From the Guidelines
Yes, Prothrombin Time (PT) is different from International Normalized Ratio (INR).
Key Differences
- PT is measured in seconds and depends on the thromboplastin reagent and measurement technique used, whereas INR is a calculated value that aims to standardize PT results across different laboratories and techniques 1.
- The INR is calculated using a mathematical formula that takes into account the PT, the international sensitivity index (ISI) of the thromboplastin reagent, and the mean normal prothrombin time (MNPT) 1.
Clinical Significance
- The use of INR allows for more accurate monitoring of patients on vitamin K antagonist therapy, as it provides a standardized measure of anticoagulation across different laboratories and techniques 1.
- The historical development of INR targets has been influenced by the need for standardization and the discovery of vitamin K antagonists, but the use of INR targets in clinical practice has been subject to varying interpretations and debates 1.
Measurement and Calculation
- The PT is typically measured using a tissue factor-induced coagulation time assay, and the result is then converted to an INR using the ISI and MNPT values 1.
- The ISI value is assigned to each batch of thromboplastin reagent, and it reflects the reagent's responsiveness to changes in coagulation factor levels 1.
From the Research
Prothrombin Time (PT) and International Normalized Ratio (INR)
- Prothrombin Time (PT) and International Normalized Ratio (INR) are related but distinct measures used in coagulation testing 2, 3.
- PT represents the time it takes for blood to clot, while INR is a calculated value that standardizes PT results for different thromboplastin reagents and instruments 2, 4.
- The INR is calculated using the mean normal PT (MNPT) and the international sensitivity index (ISI) to provide a standardized result for monitoring anticoagulant therapy with vitamin K antagonists such as warfarin 2, 5.
Differences between PT and INR
- PT results can vary between laboratories due to differences in thromboplastin reagents and instruments, while INR is intended to provide a standardized result 2, 4.
- However, even with the use of INR, results can still vary between laboratories due to differences in ISI values and MNPT 2, 5.
- The use of INR is not universally applicable and may not be suitable for all clinical settings, such as liver failure 4.
Clinical Applications
- PT and INR are commonly used to monitor anticoagulant therapy with vitamin K antagonists such as warfarin 2, 3.
- INR is also used to monitor patients with liver failure, although its use in this setting has been questioned due to variability in results 4.
- Portable coagulation monitors, such as the CoaguChek Pro, can measure PT and INR using capillary whole blood, but results may not always agree with standard laboratory methods 6.