Implications of use of low international sensitivity index thromboplastins in prothrombin time testing.

The recent introduction of thromboplastin reagents with low international sensitivity index (ISI) values into the US market for the purpose of generating a more precise international normalized ratio than high ISI thromboplastins could has necessitated an evaluation of the impact of the low ISI reagents on prothrombin time (PT) testing in general. In this study, PT testing with three thromboplastin reagents, one of which (presently used in our laboratory) has an ISI of 2.10 and the other two ISI values of 0.92 and 1.06, respectively, was performed on normal individuals, on quality control reference plasma specimens and single-factor-deficient plasma specimens, and on patients with liver disease, intravascular coagulation, and receiving oral anticoagulant therapy. We found that PTs of normal individuals determined by all three thromboplastins were virtually identical. The thromboplastins with a low ISI generated much longer PTs on abnormal reference plasma specimens than did the high ISI product. Low ISI reagents also produced longer PTs in all three groups of patients. However, the degree of prolongation was far greater for patients receiving warfarin than for the other two groups of patients. Conversion of the PT to an international normalized ratio minimized the discrepancy seen in the PT ratio in patients receiving oral anticoagulants. The two low ISI thromboplastins did not produce near-identical values of PT, PT ratio, or international normalized ratio on plasma specimens obtained from patients who received warfarin therapy. The critical value set for PT with a high ISI thromboplastin would not be adequate if the reagent is to be replaced with a low ISI product.