BACKGROUND: Poor adherence in COPD can increase the exacerbation risk. The GOLD 2023 guidelines recommend the assessment of adherence as a key action item. However, different methods to assess adherence exist and the chosen methodology could impact the outcome. 

AIM: To systematically review literature to describe and critically evaluate the different methods used to assess adherence to COPD medication in electronic healthcare databases. 

METHODS: We searched MEDLINE using the PubMed interface, Web of Science and Embase for articles published before October 11^th 2022. Screening for inclusion and data extraction were independently conducted by two reviewers. Methods to assess adherence were evaluated according to ISPOR and EPSACOMP checklists. 

RESULTS: A total of 161 studies were included, with 4 assessing initiation, 134 implementation and 48 papers assessing persistence (based on the ABC taxonomy of Vrijens, B. et al. BJCP 2012;73(5):691-705). Adherence was mainly assessed in outpatient dispensing data. In total, 18 different methods to assess implementation were mentioned, whereby the majority of the studies (72%, 97/134) reported (a variant of) the medication possession ratio (MPR, 40/97) or the proportion of days covered (PDC, 66/97). Diverse definitions of persistence were observed, with 31% (15/48) indicating a rationale for the chosen treatment gap. Only 14% (22/161) of all included studies incorporated hospitalization days in their formula. 

CONCLUSION: Despite efforts for harmonization, variations in adherence assessment methods are widespread. Attention for transparency, the impact of hospitalization stays and the rationale in choosing an adherence measure is recommended.