*Result*: Validation of the 0-to-10 Medication Adherence Scale against the Medication Possession Ratio: A comparative analysis.

*Purpose: Medication nonadherence increases healthcare costs and reduces care quality. Although there is no universally accepted gold standard for measuring medication adherence, the Pharmacy Quality Alliance defines adherence as a Medication Possession Ratio (MPR) of at least 0.80. This study sought to validate the 0-to-10 Medication Adherence Scale against the MPR.
Methods: This prospective study included adults prescribed medications for chronic diseases receiving care at primary care clinics in San Antonio, TX, in 2019. Patients were included if they were an adult (18 years of age or older), were prescribed at least one medication for a chronic condition, had 3 or more medication fills during the study period, were seen by a clinical pharmacist at one of the clinics, and were filling prescriptions at the health system's outpatient pharmacies. Patients were excluded if they did not have a chronic condition, did not give consent, were hospitalized for more than 14 days, or did not speak English or Spanish. Participants completed an 11-item questionnaire, including the adherence scale, and their MPR was calculated from pharmacy records. Statistical analyses evaluated the scale's correlation with the MPR and its ability to classify adherence.
Results: Among 95 participants (mean age, 54 years; 73.7% Hispanic; 66.3% female), self-reported adherence scores of 9 or higher correlated significantly with an MPR of at least 0.80 (χ² = 10.009, P = 0.0185). Spearman's correlation (r = 0.3569, P = 0.0004), linear regression (R² = 0.1515), and receiver operating characteristic (area under the curve = 1.00) analyses confirmed the scale's validity, while Cohen's κ showed fair agreement (κ = 0.3226).
Conclusion: The 0-to-10 Medication Adherence Scale is a quick, effective tool for assessing medication adherence, correlating strongly with the MPR. Its simplicity supports its integration into clinical practice.
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