• Mir Sadat-Ali, Abdulmohsen H Al-Elq, Iman H Al-Shaikh, Haifa A Al-Turki, Amein K Al-Ali, and Abdallah A Al-Othman.
• Department of Orthopedic Surgery, College of Medicine, King Fahd Hospital of the University, University of Dammam, Dammam, Kingdom of Saudi Arabia. E-mail. drsadat@hotmail.com.
• Saudi Med J. 2014 Oct 1; 35 (10): 1243-9.

ObjectivesTo compare the performance of 3 commonly used 25-hydroxyvitamin D (25-OHD) assays among a sample of the Saudi population.MethodsThis 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 levels 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.ResultsThe 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.ConclusionLevels 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.

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