Cost-effectiveness of adherence-enhancing interventions: a quality assessment of the evidence.

To determine whether the current cost-effectiveness evidence on adherence-enhancing interventions (AEIs) was of sufficient quality to aid in decision-making regarding medication adherence policies. A computerized search of Embase, MEDLINE, Cinahl, Econlit, NHSEED, Psychlit, EPIC, and Cochrane databases (1980-April 2004) was performed. English-language human subject articles were identified using the key words compliance, adherence, concordance, patient assistance, therapeutic alliance, costs, economics, efficiency, resource use/utilization, cost-of-illness, cost-effectiveness, cost-minimization, cost-utility, and cost-benefit. Studies that appeared to assess the cost-effectiveness of medication AEIs were included. Methodologic rigor was assessed using 15 minimum quality criteria. We found 45 comparative studies in 43 publications. Asthma (14 studies) and psychiatric illness (12 studies) were most commonly investigated. In 33 studies, interventions were educational, 18 had multiple components, and 23 did not appear to be linked to proven reasons for nonadherence. Reporting of adherence and outcome results was often unclear. Cost data were poorer quality than outcome data, using average or estimated costs and omitting some cost elements. Nine studies carried out incremental economic analysis. No study met all quality criteria. We were not able to make definitive conclusions about the cost-effectiveness of AEIs due to the heterogeneity of the studies found and incomplete reporting of results. Important policy decisions need to be made about nonadherence; however, they are currently being made in a vacuum of adequate information. AEIs must be based on reasons for nonadherence and be evaluated using accepted clinical and economic quality criteria.