131 - The Prothrombin time: review
Autor(s): F. Manzato
Issue: RIMeL - IJLaM, Vol. 5, N. 2, 2009 (MAF Servizi srl ed.)
Page(s): 131-137
Quick’s prothrombin time (PT) test, originally introduced in 1935, remains the most universally popular method for monitoring anticoagulant therapy. Quick set out to device a means of assaying “prothrombin” by measuring the recalcification time of citrated plasma in presence of thromboplastin reagent. At that time many clinicians refused to see any practical value in the PT test and even doubted its validity on theoretical ground. Today it is known that the “prothrombin” measured by Quick’s test is a complex of factors and, among them, 3 are vitamin K dependent. With the introduction of oral anticoagulant therapy the PT was adopted as the test of choice for dosage prescription. However, thromboplastins can vary markedly in their responsiveness to the defects induced by vitamin K antagonist therapy. There were wide variation in la- boratory techniques and reagents, methods of repor- ting results and therapeutic ranges advocated. Standar- dization of the PT had begun in 1962 with the supply of a standardized human thromboplastin to clinical laboratories, the so called Manchester comparative re- agent. In 1983, owing to the need for worldwide stan- dardization, the World Health Organization (WHO) established a scheme based on the International Sensi- tivity Index (ISI) of thromboplastin reagent to provide International Normalized Ratios (INR). Although the ISI corrects for major differences in PT results between test systems (thromboplstin/coagulometer combinations), persistent INR disagreement between results with different test systems is frequently observed. Local PT system ISI calibration, therefore, appears essential. Moreover, the use of INR for reporting PT in patients with liver disease fails to yield standardization. After more than 70 years since its introduction it is now time to think about its standardization for patients with liver diseases with the introduction of a new INR, the INR Liver.