PREVENT

Background

Randomized, double-blind, placebo-controlled trial (PREVENT). 508 patients with idiopathic (unprovoked) venous thromboembolism who had completed a median of 6.5 months of full-dose anticoagulation therapy (INR 2.0–3.0). Randomized to low-intensity warfarin (target INR 1.5–2.0) or placebo for up to 4.3 years. The study was designed around the hypothesis that a lower INR target could suppress the hypercoagulable state associated with thrombophilia and unprovoked VTE while substantially reducing bleeding risk compared to conventional-intensity warfarin. Trial terminated early due to overwhelming benefit of low-intensity warfarin.

Interventions and follow up

Arm A: Low-intensity warfarin, target INR 1.5–2.0, indefinite (up to 4.3 years)
Arm B: Placebo, indefinite
Primary endpoint: Recurrent symptomatic VTE
mFollow up: Mean 2.1 years (up to 4.3 years)

Results

Recurrent VTE: 2.6/100 pt-yrs (warfarin) vs 7.2/100 pt-yrs (placebo), HR 0.36 (95% CI 0.19–0.67), P<.001; 64% relative risk reduction
Major hemorrhage: 5 (warfarin) vs 2 (placebo), P=.25 — not significant
Composite (recurrent VTE + major bleed + death): 48% reduction with warfarin
Risk reduction in thrombophilia subgroup: Similar to overall population

Adverse events

Main adverse events: Major hemorrhage was numerically higher with low-intensity warfarin (5 vs 2 patients) but not statistically significant given the small sample. No intracranial hemorrhages in either group. Minor bleeding events were more common with warfarin (not tabulated separately). INR monitoring required throughout treatment; patient burden of INR monitoring not assessed.

Conclusions

Long-term low-intensity warfarin (INR 1.5–2.0) reduces recurrent VTE by 64% versus placebo after completing initial anticoagulation, with a modest and non-significant increase in major bleeding, demonstrating that extended anticoagulation is highly effective for secondary VTE prevention.

Key Limitations

Key Limitations: Compared against placebo, not conventional-intensity warfarin or DOAC — does not answer which extended anticoagulation intensity or agent is optimal (ELATE directly compared low- vs conventional-intensity warfarin and found conventional superior). Low-intensity INR 1.5–2.0 is difficult to maintain in practice — the target range is narrow and patients frequently drift below 1.5 or above 2.0, requiring frequent monitoring similar to conventional warfarin. Small sample (n=508) reduces power for safety endpoints. All patients had idiopathic VTE — results may not apply to provoked VTE. Conducted before the DOAC era; low-intensity warfarin is now largely of historical interest compared to reduced-dose DOAC extended therapy (apixaban 2.5 mg BID, rivaroxaban 10 mg).

Clinical Context

PREVENT established the principle that extended anticoagulation effectively prevents recurrent VTE after unprovoked events — a foundation for modern practice. However, ELATE demonstrated that conventional INR 2.0–3.0 is more effective than INR 1.5–2.0 without more bleeding, and the DOAC era (AMPLIFY-EXT, EINSTEIN-CHOICE) has supplanted warfarin-based extended therapy. PREVENT remains historically important as the first major trial showing that continuing anticoagulation — even at reduced intensity — dramatically reduces secondary VTE recurrence compared to stopping entirely.

References

References: Ridker PM et al, NEJM 2003 (PREVENT primary)