Millions of patients receive oral anticoagulation therapy to treat or prevent thromboembolic disease.¹ While effective management of warfarin sodium therapy is not easy, better anticoagulation control can improve outcomes and reduce adverse events.² Indeed, a growing body of literature has shown that patients whose care is managed in  dedicated anticoagulation clinics (ACCs) have better outcomes than those managed in usual care.^3-6 Clinical guidelines^1,7 recommend that all ACCs should have certain features, including quality improvement programs, adequate documentation of care, sufficient training for providers, and a ratio of patients to providers of less than 400:1. However, there is little evidence as to which, if any, of these features contribute to the improved outcomes seen with ACC care.

Although oral anticoagulation therapy has been used for more than 5 decades, quality measurement has had little penetration into this field.² Our group recently undertook the first systematic effort to profile riskadjusted anticoagulation control in an integrated health system. Within the Veterans Health Administration (VA), a system in which essentially all anticoagulation therapy is managed in an ACC, the mean percentage time in therapeutic range (TTR) varied widely among sites,⁸ with some sites performing more than 10% better or worse than would be expected based on the risk-adjustment model. Variations of this magnitude are associated with important differences in rates of stroke, venous thromboembolism, and major hemorrhage.^9-13 To help all sites in the VA improve their performance, it is necessary to understand the site-level correlates of better or worse anticoagulation control.

Therefore, this study had the following 2 objectives: first, to describe differences in the organization and management of ACCs within the VA, and second, to assess whether these differences help explain sitelevel differences in anticoagulation control. We measured site characteristics using a survey of VA ACCs that examined structural features that seem likely to affect anticoagulation control, including staffing ratios, provider training protocols, and the existence of quality improvement programs.^1,7 We hypothesized that these variables would be associated with anticoagulation control. In this study, either a positive finding or a negative finding would be useful because it can help determine whether enforcing conformity to any specific feature is likely to improve anticoagulation care and control in the VA.

METHODS

Data

The data for this study have been described elsewhere. The Veterans Affairs Study to Improve Anticoagulation^8,14 included all patients who received oral anticoagulation therapy from the VA between October 1, 2006, and September 30, 2008. The study was approved by the institutional review board of the Bedford VA Medical Center, Bedford, Massachusetts.

We included international normalized ratio (INR) values when patients were “on warfarin” (ie, when a patient was in possession of warfarin or was having INR tests performed at least every 42 days). We defined the period of warfarin possession as the duration of the most recent VA prescription for warfarin, plus 30 days. We calculated percentage TTR using the method by Rosendaal et al,¹⁵ which assigns an INR value to each day by linear interpolation between successively observed INR values. Gaps of 56 days or more between INR values are not interpolated. The patient’s TTR equals the percentage of days for which the interpolated INR values lie between 2.0 and 3.0 (from 0%-100%).

Independent Variables of Site-Level Characteristics

There are 128 sites of care within the VA, each of which includes a hospital, an outpatient care center, and several outlying community-based clinics. Each site has a specialized ACC, which is usually run by clinical pharmacists under the supervision of a medical director.16 Therefore, essentially all anticoagulation care within the VA is delivered within specialized ACCs. In October 2006, VA Pharmacy Benefits Management surveyed all 128 VA sites of care for the organization and management of their ACCs. Topics included visit modalities (face to face, telephone, and mail), quality improvement programs, clinic staffing, provider training, documentation, and care coordination (the verbatim text of the questionnaire is given in the eAppendix [available at www.ajmc.com]). We abstracted our independent variables from the responses to the questionnaire.

Dependent Variable of Risk-Adjusted Anticoagulation Control

Site mean TTR was adjusted for case mix using a model that incorporates patient demographics, comorbid conditions that have an adverse effect on TTR, and general measures of comorbidity, including the number of medications and the number of hospitalizations. The derivation and validation of this model, which achieved an R2 of 13.3%, has been described previously.^8,14 The model was used to calculate the expected mean TTR for each site (“E”) based on patient characteristics. The expected mean TTR was compared with the observed mean TTR for each site (“O”). Therefore, each site’s performance was characterized by an O minus E (“O − E”) score, our measure of risk-adjusted TTR (RA-TTR).

Statistical Analysis and Power

We began by first calculating the mean observed TTR and the mean expected TTR for each site of care and then computing an O − E score for each site. We compared O − E scores between sites with and without various organizational characteristics using unpaired t test and compared O − E scores for multilevel variables using analysis of variance. We did not adjust for multiple comparisons. With regard to statistical power, for a characteristic present at half of the sites, t test would have 80% power to detect a 3% difference in the O − E score (a small-to-moderate effect size). Analyses were performed using commercially available statistical software (SAS version 9.1; SAS Institute, Cary, North Carolina).

RESULTS

Study Sample