The anomalous behaviour of exogenous 25-hydroxyvitamin D in competitive binding assays

The Vitamin D International External Quality Assessment Scheme (DEQAS) was established in 1989 to monitor the performance of assays for 25-hydroxyvitamin D (25-OHD) and 1,25-dihydroxyvitamin D (1,25(OH)(2)D). This is achieved through the quarterly distribution of five samples of human serum. Results are used to calculate an All-Laboratory Trimmed Mean and a Method Mean for each of the methods used by participants. In July 2005, participants were asked to assay serum to which 50.9 nmol of either 25-OHD3 or 25-OHD2 had been added as ethanolic solutions. The final concentration of ethanol in the serum was 0.7%. The distribution also included a sample of the original serum (OS) containing 0.7% pure ethanol. The percentage recoveries of exogenous 25-OHD3 (R1) and 25-OHD2 (R2) were calculated for each method. Results (OS nM, R1 and R2) were as follows: DiaSorin RIA (n = 53); 39.2, 82.1%, 83.3%, DiaSorin Liason (n = 16); 36.8, 81.4%, 88.6%, IDS RIA (n = 2 1); 36.4, 54.2%, 29.1 %, IDS OCTEIA (n = 16); 47.3, 78.8%, 56.4%, Nichols Advantage (n = 2 1); 58.9, 46.4%, 43.2%, HPLC (n = 9); 42.6, 112.2%, 97.1 %, LC-MS (n = 4); 34.0, 111.5 %, 118. 1 %. The IDS RIA and Nichols assays gave unexpectedly low recoveries. This does not appear to be a calibration problem or the effect of ethanol. (c) 2006 Elsevier Ltd. All rights reserved.