A partial thromboplastin time (PTT) test measures how long it takes for a clot to form in a blood sample. A clot is a thick lump of blood that the body produces to seal leaks, wounds, cuts, and scratches and prevent excessive bleeding.
The blood's ability to clot involves platelets (also called thrombocytes) and proteins called clotting factors. Platelets are oval-shaped cells made in the bone marrow. Most clotting factors are made in the liver.
When a blood vessel breaks, platelets are first to the area to help seal off the leak and temporarily stop or slow the bleeding. But for the clot to become strong and stable, the action of clotting factors is required.
The body's clotting factors are numbered using the Roman numerals I through XII. They work together in a specialized sequence, almost like pieces of a puzzle. When the last piece is in place, the clot develops — but if even one piece is missing or defective, the clot can't form.
The PTT test is used to evaluate the ability of a person's blood to clot. If it takes an abnormally long time for the blood to clot, it can indicate a problem with one or more of several different clotting factors. This may be a sign of:
a missing, deficient, or defective clotting factor or factors
liver disease (because many clotting factors are made in the liver)
treatment with heparin, a blood-thinning medication
Why It's Done
Doctors may order the PTT test as part of an evaluation for a bleeding disorder such as hemophilia or von Willebrand disease. Symptoms of a bleeding disorder can include easy bruising, nosebleeds that won't stop, excessive bleeding after dental procedures, gums bleeding easily, heavy menstrual periods, blood in the urine, or swollen or painful joints.
Even in the absence of symptoms, doctors may use the test to ensure that clotting ability is normal before a patient undergoes a major procedure such as surgery.
The PTT test is especially useful in monitoring the effects of the blood-thinning medication heparin. Blood thinners are frequently given to prevent clots in patients who've had a heart attack or stroke, or who have an artificial heart valve. Because dosing is critical — enough medication must be given to prevent dangerous clots, but not so much so as to cause excessive bleeding — close monitoring is necessary.
In many cases, the PTT test is performed with a prothrombin time (PT) test to give doctors a more complete picture of clotting factor function.
No special preparations are needed for this test. If your child takes blood-thinning medication, antihistamines, or aspirin, you should tell the doctor because these can affect test results.
On the day of the test, having your child wear a T-shirt or short-sleeved shirt can make things easier for the technician drawing the blood.
A health professional will clean the skin surface with antiseptic, and place an elastic band (tourniquet) around the upper arm to apply pressure and cause the veins to swell with blood. Then a needle is inserted into a vein (usually in the arm inside of the elbow or on the back of the hand) and blood is withdrawn and collected in a vial or syringe.
After the procedure, the elastic band is removed. Once the blood has been collected, the needle is removed and the area is covered with cotton or a bandage to stop the bleeding. Collecting the blood for the test will only take a few minutes.
What to Expect
Collecting a sample of blood is only temporarily uncomfortable and can feel like a quick pinprick. Afterward, there may be some mild bruising, which should go away in a day or so.
Partial thromboplastin time is measured in seconds. PTT test results are compared with the average clotting time of healthy people.
The time is longer in people who take blood thinners. The blood sample will be processed by a machine and the results are usually available after a few hours or the next day.
The PTT test is considered a safe procedure. However, as with many medical tests, some problems can occur with having blood drawn. These include:
fainting or feeling lightheaded
hematoma (blood accumulating under the skin causing a lump or bruise)
pain associated with multiple punctures to locate a vein
Helping Your Child
Having a blood test is relatively painless. Still, many kids are afraid of needles. Explaining the test in terms your child can understand might help ease some of the fear.
Allow your child to ask the technician any questions he or she might have. Tell your child to try to relax and stay still during the procedure, as tensing muscles and moving can make it harder and more painful to draw blood. It also may help if your child looks away when the needle is being inserted into the skin.
If You Have Questions
If you have questions about the PTT test, speak with your doctor.