Which statement about ISI is true?

Which statement about ISI is true?

• A. Lower ISI indicates greater sensitivity
• B. Higher ISI indicates greater sensitivity
• C. ISI has no effect on sensitivity
• D. ISI indicates clot time

Answer: (A)

ISI tells you how sensitive a thromboplastin reagent is in a prothrombin time test. A lower ISI means the reagent is more sensitive to changes in coagulation factors (or to warfarin effect). Because of that heightened sensitivity, the same drop in factor activity will cause a larger increase in clotting time with a low-ISI reagent, making the test more discriminating. Conversely, a higher ISI indicates less sensitivity, so the clotting time changes less for the same factor alteration. ISI is a property of the reagent and affects sensitivity, not the actual clot time by itself. Therefore, the statement that a lower ISI indicates greater sensitivity is the correct interpretation.

ISI tells you how sensitive a thromboplastin reagent is in a prothrombin time test. A lower ISI means the reagent is more sensitive to changes in coagulation factors (or to warfarin effect). Because of that heightened sensitivity, the same drop in factor activity will cause a larger increase in clotting time with a low-ISI reagent, making the test more discriminating. Conversely, a higher ISI indicates less sensitivity, so the clotting time changes less for the same factor alteration. ISI is a property of the reagent and affects sensitivity, not the actual clot time by itself. Therefore, the statement that a lower ISI indicates greater sensitivity is the correct interpretation.