Transition probabilities and predictors of adherence in a California Medicaid population using antihypertensive and lipid-lowering medications.

OBJECTIVES: To determine adherence rates, transition probabilities, and factors associated with transition from higher to lower adherence in antihypertensive (AH) and lipid-lowering (LL) medications.
METHODS: California Medicaid data (1995-2003) were used to identify hypertensive patients with prescriptions for both AH and LL medications. Proportion of days covered (PDC) was used to define three adherence classifications: fully adherent (FA, PDC >or= 0.8), partially adherent (PA, 0.2 <or= PDC < 0.8), and nonadherent (NA, PDC < 0.2). Annual transition matrices documented the probability of adherence status changes.
RESULTS: Only 13% of the 5943 patients were FA to both drugs at baseline. Patients who were FA (60%) or NA (84%) to both drugs had high probability of maintaining status at year two (Y2). Significant variables associated with a transition from adherent to NA at Y2 included African American race (odds ratio [OR] 1.5), other race groups (OR 1.2), lack of Medicare eligibility (OR 1.3), and initiating LL therapy of fibric acid derivatives (OR 1.3) or niacin (OR 1.8).
CONCLUSIONS: Patients FA or NA with both drugs at baseline were more likely to maintain their adherence status. Race, insurance coverage, and type of LL medication were significantly associated with transitioning from any adherence status to nonadherence. These findings may be useful in guiding cost-effectiveness analyses incorporating adherence estimates.