How to Calculate Protime

Health care professionals measure the time it takes for blood to clot using patient blood samples.

Protime -- a nickname for prothrombin time -- measures the blood's clotting tendency. This is critical for individuals taking anticoagulants to treat blood clots and other disorders. Some people undergo regular blood tests and wait for results, while others use take-home kits. Specifically, protime tells doctors and patients how long it takes the blood to clot, compared to average. This result is known as the International Normalized Ratio (INR). Blood that clots too quickly is at risk of developing clots for no reason, while blood that clots too slowly puts the individual at risk of internal bleeding.

Things You'll Need

• Test tubes
• Plasma sample
• 0.2 ml thromboplastin, with chloride calcium
• Stopwatch

Instructions

1. Warm the plasma sample to 98.6 degrees Fahrenheit for two minutes. If possible, use two samples to check for accuracy. Also, warm 0.2 ml of the clotting reagent thromboplastin with chloride calcium to the same temperature. If the thromboplastin does not already contain calcium, add 25 mM of calcium chloride to it.

2. With your stopwatch handy, add the thromboplastin -- also known as Factor II -- and calcium chloride to the plasma inside the test tube. This initiates clot formation.

3. Start your stopwatch and time how long it takes for a fibrin clot to form in the plasma sample. According to MedlinePlus, the normal range for clotting is 11 to 13.5 seconds. A clot that takes longer than average to form indicates that the blood lacks one or more clotting factors, such as prothrombin, fibrinogen, Factor X, VII or V. It may also be a symptom of a vitamin K deficiency or problems like hepatitis, liver disease or bile duct obstruction.

Tips & Warnings

• If performing more than one protime test to ensure accuracy, perform them one at a time. This keeps the results clear.

• To calculate INR, divide the patient's blood clotting time by the average. For instance, 26 seconds over the average 13 seconds gives a patient an INR of 2.

• Keep in mind test results are not always definitive. Different methods of determining coagulation have led to widely different results, according to Tampere University Hospital research published in a 2005 edition of "Clinical Chemistry."

References

• University of Maryland Medical Center: Prothrombin Time (PT) - Overview
• World Federation of Hemophilia: Prothrombin Time (PT)
• "Clinical Chemistry": Poor Agreement Among Prothrombin Time International Normalized Ratio Methods: Comparison of Seven Commercial Reagents
• "Understanding Laboratory Investigations for Nurses and Health Professionals"; Chris Higgins; 2007
• Medline Plus: Prothrombin Time (PT)
• "District Laboratory Practice in Tropical Countries, Part 2"; Monica Cheesbrough; 2006

Resources

• University of Virginia Health System: Reference Range for Prothrombin Time - Coumadin