About a quarter of this population of older patients with DVT or PE did not fill anticoagulant prescriptions during the study period for the minimum 90 days of therapy recommended. This is a conservative estimate, as it includes only those who actually began oral anticoagulants following hospital discharge. Some of the 270 patients who did not initiate therapy (and were thus excluded from the main analysis) may also have required 3 months of therapy, although exactly how many cannot be determined. African-American patients and patients with DVT rather than PE were significantly less likely to complete the recommended minimum course of therapy. The presence of reversible risk factors did not explain shorter duration of therapy in this cohort.
A population-based study from Sweden has examined duration of oral anticoagulant therapy after discharge.15
In this study, 8% of patients received 1 month or less of anticoagulant therapy, 37% were treated for 2 to 3 months, 32% for 4 to 6 months, and 20% for 7 months or more. Another study examined adherence to recommendations for anticoagulation during the hospitalization period itself, but did not cover the period after hospitalization.16
Observational studies such as these can complement randomized controlled trials and evidence-based guidelines by measuring how well recommendations are being followed in practice.
Our finding of an association between race and inadequate duration of therapy is consistent with prior studies showing undertreatment of African Americans for ischemic heart disease,17,18
and other conditions. This association is unlikely to be confounded by income, because enrollment in Medicaid did not emerge as an important determinant of short duration of therapy in either univariate or multivariate analyses. A recent study using data on Medicare beneficiaries has shown that race remains a persistent determinant of disparities in the use of a variety of health services, even after adjusting for income.21
Nonwhite race has also been found to be a marginally significant predictor of noncompliance with anticoagulant therapy in an anticoagulation clinic,22
but the generalizability of this finding to other settings is unclear. Because African Americans may be at an elevated risk of DVT as compared with whites,23
the consequences of undertreatment may be even more important.
We found that 1.4% of patients went on to have recurrent events within the first 90 days of therapy, a figure consistent with other studies, which have shown recurrence rates ranging from 0.6% to 5% at 90 days.24
Interestingly, all patients in the study sample with recurrent events in the first 90 days were covered by a prescription for anticoagulant therapy; this suggests that failure to follow recommendations for duration of therapy may not be the source of recurrent events in this population. Rather, patients may have been anticoagulated at a subtherapeutic international normalized ratio or may have been noncompliant with therapy. Randomized clinical trials25,26
and a population-based study15
have demonstrated that shorter duration of anticoagulant therapy is a risk factor for recurrent events.
The problem of diagnostic validity is a necessary concern in any study based on health care claims data. We addressed this concern in two ways. First, we included only hospitalized patients for whom DVT or PE was the principal or admission diagnosis by using DRG codes, instead of ICD-9 codes, for DVT or PE. Second, we required that patients fill at least one prescription for oral anticoagulants within 14 days of discharge, thus eliminating patients in whom oral anticoagulants would not be the preferred therapy (e.g., patients with isolated calf vein thrombosis). The study sample is therefore likely to be specific for patients with DVT or PE, enhancing the validity of our conclusions.
The duration of therapy for each patient is likely to be a function of both patient and physician characteristics. Inadequate duration of therapy may result from the patient not refilling a prescription or the physician not prescribing 90 days of therapy, decisions which may have many determinants. We cannot discern the relative contributions of the physician and the patient to the outcome observed; future research is needed to examine physician characteristics (e.g., generalist vs specialist) as potential predictors of duration of therapy as well as health care system issues (such as the presence of managed care or the availability of an anticoagulation clinic).
One limitation affecting this study's generalizability is that the patient sample was drawn from a lower income population. However, we found no difference in anticoagulant utilization between patients of modest income (PAAD) and those who were poor (Medicaid), suggesting that income is probably not a major determinant. Further, it is unlikely that physicians would prescribe different durations of therapy to poor versus modest income patients, especially if the poor had comprehensive drug benefits, as was the case here. In fact, the broad coverage of drug expenses in both the Medicaid and PAAD programs effectively removes cost as a barrier for such patients; utilization rates in patients with less generous medication insurance could be considerably worse.
Another limitation of this study concerns the frequent dosage adjustments often required of patients on oral anticoagulant therapy. If dosage is adjusted downward without a new prescription being written, a duration of therapy calculated based on filled prescriptions will underestimate the true duration of therapy. If dosage is adjusted upward without a new prescription being written, duration of therapy will be overestimated. Thus, patients with durations of therapy close to 90 days might be misclassified, although the direction this bias would take is not clear. However, when we examined factors associated with inadequate duration of therapy in sensitivity analyses using stricter definitions of inadequate duration (less than 75 days and less than 60 days), the findings were virtually identical.
Given that about one quarter of patients studied did not receive 90 days of oral anticoagulant therapy after DVT and PE and that African-American race was associated with an increase in the risk of inadequate duration of therapy, further studies are needed to evaluate the reasons for inadequate duration of therapy after DVT and PE as well as the causes of racial disparities in anticoagulant use.