What is Prothrombin Time?

Mary McMahon
Mary McMahon
Mary McMahon
Mary McMahon
In a prothrombin time test, a technician spins a blood sample to separate out the plasma before adding the thromboplastin and calcium.
In a prothrombin time test, a technician spins a blood sample to separate out the plasma before adding the thromboplastin and calcium.
In a prothrombin time test, a technician spins a blood sample to separate out the plasma before adding the thromboplastin and calcium.

Prothrombin time is a measurement indicating how long it takes for a patient's blood plasma to clot. A laboratory technician can analyze a sample to determine prothrombin time, or a patient can use a kit to do it at home. Other tests to measure clotting may be part of a diagnostic workup to get a complete picture of a patient's overall health and to identify specific issues of concern with his blood. Many clinics and hospitals can do this test in their own lab to rapidly provide results.

Prothrombin time is the time it takes a blood clot to form.
Prothrombin time is the time it takes a blood clot to form.
Prothrombin time is the time it takes a blood clot to form.

In the prothrombin time test, the technician spins a blood sample to separate out plasma before adding calcium and thromboplastin. The technician sets a timer and waits for the blood to clot. In healthy people, it takes around 11 to 13 seconds. Many labs use a measurement called the international normalized ratio, comparing the lab's results to a control sample, to express prothrombin time values. Normal values fall between 1.0 and 1.4. Using a ratio allows doctors to interpret readings from different labs or different batches of thromboplastin consistently.

One reason to request this lab test is to check on a patient's response to anticoagulant medications. A patient may take these drugs to bust clots or address unusually thick, rapidly clotting blood. While on anticoagulants, people need regular blood tests to check their prothrombin time. Especially thin blood can be a cause for concern, as it may predispose the patient to serious risks of internal bleeding. It can take several days for medication or dosage changes to take effect and a doctor may monitor a patient closely until the blood clots consistently.

Before surgery, a prothrombin time test can be part of the presurgical evaluation to make sure the patient is a good candidate. If the patient's blood is unusually thin, she could experience excessive bleeding during surgery. It may be necessary to proceed anyway, but special care is necessary to protect the patient's health. For elective procedures, the surgeon may want to wait until the patient's blood clots more reliably.

In patients with suspected bleeding disorders, this test can be part of the diagnostic workup. Unusually fast or slow clotting rates are a sign of a problem with the blood. Other tests may reveal the source of the problem, or offer clues a doctor can use to pursue the cause. As the patient receives treatment, the doctor can order periodic blood screening to see if the patient is responding and to identify potential complications or dangerous side effects.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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    • In a prothrombin time test, a technician spins a blood sample to separate out the plasma before adding the thromboplastin and calcium.
      In a prothrombin time test, a technician spins a blood sample to separate out the plasma before adding the thromboplastin and calcium.
    • Prothrombin time is the time it takes a blood clot to form.
      Prothrombin time is the time it takes a blood clot to form.