Anticoagulation and Prothrombin Time Ratios-Reply

Jack Hirsh, MD
Arch Intern Med. 1992;152(8):1720. doi:10.1001/archinte.1992.00400200144027.
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In Reply.—  I think that Hoskison and Boomer may have missed the point of the argument made for using the international sensitivity index (ISI) and international normalized ratio (INR). The study by Bussey and associates1 has demonstrated that the thromboplastins used to measure the prothrombin time (PT) ratios vary markedly across laboratories in the United States. Other studies have shown that the responsiveness (ISI values) of thromboplastins can vary when the laboratory purchases a new batch of thromboplastin from the same vendor. Therefore, using the example given by Hoskison and Boomer, for a control PT of 11.8 seconds and a patient PT of 15.5 seconds, the PT ratio would be 1.3. If a very responsive thromboplastin is used (for example, one that is similar in responsiveness to the World Health Organization Standard [ISI of 1.0]), this ratio of 1.3 would constitute an INR of 1.3. If, on the other


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