Serum Vitamin D Measurement May Not Reflect What You Give to Your Patients

The recognized index of vitamin D (VTD) status is the measurement of circulating concentrations of 25‐OH VTD (25VTD). A concentration of 30 ng/ml 25VTD (75 nM) is considered by many experts as the minimum optimal concentration.(1) There is currently a growing interest in VTD far beyond bone and calcium metabolism,(2) including cancer, immunology, and hypertension, which has caused a recent upsurge in requests for 25VTD evaluation,(3) necessitating the need for accurate measurement. We report here the case of a 60‐yr‐old woman diagnosed as having VTD deficiency (serum 25VTD measured with the automated Roche Elecsys method at 12 ng/ml). She was given a single 600,000U VTD2 oral dose. Because serum 25VTD measured with the same assay 2 wk later was still low (11 ng/ml), she was referred to our unit for extensive laboratory testing. All biochemical parameters were normal, including 25VTD (50 ng/ml), but this time the Diasorin radioimmunoassay (RIA) was used to quantify 25VTD. To study the cause for these discrepant results further, we conducted measurements of 25VTD by a specific liquid chromatography‐mass spectrometry (LC/MS/MS) method. The LC/MS/MS method separates and quantifies 25‐hydroxylated metabolites of both VTD2 and VTD3, which are summed to get the total 25VTD concentration. The LC/MS/MS is considered by many as the candidate reference method for 25(OH)D measurement,(4) although drawbacks because of the recognition of other compounds such as epimers have been highlighted, especially in pediatric subjects.(5,6) In addition to the index case, all three methods were used to measure 25VTD in serum collected from 11 healthy subjects (5 men and 6 women; age, 21–62 yr) before (D0) and 7 and 28 days after a single 600,000U VTD2 dose to mimic the above‐mentioned case. Pooling the results from the three time‐points, we found that the LC/MS/MS results were highly correlated with the RIA values (Spearman's ρ = 0.94; p < 0.0001) but not with the Elecsys values (ρ = 0.16; not significant).