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.
To test your body’s blood clotting abilities, the laboratory collects a sample of your blood in a vial and adds chemicals that will make your blood clot. The test measures how many seconds it takes for a clot to form.
This test is sometimes called an activated partial thromboplastin time (APTT) test.
Your doctor may order a PTT test to investigate the cause of prolonged or excessive bleeding. Symptoms that may prompt your doctor to order this test include:
- 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 does 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.
Several medications can affect the results of a PTT test. These include:
- vitamin C
Make sure you tell your doctor about all the medications you take. You may need to stop taking them before the test.
As with any blood test, there’s a slight risk of bruising, bleeding, or infection at the puncture site. In rare cases, your vein may become swollen after a blood draw. This condition is known as phlebitis. Applying a warm compress several times a day can treat phlebitis.
Ongoing bleeding could be a problem if you have a bleeding disorder or are taking blood-thinning medication, such as warfarin or aspirin.
To perform the test, the phlebotomist or nurse takes a sample of blood from your arm. They clean the site with an alcohol swab and insert a needle into your vein. A tube attached to the needle collects the blood.
After collecting enough blood, they remove the needle and cover the puncture site with a gauze pad.
The lab technician adds chemicals to this blood sample and measures the number of seconds it takes for the sample to clot.
Normal PTT test results
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 particular disease. It only provides insight about 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:
- hemophilia 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
- liver disease
The wide range of possible 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.