**Question:** Prothrombin time (PT) of a patient is 26 seconds, control PT is 13 seconds, and sensitivity index is 1.41. What will be the INR of this patient?

**Core Concept:**
The prothrombin time (PT) is an international normalized ratio (INR) test that measures the clotting time of a patient's blood in comparison to a control sample. The sensitivity index (SI) is a factor used to adjust the PT values to obtain the INR, which is a standardized measure of bleeding risk. The INR is calculated using the formula: INR = (PT-control PT) / control PT x sensitivity index.

**Why the Correct Answer is Right:**
The INR is calculated by subtracting the patient's PT from the control PT, then dividing the result by the control PT and multiplying by the sensitivity index. In this case, the patient's PT is 26 seconds, control PT is 13 seconds, and the sensitivity index is 1.41. Applying the formula:

INR = [(26-13) / 13] x 1.41
INR = 1.67 x 1.41
INR ≈ 2.2

**Why Each Wrong Option is Incorrect:**
A. 26 seconds PT divided by 13 seconds control PT multiplied by 1.41 is not the correct calculation for determining the INR.
B. The INR calculation involves PT, control PT, and sensitivity index, but this option omits the sensitivity index, which is essential for accurate INR determination.
C. The calculation process is similar to option A, but the specific values used are different, leading to an incorrect INR result.
D. The calculation process is similar to option A, but the specific values used are different, leading to an incorrect INR result.

**Clinical Pearl:**
The international normalized ratio (INR) provides a standardized measure of bleeding risk and is often used in anticoagulation monitoring for patients on warfarin therapy. A higher INR indicates a higher risk of bleeding. A value of INR between 2.0 and 3.0 is considered therapeutic for most patients on warfarin therapy.