ABSTRACT
To compare the performance of 3 commonly used 25-hydroxyvitamin D [25-OHD] assays among a sample of the Saudi population. This cross-sectional study was carried out between January 2011 and December 2012 at King Fahd Hospital of the University, Al-Khobar, Saudi Arabia. After informed consent, blood samples for measurement of 25-OHD level was extracted from 200 adults. The vitamin D level of each individual were determined using chemiluminescence immunoassay [CLIA], radio-immuno assay [RIA], and liquid chromatography-tandem mass spectrometry [LC-MS/MS] assay. Assays were also compared through commonly used cut-points for classification of vitamin D deficiency. Bias between assays was evaluated using Bland-Altman plots. The average age of patients was 45.7 +/- 16.1 years. A significant difference between the assays was found. The mean 25-OHD levels were highest for the LC-MS/MS [21.65 ng/mL, 95% CI 19.74-23.56], intermediate for RIA [16.607 ng/mL, 95% CI 14.87-18.32], and lowest for CLIA method [13.864 ng/mL, 95% CI 12.109-15.618]. Using 30 ng/mL as a cutoff value, only 6% was found to have normal levels of 25-OHD using CLIA, 9% using RIA, and 22% using LC-MS/MS. Levels of 25-OHD and the prevalence of vitamin D deficiency are dependent on the assay used. The reported high prevalence of hypovitaminosis D among the Saudi population can be partially explained by the use of assays that underestimate vitamin D levels