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Letters to the Editor |
1951 Northwestern Ave, DiaSorin Inc., P.O. Box 285, Stillwater, MN 55082-0285, E-mail jane.schmidt{at}diasorin.com
To the Editor:
A notable shortcoming of the HPLC method for measuring 25-hydroxyvitamin D described by Lensmeyer et al. (1) is its failure to adequately remove interference by the C3-epimer. Although the authors dismiss this because the interference was less than the lowest limit of quantification, this would not be true in some samples collected from children >1 year old, in which the concentration of this metabolite can be as high as 92 µg/L(2). The only methods demonstrated not to cross-react with the 3-epimer are a specifically modified liquid chromatography-tandem mass spectrometry (LC-MS/MS) method and the DiaSorin RIA assay(2).
The relatively poor correlation between HPLC and the DiaSorin RIA for 25-OH vitamin D reported by Lensmeyer et al. (1) contradicts observations by several investigators(2)(3)(4)(5)(6)(7) of good agreement between both the DiaSorin RIA and LIAISON® assays and either HPLC or LC-MS/MS methods. Indeed, several groups have used the DiaSorin RIA as the reference method to validate HPLC or LC-MS/MS methods with good agreement(3)(4)(5). The study of assay variability by Binkley et al.(7) with a small number of samples demonstrated good agreement and no significant bias between the DiaSorin RIA and an HPLC method. Furthermore, results from the International Vitamin D External Quality Assessment Scheme confirm the accuracy of the DiaSorin RIA in measuring both D3 and D2 forms of 25 OH vitamin D(8). The DiaSorin RIA gave results for 3 samples containing primarily D3 that were very close to the all-laboratory trimmed mean, and although the DiaSorin RIA method means for the 2 samples containing endogenous D2 were lower than that for HPLC, the difference was not statistically significant based on the high CV of the latter method.
DiaSorin has recently incorporated a panel of serum samples with 25-hydroxyvitamin D values measured by LC-MS/MS into the routine QC testing for both the RIA and LIAISON 25-OH vitamin D assays. The use of these samples assures that both methods will continue to have the good correlation to a reference method that was demonstrated by Fenske et al. (6).
References
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