Comparison of Pharmacy Refill Data With Chemical Adherence Testing in Assessing Medication Nonadherence in a Safety Net Hospital Setting

Background Pharmacy fill data are a practical tool for assessing medication nonadherence. However, previous studies have not compared the accuracy of pharmacy fill data to measurement of plasma drug levels, or chemical adherence testing (CAT). Methods and Results We performed a cross-sectional study in patients with uncontrolled hypertension in outpatient clinics in a safety net health system. Plasma samples were obtained for measurement of common cardiovascular drugs, including calcium channel blockers, thiazide diuretics, beta blockers, angiotensin-converting enzyme inhibitors/angiotensin receptor blockers, and statins, using liquid chromatography mass spectrometry. Proportion of days covered (PDC), a method for tracking pharmacy fill data, was calculated via linkages with Surescripts, and its diagnostic test characteristics were compared with CAT. Among 77 patients with uncontrolled hypertension, 13 (17%) were nonadherent to at least 1 antihypertensive drug and 23 (37%) were nonadherent to statins by CAT. PDC was significantly lower in the nonadherent versus the adherent group by CAT only among patients prescribed an angiotensin-converting enzyme inhibitor/angiotensin receptor blocker or statin (all P<0.05) but not in patients prescribed other drug classes. The sensitivity and specificity of PDC in detecting nonadherence to angiotensin-converting enzyme inhibitors/angiotensin receptor blockers and statin drugs by CAT were 75% to 82% and 56% to 79%, respectively. The positive predictive value of PDC in detecting nonadherence was only 11% to 27% for antihypertensive drugs and 45% for statins. Conclusions PDC is useful in detecting nonadherence to angiotensin-converting enzyme inhibitors/angiotensin receptor blockers and statins but has limited usefulness in detecting nonadherence to calcium channel blockers, beta blockers, or thiazide diuretics and has a low positive predictive value for all drug classes.

Abstract P133: Prevalence Of Medication Non-adherence Via Therapeutic Drug Monitoring In Patients With Uncontrolled Hypertension Treated At A Safety Net Hospital

Background: Medication nonadherence presents one of the greatest challenges to hypertension management. Previous studies from our group have shown therapeutic drug monitoring (TDM), biochemical monitoring of drug levels, is more accurate than self-report, detailed questionnaires, or prescription fill rate in detecting nonadherence. Prior studies have also shown a prevalence of nonadherence from 45% to 80% among insured patients with uncontrolled hypertension despite having been prescribed a multi-drug regimen. Previous studies have not assessed adherence to antihypertensive drugs in a safety net population by TDM.

Methods: We performed a cross-sectional study in patients with uncontrolled hypertension in the primary care Internal Medicine and Cardiology Clinics at Parkland Health& Hospital System (PHHS). Patients with BP of 130/80 mmHg, prescribed ≥2 antihypertensive medications, and self-reporting medication adherence were enrolled after informed consent. Plasma samples were obtained for measurement of 44 cardiovascular drugs using liquid chromatography mass spectrometry.

Results: Among 77 patients with uncontrolled hypertension (57% female, 65% Black, 12% Hispanic), 13 (17%) were nonadherent to at least one anti-hypertensive drug by TDM. There was no difference in baseline characteristics between the two groups. The adherent and nonadherent groups had similar systolic (146±13 vs 148±18, p=0.67) and diastolic (83±9 vs 88±10 mmHg, p=0.13) BP. All patients had either medical insurance or some form of prescription financial assistance. 15 patients had prescription financial assistance, while 62 patients had medical insurance. Nonadherence rates for patients on financial assistance programs did not significantly differ from patients with medical insurance (13% vs 18%, p>0.999).

Conclusions: We found a surprisingly low prevalence of anti-hypertensive medication nonadherence in this uncontrolled hypertension population despite many barriers to treatment. Our study suggests a lesser role of medication nonadherence in uncontrolled hypertension in the safety net population.