An international comparison of serum 25-hydroxyvitamin D measurements

Vitamin D status is usually assessed by measuring the serum 25-hydroxyvitam in D (25(OH)D) concentration. This mainly depends on sunshine exposure, nut rition and age. Interlaboratory variation may hamper comparison between res ults from different populations. This study reports cross-calibration of th e 25(OH)D assays of five laboratories. In study 1, serum 25(OH)D was measur ed with three different assays in 104 serum samples from a large vitamin D supplementation study. The mean serum 25(OH)D level was 80% higher when mea sured by competitive protein binding (CPB) assay than by high-performance l iquid chromatography (HPLC), while radioimmunoassay (RIA) gave intermediate values. The highest correlation was observed between RIA and HPLC (r = 0.8 4, p<0.01). Of the serum 25(OH)D values in the lowest quartile by HPLC, 25% were: not recognized by CPB and 21% were not recognized by RIA as belongin g to the lowest quartile. In study 2, the five laboratories analyzed serum 25(OH)D in eight serum samples covering the concentration range very low to high, with five different assays. The differences between the mean values for serum 25(OH)D between the laboratories with the highest and lowest valu es was 38%. The ranking order of individual samples according to the serum 25(OH)D value was very similar in all laboratories. The results show that 2 5(OH)D values from different laboratories can not be assumed to be comparab le unless a careful cross-calibration has been performed.