonal cycle of 25-hydroxyvitamin D [25(OH)D], is unresolved because earlier studies were cross-sectional or small. METHODS: We analysed 268,877 health-screening examinations from 69,131 adults between January 2014 and December 2025 at a single Korean centre. Because no participant was examined twice in a calendar year, within-person seasonal contrasts span years; all models therefore combined person with calendar-year fixed effects. Seasonality was quantified by cosinor regression with cluster-robust standard errors. RESULTS: The crude amplitude of tHcy (0.473 μmol/L) fell to 0.255 μmol/L under person fixed effects, so about half of the apparent seasonality reflected who attended. After the secular trend was also removed the within-person amplitude was 0.301 μmol/L (95 % CI 0.281-0.324), peaking in early November, and was unchanged by time-varying covariates. The 25(OH)D acrophase was in August, 2.6 months away, and adjustment for 25(OH)D did not attenuate the tHcy amplitude (retention 108.0 %); the rhythm persisted where within-person 25(OH)D barely varied. Spring examinations carried lower within-person odds of exceeding 15 μmol/L (odds ratio 0.699, 95 % CI 0.654-0.748). CONCLUSIONS: Month of sampling is a small but reproducible systematic source of within-person variation in tHcy that is not explained by 25(OH)D. It is modest relative to within-subject biological variation, but sufficient to alter classification near the decision limit and to bias reference intervals derived from seasonally unbalanced data.

The authors' abstract, as published at the source. Clinical Chemistry and Laboratory Medicine (CCLM), 2026 · DOI ↗