Effect of smoking on the serum levels of 25-hydroxyvitamin D depends on the assay employed

Objective: Because we found higher serum 25-hydroxyvitamin D (25(OH) D) levels among smokers than among non-smokers with analyses using an electrochemiluminescence immunoassay (ECLIA) from Roche, the purpose of the present study was to examine whether this difference between smokers and non-smokers was maintained using other serum 25(OH) D assays. Design: A cross-sectional population-based study on 6932 participants from the Tromso study, 1994-1995, and one validation study comparing six different serum 25(OH) D assays in 53 non-smokers and 54 smokers were performed. Methods: The association between smoking, season and serum 25(OH) D as measured by ECLIA (Roche) was assessed in the population-based study using general linear models with multivariate adjustments. In the validation study, serum levels of 25(OH) D were analysed with liquid chromatography coupled with mass spectrometry assay from two different laboratories, RIA (DiaSorin), HPLC, RIA (IDS) and ECLIA (Roche). T-tests and linear mixed model analyses were performed to compare the serum 25(OH) D levels in smokers and non-smokers within and between the methods. Results: In the population-based study, the serum levels of 25(OH) D using the ECLIA method were 51.9, 53.2 and 72.0 nmol/l in never, former and current smokers (P<0.01). In the validation study, the serum concentration of 25(OH) D was 10.3 nmol/l higher in smokers than in non-smokers (P<0.01) using the ECLIA (Roche), while non-significantly lower serum levels of 25(OH) D were found in smokers using the other five methods. Conclusions: These two studies indicate that the ECLIA (Roche) overestimates serum 25(OH) D levels in smokers by unknown mechanisms. If confirmed, this might have clinical consequences, and the issue needs further exploration.