Drug use among complete responders, partial responders and non-responders in a longitudinal survey of nonagenarians: analysis of prescription register data.

PURPOSE In observational studies, non-response can limit representativity and introduce bias. We aimed to investigate the longitudinal changes in the number of used drugs among complete responders, partial responders, and non-responders in a whole birth cohort of Danish nonagenarians participating in a longitudinal survey. METHODS We obtained prescription data on all individuals born in 1905 and living in Denmark when the Danish 1905 cohort study was initiated in 1998 (n = 3600) using the Danish National Prescription Registry. Drug use was assessed for complete responders, non-responders at baseline, and partial responders (i.e., dropouts) in the 4-month period preceding each wave of the study (1998, 2000, 2003, and 2005), that is, as the cohort aged from 92-93 to 99-100 years. RESULTS Complete responders, non-responders, and partial responders used a similar number of drugs at baseline, on average 4.4, increasing to 5.6 at the age of 99-100 years. In all groups, the number of used drugs increased over time; partial responders had the largest increase of 0.39 drugs per year (95% confidence interval (CI): 0.33-0.44) compared with 0.32 (95%CI: 0.27-0.37) and 0.30 (95%CI: 0.25-0.35) in the other groups. Furthermore, the most frequently used drug classes (e.g., loop diuretics and paracetamol) and the drug classes with the largest change (e.g., increase: laxatives and paracetamol; decrease: benzodiazepines) were similar across response groups. CONCLUSIONS The number of used drugs increased in all response groups between the age of 92 and 100 years. In this study, drug use among complete responders was representative of the general drug use in the entire cohort. Copyright © 2016 John Wiley & Sons, Ltd.