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Improving Hypertension Control in a Behavioral Clinical Trial: What Role did Medication Adherence Play?

Kaboli PJ, Vander Weg MW, Lund BC, Holman JE, Egts S. Improving Hypertension Control in a Behavioral Clinical Trial: What Role did Medication Adherence Play? Poster session presented at: Society of General Internal Medicine Annual Meeting; 2010 Apr 30; Minneapolis, MN.


Background: Medication adherence rates achieved in clinical trials are generally higher than those observed in practice. It is less clear whether similar effects are seen in other types of intervention trials. While an intervention may not specifically focus on medication adherence, it is possible that adherence may improve more in intervention participants than in controls because of increased attention to their medication regimen. If so, positive results in such trials may be skewed by increased medication adherence. Our study aims to further examine the role of medication adherence on improved hypertension control during the V A project to implement diuretics (VAPID), a study to increase the prescription rate of thiazide-type diuretics. Methods: Patients with uncontrolled hypertension and not currently taking a thiazide diuretic were selected from 13 Midwest V A outpatient clinics and randomized into intervention and control groups. All intervention patients received patient-tailored infonnation that stated they were above goal for blood pressure, listed their antihypertensive agents, and encouraged them to ask their provider at their upcoming appointment whether a thiazide diuretic would be appropriate; control patients received usual care. Medication adherence for antihypertensive medications was assessed using two previously validated methods: a 4-item self-reported adherence measure (Morisky scale) and a medication possession ratio (MPR) using V A pharmacy refill data. The Morisky scale was administered at three points (0, 6, 12 months) and considered "high adherence" with a score ofO. MPRs were detem1ined using two 12-month time frames before and after enrollment and reported as the proportion of days patients had antihypertensive medications available. Results: A total of 573 subjects were enrolled and completed a visit with their provider. Although there was no significant difference in self-reported medication adherence between intervention and control patients at any time point, there was a significant increase in self-reported adherence during the study in both intervention (39.5% to 59.5%) and control patients (40.7% to 55.4%) from 0 to 6 months (p < 0.001 for both). There was no difference in adherence between 6 and 12 months. Meanwhile, MPRs were no different between study groups in the 12 months pre-intervention and 12 months post, but showed a contradictory decrease in adherence in both lI1terventiol1 (0.93 to 0.87 days) and control patients (0.93 to 0.88 days) in the 12 months pre and postintervention, respectively (p < 0.001 for both). Neither the Morisky scale nor the MPR was associated with the observed increase in blood pressure control, from 32.5% ofpatients controlled at their initial visit compared to 45.2% controlled at 12 months (p < O.OOl). Conclusions: The VAPID study intervention achieved a significant improvement in blood pressure control that was independent of medication adherence. Paradoxically, self-reported adherence significantly increased, while phannacy refill adherence significantly decreased; these changes were equivalent between intervention and control patients. The global increase in self-reported medication adherence may be attributed to a combination of patients' tendency to over-report adherence as well as a possible social desirability bias. While phannacy refill adherence decreased, it remained high, a change that was not likely clinically relevant.

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