Accuracy of three automated 25-hydroxyvitamin D assays in hemodialysis patients

We evaluated the accuracy of three automated assays for 25(OH)D measurement in comparison to ID-XLC-MS/MS in hemodialysis patients, considering the importance of their vitamin D status and reported discrepant results obtained with automated assays.

All three assays were heterogeneous, competitive immunoassays or vitamin D binding protein assays on Architect (Abbott), Modular E170 (Roche) and iSYS (IDS). Measurements were performed in serum of 99 hemodialysis patients and 50 healthy subjects, double blind with a different operator and aliquot for each method.

Architect showed the highest deviation for hemodialysis (slope 0.3864, intercept 8.7409) and healthy subjects (slope 0.5024, intercept 6.8426) and reported significant lower results. Considering 30 ng/ml as cut-off for optimal 25(OH)D concentration, Architect falsely assigned 48.5% of the hemodialysis and 6% of the healthy subgroup a suboptimal vitamin D status. iSYS results of hemodialysis patients also deviated (slope 0.6136, intercept 8.6604) but showed less discordant values than Modular E170 in patients with 25(OH)D concentrations between 10 and 40 ng/ml.

We conclude that not all automated 25(OH)D assays may be considered equally accurate in samples from hemodialysis patients compared to samples from healthy subjects. We found most deviating results with Abbott (Architect) measurements compared to ID-XLC-MS/MS in hemodialysis patients as well as in healthy subjects. We suggest a possible role of matrix effects like elevated urea or other retained metabolites in hemodialysis sera, causing incomplete binding disruption between 25(OH)D and DBP, in the poor assay accuracy.