Blood Flow and Veins

What is Prothrombin Time (PT)? How Quickly Blood Clots?

Last updated on October 20th, 2022 at 07:38 am

Prothrombin time is used to determine how quickly blood clots are. PT INR is the International Normalized ratio (INR) and a standard indicator of blood clotting function. A Prothrombin Time or PT INR test is a lab test that measures the time it takes for a clot to form. It’s an important measure for people on blood thinners or who are at risk.

Prothrombin Time (PT) definition

A prothrombin time (PT) test measures the time it takes for your blood plasma to clot. Prothrombin, also known as factor II, is just one of many plasma proteins involved in the clotting process.

What prothrombin time test do?

When you get a cut and your blood vessel ruptures, blood platelets collect at the site of the wound. They create a temporary plug to stop the bleeding. In order to produce a strong blood clot, a series of 12 plasma proteins, or coagulation “factors,” act together to make a substance called fibrin, which seals the wound.

A bleeding disorder known as haemophilia could cause your body to create certain coagulation factors incorrectly, or not at all. Some medications, liver disease, or vitamin K deficiency may also cause abnormal clot formation.

6 Symptoms of a bleeding disorder

  1. easy bruising
  2. bleeding that won’t stop, even after applying pressure to the wound
  3. heavy menstrual periods
  4. blood in the urine
  5. swollen or painful joints
  6. nosebleeds

What causes bleeding disorder?

If your doctor suspects you have a bleeding disorder, they may order a PT test to help them make a diagnosis. Even if you have no symptoms of a bleeding disorder, your doctor may order a PT test to make sure your blood is clotting normally before you undergo major surgery.

If you’re taking the blood-thinning medication warfarin, your doctor will order regular PT tests to ensure you’re not taking too much medication. Taking too much warfarin can cause excessive bleeding.

Liver disease or vitamin K deficiency can cause a bleeding disorder. Your doctor may order a PT to check how your blood clots are if you have one of these conditions.

What do the PT test results mean?

Blood plasma normally takes between 11 and 13.5 seconds to clot if you’re not taking blood-thinning medication. PT results often are reported as an international normalized ratio (INR) that’s expressed as a number. A typical range for a person not taking blood thinner medication is 0.9 to about 1.1. For someone taking warfarin, the planned INR is usually between 2 and 3.5.

If your blood clots within the normal time, you probably don’t have a bleeding disorder. If you are taking a blood thinner, a clot will take longer to form.

Your doctor will determine your goal of clotting time.

If your blood doesn’t clot in the normal time, you may:

  • be on the wrong dose of warfarin
  • have liver disease
  • have vitamin K deficiency
  • have a bleeding disorder, such as factor II deficiency

If you have a bleeding disorder, your doctor may recommend factor replacement therapy or a transfusion of blood platelets or fresh frozen plasma.

What is a partial thromboplastin time (PTT) test?

A partial thromboplastin time (PTT) test is a blood test that helps doctors assess your body’s ability to form blood clots.

Bleeding triggers a series of reactions known as the coagulation cascade. Coagulation is the process your body uses to stop bleeding. Cells called platelets create a plug to cover the damaged tissue. Then your body’s clotting factors interact to form a blood clot. Low levels of clotting factors can prevent a clot from forming. A deficiency in clotting factors can lead to symptoms such as excessive bleeding, persistent nosebleeds, and easy bruising.

Why do I need a PTT test?

Your doctor may order a PTT test to investigate the cause of prolonged or excessive bleeding.

Symptoms to do prothrombin time test
  • frequent or heavy nosebleeds
  • heavy or prolonged menstrual periods
  • blood in the urine
  • swollen and painful joints (caused by bleeding into your joint spaces)
  • easy bruising

The PTT test can’t diagnose a specific condition. But it helps your doctor learn whether your blood clotting factors are deficient. If your test results are abnormal, your doctor will probably need to order more tests to see which factor your body isn’t producing.

Your doctor might also use this test to monitor your condition when you take the blood thinner heparin.

Medicines which affect the prothrombin time test result

Several medications can affect the results of a PTT test. These include:

  • heparin
  • warfarin
  • aspirin
  • antihistamines
  • vitamin C
  • chlorpromazine

Make sure you tell your doctor about all the medications you take. You may need to stop taking them before the test.

What are normal values of PTT test result?

PTT test results are measured in seconds. Normal results are typically 25 to 35 seconds. This means that it took your blood sample 25 to 35 seconds to clot after adding the chemicals.

The exact standards for normal results may vary depending on your doctor and lab, so ask your doctor if you have any concerns.

Abnormal PTT test results

Remember that an abnormal PTT result doesn’t diagnose any disease. It only provides insight into the time it takes for your blood to clot. Multiple diseases and conditions can cause abnormal PTT results.

A prolonged PTT result may be due to
  • reproductive conditions, such as recent pregnancy, current pregnancy, or recent miscarriage
  • haemophilia A or B
  • deficiency of blood clotting factors
  • Von Willebrand disease(a disorder that causes abnormal blood clotting)
  • disseminated intravascular coagulation (a disease in which the proteins responsible for blood clotting are abnormally active)
  • hypofibrinogenemia (deficiency of the blood clotting factor fibrinogen)
  • certain medications, such as the blood thinners heparin and warfarin
  • nutritional issues, such as vitamin K deficiency and malabsorption
  • antibodies, including cardiolipin antibodies
  • lupus anticoagulants
  • leukaemia
  • liver disease

The wide range of probable causes for abnormal results means that this test alone is not enough to determine what condition you have. An abnormal result will probably prompt your doctor to order more tests.

What is activated partial thromboplastin time (aPTT) test?

The aPTT is one of several blood coagulation tests. It measures how long it takes your blood to form a clot.

Normally, when one of your blood vessels is damaged, proteins in your blood called clotting factors to come together in a certain order to form blood clots and quickly stop bleeding. The aPTT test can look at how well those clotting factors are working. It’s often used with other tests that monitor clotting factors.

Together with PT, PTT and aPTT, give your doctor a more complete picture of what happens in your body when a clot forms.

Introduction to the Pathology Laboratory testing your PT INR level

PTT is ‘partial thromboplastin time’, and PT is ‘prothrombin time.’ Both PTT and PT are tests used for measuring the time taken for the blood to clot. These two tests are mainly conducted to check for bleeding problems or the chances of excessive bleeding during surgery.


The prothrombin time (PT) is the time it takes blood to clot after the addition of tissue factor. The PT measures some of the blood clotting factors. It is used in the management of clotting disorders. (PT INR)


The international normalized ratio (INR) is a calculation based on the results of a PT. (PT INR)


The partial thromboplastin time (PTT) evaluates different coagulation factors than PT does. It is often used in the management of bleeding disorders.

By evaluating the results of the PT, INR, and PTT together, your doctor can gain clues about what bleeding or clotting disorder may be present.

Note: order none of above test for self-medication. Your doctor can only assess the need for the PT, PTT, aPTT tests.

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