Aim: Method evaluation studies should be carried out for every new method planned to be used in laboratories. The aim of this study is to determine the analytical performance of the immunoassay method used for determining of plasma 25-hydroxy vitamin D levels and also to compare to high performance liquid chromatography (HPLC) method.

Materials and Methods: 113 plasma samples were used to carry out repeatability, linearity and recovery studies for 25-hydroxy vitamin D measurement with the Abbott Architect chemiluminescence immunoassay (CLIA) method. HPLC was accepted as reference method. Method comparison study was carried out.

Results: The coefficient of variations for the Architect 25-hydroxy vitamin D method were 4.07% and 2.42 % within-day; 4.77% and 3.93% between-day for low and intermediate plasma levels respectively. The results obtained from the recovery studies were 92.5% and 105.9% for low levels; and 108.3% and 105.5% for intermediate levels. As a result of the method comparison study, Architect method was found to have a significant negative bias [As mean, 9 ng/mL lower, ±2SD interval: (-34.1) – (16.1) ng/mL] and it was also observed that there was a constant and proportional difference between that two methods (HPLC= -3.2657 + 1.7388 CLIA; rho= 0.656).

Conclusion: Architect 25-hydroxy vitamin D method has met the precision, linearity and recovary targets but not the bias targets. For the accurate diagnosis and treatment follow-up of Vitamin D deficiency, it is obvious that to carry out analytical performance studies and to standardize of the measurement methods are essential. As the use of standard reference materials developed for the standardization of vitamin methods becomes widespread, the inter-method variability will also decrease. In addition, participation in laboratories' external quality assessment programs and their assessment of results will provide significant contributions to the use of laboratory findings for patient benefit.