BACKGROUND: The National Quality Forum recently endorsed the proportion of days covered (PDC)€”a measure of medication adherence€”as an indicator of quality in drug therapy management.

OBJECTIVE: To inform initial efforts to improve the quality of drug therapy management, we compared PDC and persistence among new users of 6 commonly used chronic medication categories.

METHODS: A retrospective analysis of pharmacy claims in a database of more than 64 million members enrolled in 100 health plans assessed persistence and adherence to drug therapy in 6 chronic conditions. Patients were included in the analysis if they initiated a prescription drug of interest in any of 6 drug classes€”prostaglandin analogs, statins, bisphosphonates,oral antidiabetics, angiotensin II receptor blockers (ARBs), and overactive bladder (OAB) medications€”between January 1 and December 31, 2005.The first claim for a drug of interest during this period was considered a patient€™s index date. Patients were required to have a minimum of 12months of continuous enrollment both preceding and following their index date. New users of a treatment were identified by excluding patients who filled a prescription for any drug in the same class during the previous 12months and were followed for a minimum of 12 months. Nonpersistence was defined as discontinuation of the therapy class following an allowed gap between refills€”30-, 60-, and 90-day refill gaps were used. Adherence was defined as a continuous measure of the proportion of days covered (PDC) during the 12-month post-index period. Logistic regression analyses predicted (a) nonpersistence during the 12-month post-index period and (b) adherence (PDC) of at least 80%, with drug class as the predictor variable of interest, controlling for demographic variables, insurance and plan type, history of hospitalization, Charlson comorbidity score,copayment for index medication, and number of medications at index.

RESULTS: A total of 167,907 patients were identified across 6 cohorts.Using the 60-day gap, 6-month persistence rates were prostagl and in analogs 47%, statins 56%, bisphosphonates 56%, oral antidiabetics 66%,ARBs 63%, and OAB medications 28%. After the first 90 days of therapy,relative persistence was stable across cohorts, and rates declined consistently from 6 months post-index to study end. Logistic regression models showed that oral antidiabetic users had a 59%, 36%, 37%, and 79% decreased risk of nonpersistence in a 12-month follow-up period compared with patients taking prostaglandin analogs, statins, bisphosphonates, orOAB medications, respectively. Risk of nonpersistence decreased with increasing age. Mean (SD) 12-month adherence rates were: prostagl and in analogs 37% (26%), statins 61% (33%), bisphosphonates 60% (34%), oral antidiabetics 72% (32%), ARBs 66% (32%), and OAB medications 35%(32%). Logistic regression indicated that oral antidiabetic use was a significantpredictor of adherence (PDC) of at least 80% compared with other therapy classes. Adjusted odds ratios for oral antidiabetics were 17.60(95% confidence interval [CI]=15.38-20.14) versus prostaglandin analogs,2.06 (95% CI=1.99-2.12) versus statins, 1.92 (95% CI=1.83-2.02) versus bisphosphonates, 1.29 (95% CI=1.24-1.34) versus ARBs, and 5.77 (95%CI=5.38-6.19) versus OAB medications.