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Prothrombin Time (PT) Unit Converter
Prothrombin Time (PT) is a blood test that measures the time it takes for plasma to clot. It is the primary test used to monitor oral anticoagulant therapy (Warfarin) and to assess the extrinsic pathway of coagulation.
Because PT results vary depending on the laboratory reagent used, results are standardized using the International Normalized Ratio (INR). This converter facilitates switching between the raw clotting time (Seconds), the standardized ratio (INR), and the older activity percentage (Quick %).
Reporting Units
StandardCalculated using default MNPT = 12.0s and ISI = 1.0 for demonstration.
Activity / Percentage
Clinical Context
Reference Range (Adult):
• Seconds: 11.0 – 13.5 seconds (varies by lab).
• INR (Healthy): 0.8 – 1.1.
• INR (Warfarin Therapy): Target is typically 2.0 – 3.0.
Calculations Used:
INR = (Patient PT / MNPT)ISI
% Activity ≈ (MNPT / Patient PT) × 100
Note: This tool uses a standard Mean Normal PT (MNPT) of 12.0s and an ISI of 1.0. Clinical results require the specific ISI of the reagent batch.
Clinical References
How to Use This Converter
Follow these steps to convert PT values.
Enter Values
Input your laboratory result (e.g., Seconds).
Automatic Calculation
The tool calculates the corresponding INR and % Activity using standard reference values (MNPT=12, ISI=1).
Interpret
Higher seconds or INR means slower clotting (thinner blood). Lower % activity means slower clotting.
Reset
Use the Clear button to reset all fields.
Frequently Asked Questions
This tool uses a generic ISI of 1.0. Your lab uses a specific ISI for their reagent batch, which affects the INR calculation.
Mean Normal Prothrombin Time. It is the geometric mean of PT from at least 20 healthy individuals.