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Prescribed daily dose (PDD), the number of doses prescribed to be taken per day, is used to calculate medication adherence using pharmacy claims data. PDD can be substituted by (1) 1 dose per day (1DD), (2) an estimate based on the 75th percentile of days taken by patients to refill a script (PDD75 ) or (3) the World Health Organisation's defined daily dose (DDD). We aimed to compare these approaches for estimating the duration covered by medications and whether this affects calculated 1-year adherence to antihypertensive medications post-stroke. We conducted a retrospective review of prospective cohort data from the ongoing Australian Stroke Clinical Registry linked with pharmacy claims data. Adherence was calculated as the proportion of days covered (PDC) for 1DD, PDD75 , and DDD. Differences were assessed using Wilcoxon rank-sum tests. Among 12628 eligible patients with stroke, 10057 (80%) were prescribed antihypertensive medications in the year after hospital discharge (78.2% aged ≥65 years, 45.2% female). Overall, the 75th percentile of patient time until next medication refill was 39 days. The greatest variations in dose regimens, estimated using person- and dose-level refill times, were for beta blockers (11.4% taking 2 tablets/day). There were comparable levels of adherence between 1DD and the PDD75 (median PDC 91.0% versus 91.2%; P=0.70), but adherence was slightly higher using DDD (92.3%; both P<0.001). However, this would represent a clinically non-significant difference. Adherence to antihypertensive medications shows similar estimates across standard measures of dosage in patients during the first year after an acute stroke. |
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