This blood test determines how quickly your blood clots. It measures how many seconds it takes to clot after substances (reagents) are added. It’s often used to help detect a bleeding disorder or clotting disorder and can help a doctor determine the cause, if present. It may sometimes be used before surgery or other invasive procedures to determine if excessive bleeding or clotting problems may be likely to occur.

For blood to clot, a certain sequence of activities called the coagulation cascade occurs. The clotting cascade depends on proteins called clotting factors, which activate each other in a step-by-step process. For blood to clot properly, there needs to be enough of each coagulation factor and they need to function properly. A problem with any of them can affect your body’s ability to form a clot.

The clotting cascade happens through two pathways— the intrinsic pathway and the extrinsic pathway. Each pathway involves certain factors, and while each pathway is separate, they interact with each other and finish in what’s called the common pathway.

This test includes:

• Prothrombin time (PT)—Evaluates how well the clotting factors involved in the extrinsic and common pathways of the coagulation cascade work together. These include factors I (Fibrinogen), II (Prothrombin), V, VII and X.
• International normalized ratio (INR)—A calculation from the PT, which adjusts for changes in PT reagents and allows for results from different labs to be compared.

Why Testing Is Important

Blood clotting, or coagulation, is a necessary process that prevents a person from losing too much blood if they're injured. However, sometimes clots form and can be dangerous, causing serious problems like deep vein thrombosis (DVT) or pulmonary (lung) embolism. By evaluating the time it takes your blood to clot via different methods, the PT tests help detect if you have a bleeding or clotting disorder. As a pre-surgical test, the PT with INR helps a doctor evaluate the likelihood of excessive bleeding or clotting to determine risks during or after surgery. This test is also clinically used to monitor individuals on a Warfarin (Coumadin®) anticoagulant (blood-thinner) therapy.

What Do Test Results Mean?

The prothrombin time test with international normalized ratio evaluates the ability to produce a clot within an appropriate time. If there aren't enough of any of the factors related to clotting or if they are not functioning properly, the test results will be prolonged. If you're taking warfarin, the INR target range depends on the condition that's being treated. Because PT results vary from lab to lab, the INR is calculated so results can be compared across labs.

You should talk to your doctor or hematologist about what your test results mean about your health. Prolonged clotting time may be due to a variety of causes, including underlying conditions such as liver disease, vitamin K deficiency, or coagulation factor deficiency. Taking anticoagulants slows down the process of clotting, causing prolonged PT results.

References

1. “Hemostasis: PT and PTT tests.” The McGill Physiology Virtual Lab. https://www.medicine.mcgill.ca/physio/vlab/bloodlab/PT_PTT.htm
2. “Mechanisms of Blood Coagulation.” Weber State University. http://departments.weber.edu/chpweb/hemophilia/mechanisms_of_blood_coagulation.htm
3. “How to Interpret and Pursue an Abnormal Prothrombin Time, Activated Partial Thromboplastin Time, and Bleeding Time in Adults.” Mayo Clinic Proceedings. https://www.mayoclinicproceedings.org/article/S0025-6196(11)61313-X/fulltext