National Utilization Patterns of Warfarin Use in Older Patients With Atrial Fibrillation

A Population-Based Study of Medicare Part D Beneficiaries

Mukaila A Raji MD MSc; Matthew Lowery BSc; Yu-Li Lin MS; Yong-Fang Kuo PhD; Jacques Baillargeon PhD; James S Goodwin MD

Disclosures

The Annals of Pharmacotherapy. 2013;47(1):35-42. 

In This Article

Abstract and Introduction

Abstract

Background: Although warfarin therapy reduces stroke incidence in patients with atrial fibrillation (AF), the rate of warfarin use in this population remains low. In 2008, the Medicare Part D program was expanded to pay for medications for Medicare enrollees.

Objective: To examine rates and predictors of warfarin use in Medicare Part D beneficiaries with AF.

Methods: This population-based retrospective cohort study used claims data from 41,447 Medicare beneficiaries aged 66 and older with at least 2 AF diagnoses in 2007 and at least 1 diagnosis in 2008. All subjects had continuous Medicare Part D prescription coverage in 2008. Statistical analysis using χ2 was used to examine differences in warfarin use by patient characteristics (age, ethnicity, sex, Medicaid eligibility, comorbidities, contraindications to warfarin, and whether they visited a cardiologist or a primary care physician [PCP]), CHADS2 score (congestive heart failure, hypertension, age, diabetes, and stroke or transient ischemic attack; higher scores indicate higher risks of stroke), and geographic regions. Using hierarchical generalized linear models restricted to subjects without warfarin contraindications (n = 34,947), we examined the effect of patient characteristics and geographic regions on warfarin use.

Results: The overall warfarin use rate was 66.8%. The warfarin use rates varied between hospital referral regions, with highest rates in the Midwestern states and lowest rates in the South. The regional variation persisted even after adjustment for patient characteristics. Multivariable analysis showed that the odds of being on warfarin decreased significantly with age and increasing comorbidity, in blacks, and among those with low income. Seeing a cardiologist (OR 1.10; 95% CI 1.05–1.16), having a PCP (OR 1.23; 95% CI 1.17–1.29), and CHADS2 score of 2 or greater (OR 1.09; 95% CI 1.01–1.17) were associated with increased odds of warfarin use.

Conclusions: Warfarin use rates vary by patient characteristics and region, with higher rates among residents of the Midwest and among patients seen by cardiologists and PCPs. Preventing stroke-related disability in AF requires implementation of evidence-based initiatives to increase warfarin use.

Introduction

Stroke is a leading cause of serious, long-term disability and the third leading cause of death in the US.[1] Atrial fibrillation (AF) increases stroke risk 5-fold and accounts for approximately 15% of all strokes.[2,3] AF affects 12% of adults aged 75 years and older and its prevalence is expected to double by 2050.[2,4–6] Warfarin, an oral anticoagulant, reduces annual risk of ischemic stroke risk by approximately two thirds in patients with AF, from 4.5% to 1.4%.[7–9] Except for patients at a very low risk for stroke, practice guidelines published by the American College of Cardiology Foundation, American Heart Association, and other scientific bodies recommend warfarin therapy for stroke prevention in AF patients without contraindications.[2,10] Despite this recommendation, the use of warfarin in AF patients remains low, with rates ranging from 39% to 65%.[11–14] Increasing the use of interventions (eg, warfarin and other anticoagulants) to prevent stroke is an important public health issue.

Warfarin is a complex drug to use. The required frequent blood testing and dose adjustments, along with the perceived risk of bleeding (especially gastrointestinal and intracranial bleeds) are common barriers to warfarin prescribing and optimal patient adherence.[15–19] Recently introduced oral anticoagulant agents (dabigatran [a direct thrombin inhibitor] and apixaban and rivaroxaban [factor Xa inhibitors]) have potential to reduce these barriers because they have fixed doses and require no blood testing.[20–22] Clinical trials on these new agents showed similar or better efficacy in stroke prevention and a better adverse effect profile compared with warfarin.[20–22] As more AF patients use these warfarin alternatives, it is important to understand the magnitude of potential warfarin underuse and the reasons for such underuse. Such understanding may help us anticipate (and plan for) therapeutic challenges (eg, toxicity misperceptions and anticoagulant underprescription) that may arise from use of these new anticoagulants. For example, regardless of availability of and access to oral anticoagulants, the decision to use warfarin is often based on the risks versus benefits perceived by the physician,[15,23] which vary by patient. A retrospective cohort study has shown that physicians were less likely to use warfarin therapy after patient exposure to any adverse bleeding event, as compared to before the event.[23] This lack of precision in practice patterns in warfarin prescribing contributes to variability in warfarin's use.

There is regional variation in stroke prevalence. Although studies show little evidence linking regional variation in stroke to variation in stroke risk factors, the quality of management of such risk factors as AF, hypertension, smoking, and diabetes may explain some regional and racial variation.[4,15,24,25] However, little is known about regional and statewide variations in the use of warfarin and other oral therapies for stroke prevention in Medicare enrollees with AF, in part because of the lack of a large nationwide database for outpatient oral drugs. The few studies conducted have small sample size or have been limited to restricted Medicare populations such as long-term care residents, managed-care beneficiaries, hospitalized patients, or patients in specific regions or health care settings.[11–15,26]

In 2006 the Medicare Part D program was implemented; in 2008 the program paid for outpatient prescription medications for approximately 27 million enrollees. The existence of this nationwide outpatient medication database allowed for the examination of national patterns of warfarin use in a large population-based sample. With the expanded coverage of medications by the Medicare Part D program, we wanted to know the rates and predictors of warfarin use in older patients with AF. We thus assessed the national rates of warfarin use across the US by patient characteristics and geographic location. This investigation is an important step in improving our understanding of the previously reported regional differences in stroke rate and in determining whether such regional differences parallel regional variations in the management of AF, a key risk factor for stroke.

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