Abstract P136: Comparison of Pharmacy Refill Data With Therapeutic Drug Monitoring (TDM) in Assessing Non-Adherence to Cardiovascular Drugs in Patients With Uncontrolled Hypertension

Abstract

Pharmacy refill data are frequently used to screen for medication non-adherence in clinical practice but its accuracy has not been validated against therapeutic drug monitoring (TDM) in patients with uncontrolled hypertension. Accordingly, we evaluated the accuracy of proportions of days covered (PDC) and medication possession ratio (MPR) in assessing non-adherence to cardiovascular medications against therapeutic drug monitoring (TDM) in patients with uncontrolled hypertension at outpatient clinics of Parkland Memorial Hospital. Patients who had blood pressure of >130/80 mmHg, were on ≥2 antihypertensive medications, and verbally reported full medication compliance were enrolled in the study after an informed consent. Plasma samples were obtained for measurement of 36 cardiovascular drugs including statins, using liquid chromatography mass spectrometry assay. Among 38 patients enrolled in our study, 21% (8/38 patients) were non-adherent to at least 1 antihypertensive medication, while 39% (12/31 patients) were non-adherent to statin drugs. The non-adherent patients were younger than the adherent patients (57.2±12.0 vs. 62.9±7.8, p=0.08), and were less likely to be African Americans (7/17 vs. 16/21, p<0.05). The non-adherent group to statin drugs had significantly higher LDL level than the adherent group (112±41 vs. 70±21 mg/dL, p<0.04). The adherent group to antihypertensive medications had similar blood pressures as the non-adherent group (145±12.8/82±9.8 vs.146±9.6/87±12.9 mmHg). The pharmacy refill data are available in 50% (19/38) of patients, and there was no significant correlation between PDC/MPR and TDM results (r 2 =0.15, p=0.11 for PDC; r 2 =0.15, p=0.16 for MPR). Using a threshold of < 80%, low PDC has a sensitivity of 100% and a specificity of 63% in detecting medication non-adherence when compared to TDM. Similar results were obtained for MPR: sensitivity of 100% and specificity of 65%. While medication refill data may be a sensitive screening tool for medication non-adherence, it has limited specificity and availability when compared to TDM. Thus, TDM constitutes a more efficient and reliable way to assess medication non-adherence, which is crucial in optimizing cardiovascular risk factors in this indigent population.