Adherence Trends for 3 Chronic Disease Medication Classes Among Differently Insured Populations

Adherence trends were examined for commercial, Medicaid, Medicare Part D, and self-insured health plans for oral antidiabetic medications, statins, and direct renin inhibitors.
Published Online: February 19, 2014
Sara C. Erickson, PharmD; R. Scott Leslie, MPH; Wenyi Qiu, MS; and Bimal V. Patel, PharmD, MS
Cardiovascular disease is the leading cause of death in the United States.1 By 2030, the prevalence of cardiovascular diseases, including hypertension, is projected to reach more than 40% of US adults and cost the United States $1.1 trillion annually (unadjusted 2008 dollars).2 Hypertension, dyslipidemia, and diabetes mellitus are key risk factors for cardiovascular disease and cardiovascular events. Several observational studies have demonstrated that adherence to medications indicated for the treatment of hypertension, diabetes mellitus, or dyslipidemia is associated with better cardiovascular outcomes, decreased cardiovascularrelated and all-cause hospitalizations and emergency department visits, and lower total healthcare costs.3-8 Although the validity of these results has been called into question by demonstration of the “healthy user bias,”9,10 it stands to reason that for a drug to be effective, it needs to be taken regularly. Furthermore, adherence to oral antidiabetic drugs (OADs), antihypertensives, and HMGCoA reductase inhibitors (statins) has been shown to be associated with the cardiovascular intermediate outcomes of lower glycosylated hemoglobin, blood pressure, and low-density lipoprotein cholesterol levels.11,12

Medication adherence is defi ned as “the extent to which a patient acts in accordance with the prescribed interval and dose of a dosing regimen.”13 Payers with large claims databases measure patient adherence with “medication possession” methodologies that use prescription fill dates and days’ supply. These methodologies are based on the number of days a patient has possession of medication rather than if the patient has actually taken the medication. The most commonly used measure in the literature is the Medication Possession Ratio (MPR).14,15 There are many variations of MPR used in the literature; however, MPR is generally calculated as the sum of the days’ supply of medication available in a given period of time divided by the sum of the number of days in the period or the time between the first and last fill of medication.14,15 The latter denominator may infl ate adherence measurements as it does not consider potential discontinuation of chronic therapy.15,16 Also, when shorter observation periods are used, the likelihood of adherence is increased.15

The Pharmacy Quality Alliance (PQA) developed and tested a medication possession adherence measure known as the Proportion of Days Covered (PDC) in several chronic medication classes and concluded that it is to be preferred over MPR methodologies.17 PDC is calculated as the number of days within the calendar year with medication supply divided by the number of days the patient is eligible for benefits. If fills overlap, the start date of the next fill is adjusted to be the day after the previous fill ended. Also, if patients have supply for more than 1 medication within the measured class on the given day, the day is only counted once as being “covered.” Thus, the PDC provides a more conservative estimate of adherence when there may be switching and concomitant therapy.17,18 Unfortunately, there are variations in PDC methodology in the literature and PDC sometimes is used as a misnomer for calculations consistent with MPR methodology.15,19 The definition of 80% PDC or MPR as a threshold for adherence is common in the literature.15 The PQA PDC methodology includes reporting the number of patients reaching 0.80 or greater PDC. While somewhat arbitrary, the 80% threshold for OAD, direct renin inhibitors (DRIs), and statin adherence has been shown to be predictive of clinical outcomes.20,21

Since the PQA endorsed a specific PDC methodology, there has been movement toward using it as the “gold standard.” The National Quality Forum (NQF) endorsed the PDC measure for 5 medication types in 2009.22 The Centers for Medicare & Medicaid Services (CMS) adopted this methodology in 2010 for measurement of adherence to OADs, DRIs, and statins as part of the Five-Star Ratings Quality Rating System.23 The National Committee for Quality Assurance (NCQA) has adopted PDC for adherence measures in the Healthcare Effectiveness Data and Information Set (HEDIS),24 and URAC—a healthcare accreditation agency—has adopted the methodology as part of the mandatory performancemeasures for pharmacy benefit management (PBM) and health plan accreditations.25,26 Also, PDC will be included in the battery of clinical quality measures for Qualified Health Plans (QHPs) accreditation necessary for participation on the Health Insurance Exchanges initiated by the Affordable Care Act.27,28

The inconsistency and variations in the use of MPR and PDC in the literature encumbers the comparison of adherence measurements in different medication classes and in different patient populations when not conducted in the same study. Several studies have shown differences in the adherence to various chronic therapies within the same population.5,7,11 Insurance type has been shown to be a significant predictor of adherence,29 likely because of the differences in demographic and clinical characteristics of the patients eligible for each type of benefit. It is of interest to compare adherence rates in these disparate populations. To the authors’ knowledge, such a comparative analysis does not exist. The objective of this analysis is to compare adherence rates for 3 key chronic medication categories in populations enrolled in different insurance types using the PQA-endorsed PDC methodology and to understand whether adherence may be trending over time.


A retrospective, cross-sectional analysis was performed using de-identified data from a sample of participating clients of MedImpact Healthcare Systems, Inc, a national pharmacy benefits manager, during 2012 and 2013. Four distinct cohorts were defined by type of insurance: commercial, Medicaid (a mix of Medicaid benefi ciaries and enrollees in programs for the indigent and other statefunded public programs), Medicare Part D beneficiaries (a mix of Medicare Advantage Prescription Drug [MA-PD] plans and Prescription Drug Plans [PDPs]), and selfinsured commercial health plans.

Within each insurance type, adherence was calculated as the Proportion of Days Covered (PDC) for each target medication class using specifications from the CMS Medicare 2014 Part C & D Star Rating Technical Notes (released September 27, 2013) and Pharmacy Quality Alliance Technical Specifications.30,31 Specifically, for patients 18 years and older with at least 2 claims for the target medication class, PDC was measured from first claim in the measurement period (index date) to end of the measurement period or member disenrollment. Days of medication coverage was calculated using fill dates and days of supply elements of prescription claims within each patient’s measurement period.32 Adherence was calculated for each cohort by using patients’ adherence rates while adjusting for length of member enrollment, or member years. Member years were calculated as number of months enrolled divided by months eligible in each measurement period. Target medication classes included OADs, DRIs, and statins—the 3 classes used in the CMS Part D Patient Safety measures. Patients utilizing insulin during the measurement period were excluded from the OAD measurement.

PDC and percent of patients meeting the definition of adherence of PDC greater than or equal to 0.80 were measured over the 2012 and 2013 calendar years as specified by PQA and CMS. We calculated these metrics for rolling 1-year periods at the conclusion of each quarter to observe possible temporal trends or seasonality. Analyses were performed using SAS (Cary, North Carolina) version 9.3.


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