Intensity of Warfarin Anticoagulation as an Independent Predictor of 6-Month Recurrence after Deep Vein Thrombosis or Pulmonary Embolism: A Population-Based Cohort Study.

Background: Most trials of standard heparin and warfarin therapy used the 3- to 6-month cumulative incidence of venous thromboembolism (VTE) recurrence as the primary efficacy outcome. However, the independent effect of warfarin anticoagulation intensity on VTE recurrence after controlling for other predictors of recurrence (e.g., active cancer, neurological disease with extremity paresis) is uncertain.

Objectives: To test warfarin anticoagulation intensity, using international normalized ratio (INR) data, as an independent predictor of 6-month VTE recurrence. Measurements: Using the resources of the Rochester Epidemiology Project, we identified all Olmsted County, MN residents with an incident deep vein thrombosis or pulmonary embolism over the 14-year period, 1984–1997 (n=1165). We followed each case (conditional on surviving 14 days without VTE recurrence) forward in time through their complete medical records in the community for first VTE recurrence. We collected date of incident VTE onset, start and stop dates of heparin and warfarin, and date, time and result of all INR values. Using these and other measures of standard warfarin therapy as time-dependent variables, we tested the intensity of warfarin anticoagulation (expressed as the cumulative percent of time with an INR at or above 2.0 [or at or above 1.5] while on warfarin) as an independent predictor of 6-month VTE recurrence while controlling for other baseline characteristics using Cox proportional hazards modeling.

Results: Of the 1165 patients, 1026 (88%) and 989 (85%) received heparin and warfarin, respectively, and 254 (22%) developed recurrent VTE over 5461 person-years of follow-up; 52 recurred within 15–180 days. In the multivariate analysis, the independent predictor of 6-month recurrence was active cancer (HR=3.98, 95% CI: 2.12, 7.45; p<0.01); age, gender, body mass index and neurologic disease with extremity paresis were not predictors of VTE recurrence. In models that included presence or absence of warfarin therapy along with proportion of time with an INR ≥2, this latter intensity variable was protective (HR=0.16 for 100% versus 0% of time, p=0.02) against recurrence, while mere presence of warfarin therapy (HR =0.72, p= 0.56) was not. Similar results were obtained when intensity was represented as proportion of time with an INR ≥1.5 (HR=0.17, p=0.02). Interestingly, a therapeutic APTT within the first 24 hours of heparin therapy also was protective against 6-month recurrence (HR=0.45, p=0.02).

Conclusions: Active cancer is an independent predictor of 6-month VTE recurrence. Warfarin therapy with a higher proportion of time with an INR ≥1.5 or ≥2 offers similar protection against recurrence. A rapid heparin-response (as reflected by a therapeutic APTT within 24 hours of heparin therapy) also is a predictor of reduced 6-month recurrence, possibly reflecting lower factor VIII activity at the acute incident VTE event. Gender is not a predictor of long-term recurrence.