INTERPRETATION OF SERIAL MEASUREMENTS OF INTERNATIONAL NORMALIZED RATIO FOR PROTHROMBIN TIMES IN MONITORING ORAL ANTICOAGULANT-THERAPY

Despite careful monitoring of oral anticoagulant treatment (OAT), some international normalized ratio (INR) for prothrombin time values will fall outside the therapeutic range. Considerable changes in serial IN R results from OAT patients may be caused by random fluctuation alone, and, for statistical reasons, a fraction of the INR values will fall outside therapeutic range and interfere with dose adjustments. On the basis of therapeutic intervals and statistical evaluation of reference changes, we suggest and discuss an alternative method for interpretat ion of serial INR measurements. Retrospective evaluation of serial mea surements of INR from OAT patients revealed an ''overshooting'' phenom enon. When a dose was adjusted on the basis of insignificant change in INR value, the subsequent INR value generally fell in the opposite di rection, If a further change of dose was initiated because of the new INR value, a similar course in the opposite direction was observed. Th is ''ping-pong'' effect renders patients in a fluctuating state of ant icoagulation and may introduce increased risk of complications, The su ggested method provides an objective criterion for dose adjustments in OAT, which should reduce patients' risk.