Calculating the International Normalized Ratio (INR)
The correct INR calculation for a patient with PT of 18.5 seconds, mean normal PT of 11.5 seconds, and ISI of 1.21 is 1.6 (option B).
Calculation Method
The INR is calculated using the following formula as defined by the World Health Organization (WHO) guidelines 1:
INR = (PT patient / PT normal)^ISI
Where:
- PT patient = Patient's prothrombin time (18.5 seconds)
- PT normal = Mean normal prothrombin time (MNPT) (11.5 seconds)
- ISI = International Sensitivity Index (1.21)
Substituting the values: INR = (18.5 / 11.5)^1.21 INR = (1.608)^1.21 INR = 1.6
Understanding the INR Calculation Components
Mean Normal Prothrombin Time (MNPT)
- The MNPT is defined as the geometric mean of prothrombin times from the healthy adult population 1
- For practical purposes, it can be calculated from at least 20 fresh samples from healthy individuals of both sexes 1
- In this case, the MNPT is provided as 11.5 seconds
International Sensitivity Index (ISI)
- The ISI is a measure of the sensitivity of a thromboplastin reagent relative to an international standard 1
- It's used to standardize PT results across different laboratories and reagents 1
- The ISI value of 1.21 indicates the relative sensitivity of the thromboplastin reagent being used
Clinical Significance
- The normal INR range is 0.8-1.2 for individuals not on anticoagulation therapy 2
- An INR of 1.6 indicates mild coagulation abnormality but is below the therapeutic range (2.0-3.0) typically targeted for patients on vitamin K antagonists 2
- This value may represent:
- Early/inadequate anticoagulation if the patient is on warfarin
- Mild coagulopathy from other causes (liver disease, vitamin K deficiency)
- Need for dose adjustment if therapeutic anticoagulation is the goal
Common Pitfalls in INR Calculation
Using arithmetic mean instead of geometric mean for MNPT calculation - This can lead to inaccurate INR values 1
Incorrect application of the ISI exponent - The ISI must be applied as an exponent to the PT ratio, not multiplied 1
Using inappropriate ISI values - Each thromboplastin reagent/instrument combination should have a specific ISI value 1, 3
Not accounting for special populations - The standard INR calculation may not be appropriate for patients with lupus anticoagulant or liver disease 1, 2
The INR calculation is a critical component of monitoring anticoagulation therapy, particularly for patients on vitamin K antagonists, and accurate calculation is essential for proper dosing and patient safety.