Warfarin INR Patterns Following Total Hip Arthroplasty.

Anticoagulant therapy is recommended for preventing venous thromboembolism (VTE) among patients undergoing total hip arthroplasty (THA). During warfarin therapy, monitoring of international normalized ratio (INR) is necessary to avoid underdosing (which can lead to VTE and associated events) or overdosing (which can lead to significant bleeding). The purpose of this project was to investigate the distribution of out-of-range INR results among warfarin-treated patients undergoing THA.

A retrospective cohort analysis was conducted using the MedMining^® database of electronic medical record. Data were obtained for patients with a THA procedure and a prescription for warfarin issued within 3 days of surgery from January 1, 2004 through January 31, 2009. We identified INR results following surgery during warfarin therapy for up to 90 days. INR results were categorized as in therapeutic range (2–3), below range (< 2), or above range (> 3). For each patient, we calculated the proportion of INR results in each category. Time to first in-range INR was calculated as days from warfarin start to first in-range level for those with at least 1 in-range level.

Of 1512 eligible patients, 58.4% were aged 65 years and above, 57.5% were female, and 98.5% were White/Caucasian. The most common reason for surgery was osteoarthritis (78.4%) and the mean length of hospital stay was 4.0 days (SD, 2.5 days). Most patients (77.6%) had 2 or more INR levels during warfarin exposure; 39.9% had 5 or more INR values. Among the 1173 patients with at least 2 INR levels available, patients had a mean of 15.7% of INR values within therapeutic range, 82.1% of INR levels below the therapeutic range and 2.2% of values above range. A total of 53.3% of patients with at least 2 INR levels had no INR fall within the therapeutic range. The median time to first in-range INR was 7 days (range 1-80 days).

This population-based observational study found poor warfarin anticoagulation control in THA patients with only about half of patients having INRs within the therapeutic range. INR levels above the therapeutic range were rare, but many patients received insufficient doses of warfarin to achieve therapeutic INR levels. Among those who did achieve at least one in-range INR, the time to achieve this was long, exposing the patient to the risk of VTE during the highest post-op risk period (i.e., within the first 7 days after surgery). It is possible that surgeons are targeting sub therapeutic INR ranges or patients have poor adherence to warfarin therapy; the data do not permit an investigation of physicians' targets or whether patients took the medication as prescribed. These research findings may not be generalizable to the broader US population. A study of the consequences of INR values below the therapeutic range in the THA population is warranted.

Kachroo:Ortho-McNeil Janssen Scientific Affairs, LLC: Research Funding; United BioSource Corp: Employment. Nordstrom:United BioSource Corp: Employment; Ortho-McNeil Janssen Scientific Affairs, LLC: Research Funding. Nutescu:Ortho-McNeil Janssen Scientific Affairs, LLC: Research Funding. Schein:Ortho-McNeil Janssen Scientific Affairs, LLC: Employment. Bookhart:Ortho-McNeil Janssen Scientific Affairs, LLC: Employment, Equity Ownership.