Assay-specific differences in lipemic interference in native and intralipid-supplemented samples.

Among the five samples distributed in January 2004 were two (samples 4 and 5) containing endogenous 25OHD 2. A summary of the results is presented in Table 1. The wide range of results submitted by users of the same method illustrates the degree to which 25OHD assays are operator-dependent. The one automated procedure (Nichols) produced more consistent values, but for the samples (1, 2, and 3) containing only 25OHD 3 , the results were again higher than those of other methods. For samples 4 and 5, in which 25OHD 2 is the predominant metabolite, method means for the IDS RIA and the Nichols automated assay were lower than those of other methods. The inference is that the IDS RIA and the Nichols automated procedure underestimate 25OHD 2 , the latter by a considerable margin. Indeed, the presence in these samples of 25OHD 3 , albeit in smaller amounts than 25OHD 2 , disguises the true extent of the problem. IDS acknowledge that their RIA has only a 75% cross-reactivity with 25OHD 2 (11), but Nichols claims a 100% cross-reactivity and has presented data to support this (12). The reason for the disparity between the manufac-turer's claims and the DEQAS findings is unclear. Low recovery of 25OHD 2 by the Nichols automated assay has also been observed in clinical samples (Dr.min-D-deficient neonates failed to show an increase in 25OHD, despite receiving large doses of ergocalciferol. When the samples were reanalyzed by the DiaSorin assay, the expected increase in 25OHD was observed. This suggests that the problem is not confined to DEQAS samples, an explanation originally proposed by the manufacturer to explain the overrecovery of 25OHD 3. In the IDS RIA, the underrecovery of 25OHD 2 can be attributed to differences in antibody specificity for the two forms of the metabolite. Interestingly, the same antibody is used in the nonisotopic version of the IDS assay, which despite the published cross-reactivity data of 75%, did not appear to underestimate 25OHD 2 in the DEQAS samples. In summary, an international quality assessment scheme has demonstrated that, for samples containing only 25OHD 3 , most commercial 25OHD methods are capable of giving results close to the target value, but the results are highly operator dependent. The Nichols automated assay gives more consistent results but generally produces higher values than other methods. For samples containing predominantly 25OHD 2 , the Nichols procedure and, to a lesser …