Impact of season and different vitamin D thresholds on prevalence of vitamin D deficiency in epidemiological cohorts-a note of caution

We investigated the impact of different cut-offs on the prevalence of 25-hydroxyvitamin D [25-(OH)D] deficiency. We used baseline data of 4149 participants (45-75 years, 50% women) of the population-based Heinz Nixdorf Recall study. Serum 25-(OH)D was measured with the Roche Cobas assay. Quartiles (p25, p50, and p75) were calculated. Data were stratified by months, sex, and age. According to the recommendations of 'Dachverband Osteologie', Endocrine Society and National Institute of Health we used 25-(OH)D thresholds of 12, 20, and 30 ng/ml to estimate vitamin D deficiency. Overall the median of 25-(OH)D was 19.8 ng/ml (p25 = 14.4 ng/ml, p75 = 26.6 ng/ml), with highest concentrations in July (p50 = 23.8 ng/ml, p25 = 18.2 ng/ml, and p75 = 31.2 ng/ml) and lowest in March (p50 = 15.8 ng/ml, p25 = 11.5 ng/ml, and p75 = 20.6 ng/ml). Prevalence of vitamin D deficiency rose from 16, 51 up to 83% using the cut-offs of < 12, < 20 ng/ml, and < 30 ng/ml, respectively. With respect to seasonal variance, prevalence of vitamin D deficiency rose to 92% in February/March using the cut-off < 30 ng/ml (< 12: 28%, < 20 ng/ml: 71%) whereas in June/July prevalence of vitamin D deficiency decreased to 71% (< 12: 6%, < 20 ng/ml: 30%). The chance to attest the diagnosis of vitamin D deficiency for cut-off 12 ng/ml in March is 6.4-fold higher than in June, for cut-off 20 ng/ml, 5.5-fold higher and for cut-off 30 ng/ml, 3.1-fold higher. Guidelines to define vitamin D deficiency revealed extremely different prevalence rates ranging between 6 and 92%. Accounting for collection time and antecedent sun exposure are important to reduce bias in research studies and improve decision-making in clinical care. Vitamin D thresholds have to be rethought.