Misclassification of exposure is high when interview data on drug use are used as a proxy measure of chronic drug use during follow-up

In many observational studies, the association between drugs and disease is analyzed with information from a baseline interview. We investigated the magnitude and direction of exposure misclassification by comparing interview data at baseline with prospectively gathered pharmacy data. The study population for this study consisted of a cohort of 2,487 participants aged 71 years or older from the Rotterdam Study. Data on drug use were gathered at the baseline interview and through pharmacies during the follow-up period between January 1, 1991, and January 1, 1999. We assessed the sensitivity, specificity, and positive and negative predictive value of interview data as proxy measures of chronic use of calcium channel blockers (CCB) in comparison with longitudinal medication records from the pharmacy. Only 3 of the 206 subjects (1.5%) who reported use at baseline did not use CCBs during follow-up. Of the 2,281 persons who reported no use of CCBs at baseline, however, 354 actually used CCBs during follow-up (15.5%). The difference between interview data and pharmacy records corresponded to a misclassification bias of 0.73 (95%CI: 0.52-1.02). Misclassification of exposure was high when interview data were used as a proxy measure of chronic use during follow-up.

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