STANDARDIZING THE PROTHROMBIN TIME - CALIBRATING COAGULATION INSTRUMENTS AS WELL AS THROMBOPLASTIN

Recently, indications for anticoagulation with warfarin have increased , prothrombin time (PT) monitoring at offices and homes has become ava ilable, and the international sensitivity index (ISI) has been recogni zed as a means of adjusting for differences in thromboplastins to stan dardize warfarin sodium dosing. However, different coagulation instrum ents may yield differences in PTs even after correcting for the ISI by means of the international normalized ratio (INR) (INR=[PT measured(I SI)/PT normal]). Because the PTs and INRs from our Anticoagulation Cli nic (portable PT monitor, ISI=2.04, normal PT=12.0 seconds) differed f rom the hospital reference laboratory (ISI=2.01, normal PT=12.0 second s) despite nearly identical ISIs and equivalent control or normal PTs, we systematically compared the two systems. During a 3-month period, we studied two groups of 50 consecutive patients who had been receivin g a stable dose of warfarin. After a single venipuncture, PTs and INRs were measured independently, and regression lines were calculated. Wi thin each group, the results from the different instruments were not i dentical, but they were highly correlated. In comparing INRs, the regr ession lines for the separate and combined groups were as follows: gro up 1 monitor INR=0.49 reference INR+0.81, r=.94; group 2 monitor INR=0 .57 reference INR+0.86,r=.88; and combined monitor INR=0.49 reference INR+0.95, r=.89. Only 82% of the differences for all samples were with in 1.0 INR units. We concluded that the instrumentation effect may be clinically meaningful, and coagulation instruments as well as thrombop lastins should be calibrated to standardize warfarin therapy.