Abstract P105: A Single-Center Experience with Low-Dose Warfarin in Patients Undergoing Total Hip or Total Knee Replacement Surgery

Abstract

Background: Current guidelines recommend thromboprophylaxis to prevent venous thromboembolism (VTE) (deep vein thrombosis [DVT]; pulmonary embolism [PE]) after total hip and total knee replacement (THR/TKR) surgery. We describe international normalized ratio (INR) levels and risks of VTE and bleeding among THR/TKR patients receiving low-dose warfarin postsurgery in a single center. Although such real-world observational data are limited by relatively small numbers, single-center analysis eliminates many confounding variables. Methods: Claims data and laboratory information were used to identify patients who (i) underwent TKR/THR between January 2000 and October 2009, (ii) had no warfarin use 60-30 days before surgery, (iii) were ≥18 years old at warfarin initiation, (iv) received warfarin <30 days before or ≤15 days after surgery, and (v) were continuously enrolled in a health plan from 30 days before to 90 days after warfarin initiation. INR levels and VTE/bleeding outcomes were recorded during continuous warfarin use 5-90 days after warfarin initiation. Results: Mean (SD) ages of THR (N=269) and TKR (N=423) patients were 67.7 (13.2) and 70.0 (9.8) years, respectively. During the study period, THR and TKR patients received warfarin for a mean (SD) of 30.8 (18.2) and 30.6 (17.8) days, respectively; the mean number of INRs performed in patients with ≥1 available INR measurement was 3.9 for both THR (N=110) and TKR (N=202) patients. Of those measurements, 2.3 in THR and 2.1 in TKR patients were <2.0; 47% (52 of 110) of THR and 53% (107 of 202) of TKR patients failed to ever achieve the American College of Chest Physicians-recommended INR range (2.0-3.0). Of THR patients with no prior VTE, 10% (26 of 261) had DVT and 0.8% (2 of 261) had PE; of TKR patients, 8% (32 of 401) had DVT and 1.5% (6 of 401) had PE; 2.6% (7 of 266) of THR and 1.7% (7 of 415) of TKR patients developed minor or major bleeds. Multivariate regression found that the risk of VTE in TKR patients who failed to achieve an INR of 2.0-3.0 was significantly higher than in those who did (HR=4.10; p =015), but this effect was not seen in THR patients (HR=0.51; p =0.30). Conclusion: These data suggest that ∼50% of patients receiving low-dose perioperative warfarin fail to achieve recommended INR levels.