Reference intervals for serum 24,25-dihydroxyvitamin D and the ratio with 25-hydroxyvitamin D established using a...

24,25(OH)₂D is the product of 25(OH)D catabolism by CYP24A1. The measurement of serum 24,25(OH)₂D concentration may serve as an indicator of vitamin D catabolic status and the relative ratio with 25(OH)D can be used to identify patients with inactivating mutations in CYP24A1. We describe a LC-MS/MS method to determine: (1) the relationships between serum 24,25(OH)₂D and 25(OH)D; (2) serum reference intervals in healthy individuals; (3) the diagnostic accuracy of 24,25(OH)₂D measurement as an indicator for vitamin D status; 4) 24,25(OH)₂D cut-off value for clinically significant change between inadequate and sufficient 25(OH)D status. Serum samples of healthy participants (n=1996) from Army recruits and patients (n=294) were analysed. The LC-MS/MS assay satisfied industry standards for method validation. We found a positive, concentration-dependent relationship between serum 24,25(OH)₂D and 25(OH)₂D concentrations. The 25(OH)D:24,25(OH)₂D ratio was significantly higher (P<.001) at 25(OH)D<50 nmol/L. The reference interval for 25(OH)D:24,25(OH)₂D ratio in healthy subjects was 7-23. Measurement of serum 24,25(OH)₂D can be used as predictor of vitamin D status, a concentration of>4.2 nmol/L was identified as a diagnostic cut-off for 25(OH)D replete status. One patient sample with an elevated 25(OH)D:24,25(OH)₂D ratio of 32 and hypercalcaemia who on genetic testing confirmed to have a biallelic mutation of CYP24A1. Our study demonstrated the feasibility of a combined 24,25(OH)₂D and 25(OH)D assessment profile. Our established cut-off value for 24,25(OH)₂D and ratio reference ranges can be useful to clinicians in the investigation of patients with an impaired calcium/phosphate metabolism and may point towards the existence of CYP24A1 gene abnormalities.