Measuring patient compliance in antihypertensive therapy--some methodological aspects.

The usefulness of prescription refilling patterns in assessing compliance was studied retrospectively in 331 subjects receiving antihypertensive drugs during 1 and 2 year follow-up periods in 1976 and 1977. The compliance distribution was J-shaped for both periods. The number of compliant patients decreased slightly when the follow-up period increased from 1 to 2 years. An arbitrary cutoff point (0.7) yielded 34% noncompliers during the 2 year period. By interviewing patients, an additional 5% of noncompliers so-called early drop-outs, were identified among those who had filled no antihypertensive prescriptions during the 2 year period. The second follow-up year yielded little additional information. Reliability of the results obtained with prescription monitoring was also assessed by interviewing patients concerning eventual hospital stays and physicians' verbal orders that were not recorded on the prescriptions. These factors did not affect the overall compliance distribution, even though some patients might have been classified incorrectly because of them. In research settings and in daily practice prescription monitoring is a useful tool for assessing patient compliance.