Nontherapeutic INR After Hospital Discharge: A Repeated-Measures Analysis of Warfarin-Treated Patients and Potential Drug–Drug Interactions

Background: Warfarin remains widely used in specific clinical situations. Its management is complicated by multiple factors that affect anticoagulant response, particularly during the early period after hospital discharge. This study examined the prevalence, patterns, and factors associated with nontherapeutic international normalized ratio (INR) among patients discharged on warfarin from a tertiary-care hospital. Methods: Electronic health records of adult patients discharged home with warfarin who had at least one INR measurement within 90 days (N = 1222) were retrospectively analyzed. Nontherapeutic INR was defined as INR outside the therapeutic range: 2.5–3.5 for mitral valve replacement and 2.0–3.0 otherwise. All available INR measurements were included. Major warfarin potential drug–drug interactions (pDDIs) were defined as DDIs with major severity according to the Micromedex® database. Factors associated with nontherapeutic INR were examined using repeated-measures generalized estimating equations (GEEs), with generalized linear mixed models (GLMMs) as confirmatory analyses. Results: Of 3704 INR measurements within 90 days after discharge, 49.4% were subtherapeutic, while 30.5% were therapeutic and 20.1% were supratherapeutic. The proportion of therapeutic INR values did not show a substantial improvement over time. In GEEs, discharge from surgical service (adjusted odds ratio (aOR) 1.24, 95%CI: 1.05–1.48, p = 0.014) and presence of major warfarin pDDIs at discharge (aOR 1.36, 95%CI: 1.11–1.67, p = 0.003) were associated with nontherapeutic INR. GLMM analyses produced consistent results with the GEE model. Conclusions: Suboptimal INR control was prevalent within 90 days post-discharge. Discharge from surgical services and presence of major warfarin pDDIs at discharge were associated with nontherapeutic INRs. Major warfarin pDDIs may serve as markers of medication complexity at discharge and may help identify patients requiring closer anticoagulation monitoring.