Administrative Claims Analysis of the Relationship Between Warfarin Use and Risk of Hemorrhage Including Drug-Drug and Drug-Disease Interactions

BACKGROUND: Despite the risk of hemorrhage, warfarin is the most commonly used oral anticoagulant today, both as monotherapy and when taken in combination with selected drugs. Warfarin is used most commonly for irregular heartbeat, after a heart attack, and after joint or heart valve replacement surgery. OBJECTIVES: To evaluate the relative risk of hemorrhage in health plan members who received warfarin concomitant with a drug known to cause an interaction or after diagnosis of liver disease or heart failure (HF). METHODS: A cohort study sample was drawn from an administrative database comprising medical and pharmacy claims for 1.7 million health plan members. A health plan member was defined as anyone who was eligible for pharmacy and medical benefits at any time from October 1, 2003, to September 30, 2004. To be included in the study, a member must have received at least 1 pharmacy claim for warfarin during the study period and been younger than 100 years. Hemorrhage was defined as a diagnosed bleeding episode recorded on a medical claim within 7 calendar days of a fill date for a pharmacy claim (new or refill) for warfarin. The following variables were used to predict the outcome measures: type of drug-drug or drug-disease interaction, patient age and gender, number of unique prescribers during the year for all drugs, specialty of the first prescriber for warfarin, average dose of warfarin, and days of warfarin therapy. Because individuals were followed only during the calendar year under study, the authors have interpreted the days of therapy measured primarily as a control on exposure. The outcome measures are prevalence of drug and disease interactions among members receiving warfarin therapy and the per-patient-per-year and per-member-per-month (PMPM) cost of medical treatment of hemorrhage associated with warfarin therapy including drug and disease interactions. Costs are defined as the total paid amount for a procedure or service after negotiated provider discounts and subtraction of patient copay and deductibles. Logistic regression was used to evaluate the relative risk of hemorrhage in users of warfarin monotherapy and of warfarin users with drug-drug and drug-disease interactions. The comparison group in the logistic regression comprised the members who were not diagnosed with either HF or liver disease and who received warfarin therapy but none of the drugs under study known to cause drug interactions. Therefore, the odds ratios [ORs] produced were estimates of the relative risk of hemorrhage when taking warfarin concomitant with selected drugs and diseases. RESULTS: Of the 17,895 patients who used warfarin during the study year, 2,634 (14.7%) were diagnosed with a hemorrhage event within 1 week after filling a prescription for warfarin. The factors associated with an increased risk of hemorrhage included female gender (OR 1.149; 95% confidence interval [CI], 1.053-1.253), liver disease (OR 1.764; 95% CI, 1.360-2.288), and HF (OR 1.559; 95% CI, 1.373-1.770). Compared with the use of warfarin alone, the use of either cephalosporins (OR 1.157; 95% CI, 1.043-1.285) or metronidazole (OR 1.578; 95% CI, 1.321-1.886) was associated with increased risk of hemorrhage, whereas the risk of hemorrhage was not greater for concomitant use of warfarin with amiodarone, fibric acid derivatives, or nonsteroidal anti-inflammatory drugs (NSAIDs), including cyclooxygenase-2 (COX-2) inhibitors. There was no relationship between estimated average daily warfarin dose and prevalence of hemorrhage. Other variables associated with an increased risk of hemorrhage were increased patient age, female gender, 120 days or more of warfarin therapy during the year, 2 or more unique prescriber numbers, and the medical specialty of the first prescriber of warfarin. Over the population of 1.7 million members, the cost for all hemorrhage events within 7 days of a pharmacy claim for warfarin was $0.40 PMPM.