The response of Quick's prothrombin time test to oral anticoagulation. Influence of thromboplastin source and calcium chloride concentration.

The response of the prothrombin time to oral anticoagulant treatment depends on the thromboplastin reagent and method of testing. A prothrombin time assay system may be characterized by its international sensitivity index (ISI) with which a coumarin-treated patient's international normalized ratio can be calculated. Both the anticoagulant (sodium oxalate or sodium citrate) and calcium chloride concentrations influenced the ISI. An ISI of 2.0 was obtained for Quick's prothrombin time assay using rabbit brain thromboplastin. Replacing rabbit brain by human brain thromboplastin (Aggeler's method) resulted in an ISI of 1.3. The results described herein are mainly of historical interest and may assist in the interpretation of anticoagulation intensity in early American recommendations.