Context: An automated application of IDS OCTEIA 25OH Vitamin D enzyme-immunoassay (IDS EIA) was developed for analyses of 25(OH)D in >7000 participants of the 1958 cohort. Variation between 25-hydroxyvitamin D [25(OH)D] assays hampers between-study comparisons and the definition of relevant cut-offs for hypovitaminosis D.

Objective: To evaluate the importance of assay variation on the estimated prevalence of hypovitaminosis D, and to assess the use of statistical harmonization to overcome the observed differences.

Design: Agreement analyses between two commercial 25(OH)D assays (IDS EIA and Diasorin RIA), with validation using performance data from Vitamin D External Quality Assessment Scheme (DEQAS).

Setting: England, Scotland and Wales

Participants: Members of the 1958 British birth cohort.

Main outcome measures: 25(OH)D was measured both by IDS and Diasorin in 781 samples. Additional quality control data was obtained through participation in DEQAS (5 distributions throughout the survey).

Results: Average 25(OH)D concentrations by IDS were -15.7nmol/l and -13.7nmol/l lower compared to Diasorin or DEQAS mean, respectively (both p<0.0001). Graphical examination demonstrated a dose related bias between IDS with Diasorin and DEQAS mean, but log-transformation removed the bias. After using the log-difference between the measurements as an adjustment factor, there were no differences in average 25(OH)D concentrations (p0.21 for comparison of IDS with Diasorin or DEQAS) and estimates for hypovitaminosis D obtained by IDS were similar to Diasorin.

Conclusions: Differences between assays have implications for public health messages about hypovitaminosis D. Harmonization of results with DEQAS enabled the use of previously determined cut-offs for hypovitaminosis D.