Which value is expressed as a ratio rather than seconds or minutes?

Which value is expressed as a ratio rather than seconds or minutes?

• A. Prothrombin Time
• B. Bleeding Time
• C. Partial Thromboplastin Time
• D. International Normalized Ratio

Answer: (D)

This item tests understanding that some coagulation results are expressed as a time, while others are standardized as a ratio. The International Normalized Ratio is a unitless value because it’s calculated by comparing a patient’s prothrombin time to a reference prothrombin time and adjusting for the sensitivity of the reagent (the ISI). In practice, INR = (PT_patient / PT_normal) raised to the ISI, which produces a dimensionless number that can be compared across different labs and reagents. This standardization is essential for monitoring warfarin therapy, where consistent results matter more than raw seconds. By contrast, prothrombin time, partial thromboplastin time, and bleeding time are reports of how long it takes for a clot to form, so they’re expressed in seconds or minutes.

This item tests understanding that some coagulation results are expressed as a time, while others are standardized as a ratio. The International Normalized Ratio is a unitless value because it’s calculated by comparing a patient’s prothrombin time to a reference prothrombin time and adjusting for the sensitivity of the reagent (the ISI). In practice, INR = (PT_patient / PT_normal) raised to the ISI, which produces a dimensionless number that can be compared across different labs and reagents. This standardization is essential for monitoring warfarin therapy, where consistent results matter more than raw seconds.

By contrast, prothrombin time, partial thromboplastin time, and bleeding time are reports of how long it takes for a clot to form, so they’re expressed in seconds or minutes.