Congruence of three self-report measures of medication adherence among HIV patients

OBJECTIVE: To evaluate the accordance between three measures of self-reported medication adherence. METHODS: A survey was administered to HIV patients. The three adherence measures included a four-item Morisky-type scale (Measure1) and two measures defining adherence as the percentage of doses taken as prescribed during the past two days (Measure2) or past two weeks (Measure3). RESULTS: For Measure1, 29.2% of the patients were categorized as high adherence and 61.5% as medium adherence. For Measure2 and Measure3, the mean scores were 93.6% and 96.5%, respectively. Using 90% as cutoff values, 78.5% and 95.4% of the patients were classified as adherent by Measure2 and Measure3, respectively (kappa = 0.30; p = 0.001). When 80% was used, 90.8% of the patients were classified as adherent for Measure2 and 96.9% for Measure3 (kappa = 0.48; p < 0.001). When using 90% and 80% as cutoff values to categorize Measure2 and Measure3 as three levels, there was no agreement between Measure1 and the other two measures. The accordance of Measure2 and Measure3 was significant albeit not high (kappa = 0.31; p < 0.001). CONCLUSIONS: The accordance between a Morisky-type adherence scale and measures of missed doses is unsatisfactory. However, "missed-dose" measures using two-day or two-week time periods yield fairly similar results. Researchers should be cautious when comparing adherence rates between studies that use different methods for assessing adherence.