Vitamin D Standardization Program (VDSP) intralaboratory study for the assessment of 25-hydroxyvitamin D assay variability and bias

An intralaboratory study assessing assay variability and bias for determination of serum total 25-hydroxyvitamin D [25(OH)D] was conducted by the Vitamin D Standardization Program (VDSP). Thirteen assays for serum total 25(OH)D were evaluated in a single laboratory including 11 unique immunoassays and one liquid chromatography - tandem mass spectrometry (LC-MS/MS) assay. Fifty single-donor serum samples, including eight samples with high concentrations of 25(OH)D-2 (> 30 nmol/L), were assigned target values for 25(OH)D-2 and 25(OH) D-3 using reference measurement procedures (RMP). Using four replicate measurements for each sample, the mean total percent coefficient of variation (%CV) and mean % bias from the target values were determined for each assay using the 50 single-donor samples and a 42-sample subset, which excluded 8 high 25(OH)D-2 concentration samples, and compared with VDSP performance criteria of <= 10 % CV and <= +5 % mean bias. All 12 assays achieved the performance criterion for % CV, and 9 of the 12 assays were within <= +5 % mean bias. The Fujirebio Inc. assay exhibited the lowest %CV and highest percentage of individual measurements within <= +5 % mean bias. Ten immunoassays exhibited changes in response due to the high 25(OH)D-2 samples with Abbott, Biomerieux, DiaSorin, DlAsource, and IDS-iSYS assays having the largest deviations. The Fujirebio Inc. and Beckman Coulter assays were only minimally affected by the presence of the high 25(OH)D-2 samples. Samples with high concentrations of 25(OH)D-2 provided a critical performance test for immunoassays indicating that some assays may not have equal response or recovery for 25(OH)D-2 and 25(OH)D-3.