Marie Hudson1, Elham Rahme, Hugues Richard, Louise Pilote. 1. Division of Clinical Epidemiology, The Research Institute of the McGill University Health Center, Montreal, Quebec, Canada. marie.hudson@mcgill.ca
Abstract
PURPOSE: Studies of medication persistency using drug databases use different definitions of persistency, making it difficult to compare the results from separate studies. We undertook a study of persistency to statins using various definitions to compare the results obtained with the different definitions. METHODS: All patients with an acute myocardial infarction in the province of Quebec between April 1999 and March 2004 who filled a prescription for a statin within 30 days of discharge were identified. The main outcomes were the 1-year rates of persistency defined as (1) the proportion of individuals with a medication possession ratio > or = 80%, (2) the proportion of individuals having filled a prescription in the last 60 days of the year, and (3) the proportion of individuals with continuous exposure after the initial prescription, using a grace period of 7 days or 25% of the duration of the previous prescription between successive prescriptions. Kaplan-Meier analysis was also performed to assess continuous persistency over time using a 7-day grace period. RESULTS: Of the 20,239 patients identified, 1 year persistency to statins ranged from 41% to 90%, depending on the definition used: 85% had a medication possession ratio for statins > or = 80%, 90% filled a prescription in the last 60 days of the year, 41% to 44% had continuous persistency, and, in survival analysis, the probability of continuous persistency was 41%. CONCLUSIONS: Measures of medication persistency yield different results, depending on the definition used to define persistency. Results of studies of persistency should thus be interpreted with caution.
PURPOSE: Studies of medication persistency using drug databases use different definitions of persistency, making it difficult to compare the results from separate studies. We undertook a study of persistency to statins using various definitions to compare the results obtained with the different definitions. METHODS: All patients with an acute myocardial infarction in the province of Quebec between April 1999 and March 2004 who filled a prescription for a statin within 30 days of discharge were identified. The main outcomes were the 1-year rates of persistency defined as (1) the proportion of individuals with a medication possession ratio > or = 80%, (2) the proportion of individuals having filled a prescription in the last 60 days of the year, and (3) the proportion of individuals with continuous exposure after the initial prescription, using a grace period of 7 days or 25% of the duration of the previous prescription between successive prescriptions. Kaplan-Meier analysis was also performed to assess continuous persistency over time using a 7-day grace period. RESULTS: Of the 20,239 patients identified, 1 year persistency to statins ranged from 41% to 90%, depending on the definition used: 85% had a medication possession ratio for statins > or = 80%, 90% filled a prescription in the last 60 days of the year, 41% to 44% had continuous persistency, and, in survival analysis, the probability of continuous persistency was 41%. CONCLUSIONS: Measures of medication persistency yield different results, depending on the definition used to define persistency. Results of studies of persistency should thus be interpreted with caution.
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