Prothrombin time in liver failure: time, ratio, activity percentage, or international normalized ratio?

Prothrombin time (PT) is a universal indicator of liver disease severity. However, variability in thromboplastin reagents leads to large interlaboratory differences in PT results. The aim of this study was to determine whether the use of the international normalized ratio (INR) or other modes of expression might achieve PT standardization in patients with liver failure. PT was measured with seven thromboplastin reagents with different sensitivities in plasmas from 27 patients with miscellaneous chronic and acute liver failure and, as a control population, 29 patients on oral anticoagulation therapy. PT was expressed in seconds, ratio, activity percentage, and INR. In patients with liver failure, only activity percentage expression eliminated variability in PT results obtained with the seven thromboplastins while INR, seconds, and ratio values remained significantly different (P < .01). In patients on oral anticoagulant therapy, only INR normalized PT results. We conclude that, in patients with liver failure, INR fails to yield a PT expression independent of the thromboplastin used and only activity percentage expression may provide a common international scale of PT reporting.