A comparison between six- and four-week intervals in surveillance of oral anticoagulant treatment.

We determined whether international normalized ratio (INR) monitoring at 6 weeks rather than 4 weeks would benefit patients and reduce costs. Patients receiving stable oral anticoagulation treatment (target INR, 3.0) with a prosthetic mechanical heart valve for more than 6 months were randomized for a maximum interval between INR determinations of 6 weeks (group 1, n = 59) or 4 weeks (group 2 [control], n = 65). Patients were followed up for 2 years. The primary end point of the study was the biologic risk of overanticoagulation or underanticoagulation, estimated as the rate of values at risk (INR, < 1.5 and > 5). The rates of INR values at risk for hemorrhagic (INR, > 5) or thromboembolic (INR, < 1.5) complications were 3.27% in group 1 and 3.09% in group 2 (P = .81). The INRs of patients in group 1 trended toward higher values, but no difference between groups was observed in time spent at various INR ranges by using the method of linear change. The mean time between INR determinations was 24.9 +/- 18.1 days (1.20 per month) in group 1 and 22.5 +/- 9.5 days (1.33 per month) in group 2 (P < .0003). For patients in stable condition with a prosthetic heart valve who are monitored at an anticoagulation clinic, a 6-week interval between INR determinations does not increase the biologic risk of thromboembolic or hemorrhagic events.