Healthy adherer effect – the pitfall in the interpretation of the effect of medication adherence on health outcomes

Abstract

Different designs of studies monitoring adherence may cause bias and subsequent unavailability to compare results. Healthy adherer effect (HAE) is a type of bias reflecting patient behaviour. It cannot be easily monitored in study population and can favourably affect health outcomes that may be incorrectly attributed to drug therapy. The aim of this paper was to assess the HAE impact on health outcomes of studies concerning medication adherence. Systematic review of literature from PubMed, EMBASE and Cochrane Register of Controlled Trials identified all randomized controlled or observational studies dealing with HAE. Included studies were analysed with respect to relationships between HAE, adherence to therapy and health outcomes. Seven studies were identified - two randomized controlled and five cohort studies. Significant occurrence of HAE in relation to mortality was not observed while one study indicated the presence of HAE in relation to surrogate (bone mineral density). Cohort studies were mainly based on drug class effect, but HAE was not revealed. Factors associated with patient behaviour (e.g. smoking, regular preventive screening) were also not clearly associated with the occurrence of HAE, but their inclusion in design of cohort studies can help to detect health seeking behaviour. Only a few studies concerning HAE were found, but they did not obtain any consistent conclusions. HAE impact was supposed particularly on treatment outcomes that may be easier affected by patient behaviour. However, researchers and clinicians should be still aware of HAE, interpret results carefully and verified them in further studies.