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)₂D] in serum may be both a nuisance and nutritionally valuable. METHODS: We investigated the impact of 24,25(OH)₂D₃ 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)₂D₃ 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)₂D 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)₂D₃ contributed to the positive bias observed in some immunoassays relative to LC-MS/MS-derived estimates for total 25(OH)D. A spiking experiment showed that the degree of cross-reactivity with 24, 25(OH)₂D was high and may underpin this positive bias. Adjustment for 24,25(OH)₂D₃ concentration brought estimates closer to true values. Data from the vitamin D RCT showed that the ratio of 24,25(OH)₂D₃ to 25(OH)D was associated with serum 25(OH)D₃ and with response of serum 25(OH)D to vitamin D supplementation. CONCLUSIONS: Our findings highlight that the effect of 24,25(OH)₂D₃ in serum is a double-edged sword - an interferent for some immunoassays, yet potentially informative of nutritional status.