Osińska, MagdalenaMagdalenaOsińskaMeyer-Massetti, CarlaCarlaMeyer-MassettiWelte, NaomiNaomiWelteZúñiga, FranziskaFranziskaZúñiga2026-02-042026-02-042026-02-21https://boris-portal.unibe.ch/handle/20.500.12422/230329Polypharmacy is widespread in residential long-term care (RLTC), increasing the risk of medication-related problems. Switzerland's RLTC institutions must report polypharmacy (defined as ≥ 9 active substances) as part of a national health-care quality indicator (QI) initiative. This sequential explanatory mixed-methods study assessed the quality of data collected to calculate a polypharmacy QI across 14 RLTC institutions in German-speaking Switzerland, aiming to recommend improvements. A validation study assessed inter-rater reliability, subsequently, medication lists of individual patients with different medication counts were discussed with RLTC staff to explore the issues affecting those calculations. Institutions and study researchers calculated different numbers of active substances in 65.3 % of the 580 residents assessed, and recorded an overall prevalence of polypharmacy of 45.5 % and 47.4 %, respectively. Solid master data, unambiguous counting rules, and electronic algorithms are necessary to calculate reliable polypharmacy QIs. Additional measures, like medication reviews, might facilitate distinctions between appropriate and inappropriate polypharmacy.enLong-term careNursing homesPolypharmacyQuality indicatorsReliabilityValidation600 - Technology::610 - Medicine & health300 - Social sciences, sociology & anthropology::360 - Social problems & social servicesarticle10.48620/944144157056810.1016/j.gerinurse.2026.103861