Audit of the frequency and clinical response to excessive oral anticoagulation in an out-patient population.

A retrospective review of over-anticoagulated patients with critical international normalized ratios (INRs) was undertaken in a large outpatient laboratory. In the six-month study period, 85 prothrombin times (PTs) were identified with an INR of > or = 6.0, an overall incidence of elevated PTs of 0.2% or two per 1,000 INR tests. Complete follow-up data was available on 65 patients. When compared to an age- and gender-matched control group without INR > or = 6.0, high-INR patients were significantly more likely to manifest the presence of alcoholism or liver disease, to have been anticoagulated for less than six months, to have experienced more frequent warfarin dosage changes, and to have had the addition of a medication known to interact with warfarin. In the high-INR group, a likely cause for the specific critical INR was identified in 44 patients (68%). Drug interactions followed by compliance problems were the most common factors identified. The 13 patients (20%) who received vitamin K therapy experienced no difference in the clinical outcome compared with those managed conservatively. Conservative management of critically high INR values appeared to be as efficacious as intervention with vitamin K therapy.