Significance of Serum 24,25-Dihydroxyvitamin D in the Assessment of Vitamin D Status: A Double-edged Sword?

BACKGROUND: 24,25-Dihydroxyvitamin D [24,25(OH)(2)D] in serum may be both a nuisance and nutritionally valuable. METHODS: We investigated the impact of 24,25(OH)(2)D-3 on the performance of commercially available immunoassays for serum total 25-hydroxyvitamin D [25(OH) D] using (a) serum from a nationally representative sample of adults, (b) serum from a spiking experiment, and (c) data from the UK Vitamin D External Quality Assurance Scheme (DEQAS). We also investigated the utility of the serum ratio of 24,25(OH)(2) D-3 to 25(OH) D as an index of inactivation and of response to vitamin D supplementation using randomized controlled trial (RCT) data. Measurement of 24,25(OH)(2)D in sera by a LC-MS/MS method allowed for an investigation of its impact on immunoassay-derived serum 25(OH) D values as well as its clinical utility. We report data from a nationally representative sample of adults, a recent vitamin D RCT in older adults, and DEQAS. RESULTS: 24,25(OH)(2)D-3 contributed to the positive bias observed in some immunoassays relative to LC-MS/MSderived estimates for total 25(OH) D. A spiking experiment showed that the degree of cross-reactivity with 24,25(OH)(2)D was high and may underpin this positive bias. Adjustment for 24,25(OH)(2)D-3 concentration brought estimates closer to true values. Data from the vitamin D RCT showed that the ratio of 24,25(OH)(2)D-3 to 25(OH) D was associated with serum 25(OH) D-3 and with response of serum 25(OH) D to vitamin D supplementation. CONCLUSIONS: Our findings highlight that the effect of 24,25(OH)(2)D-3 in serum is a double-edged sword-an interferent for some immunoassays, yet potentially informative of nutritional status. (C) 2015 American Association for Clinical Chemistry