Reply to the article entitled “Impact of assay design on test performance: lessons learned from 25-hydroxyvitamin D” by Farrell et al., Clin Chem Lab Med 2014;52:1579–87

Abstract

Keywords: 25-hydroxyvitamin D; 25-OHD; automated 25-OHD assay; liquid chromatography-tandem mass spectrometry (LC-MS/MS).To the Editor, In the recent article by Farrell et al. [1], the authors explain the variability in results of various vitamin D (25-OHD) immunoassays based on specific aspects of assay design. Here, we question their interpretation of the conclusion.The authors attempt to explain the variation in results obtained with commercial 25-OHD assays. To test their hypotheses, they conducted studies using samples containing 3-epimer 25-OHD, 25-OHD 2 , and human antianimal antibodies. They first tested the hypothesis that assays with one-step release and capture would show superior ability to equally recover 25-OHD 3 and 25-OHD 2 . While the authors acknowledged that none of the immunoassays demonstrated equimolar recovery of 25-OHD 2 and 25-OHD 3 , they found that the DiaSorin assay had the least bias to 25-OHD 2 by liquid chromatography-tandem mass spectrometry (LC-MS/MS). They concluded that these results supported their hypothesis, since only the DiaSorin assay released 25-OHD in the presence of capture by target analyte. We question this conclusion, because they erroneously report in table 4 that the Siemens ADVIA Centaur ® Vitamin D Total (Siemens) assay does not release 25-OHD in the presence of capture moiety. The Siemens assay is performed in one step using sequential reagent additions, not in two steps (which infers a wash step or separation step between critical reagent additions) as observed in some infectious disease assays. Therefore, as vitamin D is released from the vitamin D binding protein (DBP), the capture antibody is present.Another goal of the Farrell et al. article was to examine whether assays that used a backfill assay format (conjugate added in excess in the final incubation) showed superior precision (by amplification of the chemiluminescent signal). Abbott and DiaSorin, but not Roche and Siemens, use the backfill assay format and reportedly showed superior assay precision at higher concentrations. However, Abbott did not show superior precision at lower concentrations, and this was explained by invoking interference by additional unknown variables. As reported in the article, the Siemens assay does not backfill the reagents, yet it demonstrated superior total precision at lower concentrations compared with the DiaSorin and Abbott assays. It would seem that these results refute, rather than support, the hypothesis about backfilling. Backfilling of reagents assumes reagent excess. The Siemens reagents are not in excess. The vitamin D analog and monoclonal antibody specific to 25(OH)vitamin D 2 /D 3 are in fact carefully optimized to obtain both accuracy and precise results. Without rigorous scientific testing to show that the backfill assay format leads to lower imprecision, we question the importance of the backfill assay design and their conclusion.The authors use their hypothesis about backfill and precision to support the discussion of superior DiaSorin assay precision at t...