Warfarin prophylaxis in patients after total knee or hip arthroplasty--international normalized ratio patterns and venous thromboembolism.

OBJECTIVE: Warfarin is frequently used for the prevention of venous thromboembolism (VTE) after total hip or knee arthroplasty (THA/TKA). The current study was conducted to determine the association between international normalized ratio (INR) levels and VTE outcomes.

METHODS: Patients who received warfarin following THA/TKA were followed for up to 90 days using an electronic health record database. INR measurements were categorized based on American College of Chest Physicians (ACCP) guidelines. Cox proportional hazards models were used to compare the risk of VTE between patients with INR levels below and within the ACCP-recommended range in patients with ≥2 available INR level measurements.

RESULTS: On or after Day 5, 33.3% and 28.6% of INR levels fell within the ACCP-recommended range for THA and TKA, respectively. VTE was diagnosed in 3% of each cohort. INR levels varied over time and were frequently below the ACCP-recommended range. Below-range INR levels were associated with greater risk of VTE in both THA (hazard ratio [HR]: 5.29; 95% CI: 2.64-10.61) and TKA (HR: 4.64; 95% CI: 2.59-8.29).

CONCLUSIONS: In the current study, the majority of patients had INR levels below the ACCP-recommended range of 2.0-3.0 during warfarin exposure following orthopedic surgery. INR levels below 2.0 were associated with a four- to five-fold increase in the risk of VTE.