Influence of the immunoassay used in measurement of serum vitamin D levels

Serum 25-hydroxyvitamin D [25(OH)D] levels are the best indicator of vitamin D levels in the body. Precision, reproducibility, and lack of standardization are the main problems in such measurements. The aim of this study was to compare the 25(OH)D levels measured using Elecsys Vitamin D Total (Roche) and ADVIA Centaur Vitamin D Total (Siemens). 25(OH)D levels were tested in 166 patients using both methods. Patients were subsequently divided into two groups: a «supplemented group» consisting of patients receiving vitamin D supplements, and an «untreated group» consisting of the rest of patients. 25(OH)D mean levels measured by the Roche and Siemens methods in the overall group were 33.6±16.0 and 19.8±12.4 ng/mL respectively. 54.2% of patients were receiving vitamin D supplements. In this group, mean 25(OH)D levels measured by the Roche and Siemens methods were 40.6±14.5 and 25.4±13.1 ng/mL respectively. In the untreated group, the respective values were 24.9±13.2 and 12.8±6.6 ng/mL. Prevalence of vitamin D deficiency (serum 25(OH)D levels less than 20 ng/mL) was higher in samples analyzed using the Siemens method (60.2%) as compared to those tested using the Roche method (23.5%). The assays evaluated are not comparable to each other. Laboratory specialists should inform clinicians of the features of the method used for measuring 25(OH)D because this will have a direct impact on interpretation of the results and medical decisions.