Medication Adherence in Breast Cancer: A Comparative Analysis of Self-Report Tools and Prescription Refill Data

Purpose Adherence to oral anticancer medications (OAMs) is critical for clinical outcomes in breast cancer (BC), yet assessment methods lack standardization. The goal is to evaluate OAMs adherence in BC patients utilizing the most common adherence assessment methods on the same population, as well as to test the feasibility and concordance of these methods when used separately or in combination. Methods 500 BC patients on OAMs at the National Cancer Institute, Cairo, were randomized into a two-period crossover study evaluating self-report tools (Arm A: Morisky Medication Adherence Scale (MMAS-8) and the Simplified Medication Adherence Questionnaire (SMAQ); Arm B: the Adherence to Refills and Medications Scale (ARMS) and the Medication Adherence Report Scale (MARS-5) with a three-month interval to minimize respondent burden and order effects. Prescription refill data were calculated via Medication Possession Ratio (MPR) and Proportion of Days Covered (PDC). Agreement, diagnostic accuracy, and receiver operating characteristic analyses were performed. Results Refill-based measures estimated higher adherence than self-reported tools. A multi-method approach significantly increased the detection of non-adherent patients (49.4%–69.0%). PDC-based combinations detected more nonadherent cases than MPR. Among self-reported measures, ARMS demonstrated the strongest concordance with refill-based metrics (κ=0.51–0.55; AUC≈0.80), followed by MARS-5 and MMAS-8, whereas SMAQ showed weaker performance. Conclusion Methodological variability significantly impacts reported adherence rates. While no single "gold standard" exists, integrating brief self-reported scales with objective metrics is feasible and provides a more accurate assessment of medication use. This multi-method approach is essential for identifying at-risk patients and tailoring adherence-support interventions in oncology.