A partial thromboplastin time (PTT) test helps doctors assess your body’s ability to form blood clots.
Each time you bleed, it triggers a series of reactions known as the “coagulation cascade.” Coagulation is the process your body uses to stop blood loss. Cells called platelets create a plug to cover the damaged tissue and then your body’s 13 clotting factors interact to produce a blood clot. Low levels of clotting factors can prevent a clot from forming. Factor deficiency can lead to symptoms such as excessive bleeding, persistent nosebleeds, and easy bruising.
For this test, a sample of your blood is collected in a vial. Chemicals are then added to the blood. The test results are based on how many seconds it takes for your blood to clot after the chemicals are added.
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. Instead, it helps your doctor figure out whether your blood clotting factors may be deficient. If your test results are abnormal, your doctor will probably need to order more tests to figure out which factor your body is not producing.
Your doctor might also use this test to monitor your condition if you are taking 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 are taking because you may need to stop taking them before the test.
To perform the test, your doctor will need to take a sample of blood from your arm. First, the site will be cleaned with an alcohol swab. Then, the doctor will insert a needle into your vein and attach a tube to the needle to collect blood. When enough blood has been collected, the needle will be removed and the site covered with a gauze pad.
Your doctor will then add chemicals to this blood sample and measure the number of seconds it takes for the sample to clot.
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 the chemicals were added.
The exact standards for normal results may vary depending on your doctor and lab, so be sure to ask your doctor if you have any concerns.
Remember that an abnormal PTT result does not 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 high PTT result may be caused by:
- reproductive conditions, such as:
- recent pregnancy
- current pregnancy
- recent miscarriage
- blood clotting problems, such as:
- hemophilia A or B
- deficiency of blood clotting factors
- von Willebrand’s 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
- antibodies, including:
- cardiolipin antibodies
- lupus anticoagulants
- diseases, such as:
- liver disease
The wide range of possible causes for abnormal results means that this test alone is not enough to tell your doctor what condition you have. An abnormal result will probably prompt your doctor to order additional tests.
As with any blood test, there is a slight risk of bruising, bleeding, or infection at the puncture site. In rare cases, the vein may become swollen after blood is drawn. This condition, known as phlebitis, can be treated by applying a warm compress several times a day.
Ongoing bleeding could be a problem if you suffer from a bleeding disorder or are taking blood-thinning medication, such as warfarin or aspirin.