Blood Test: Prothrombin Time (PT)
What It Is
A prothrombin time (PT) 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 is a complex process involving 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 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 PT test can be used to evaluate the action of five different clotting factors (I, II, V, VII, and X). If any factor is missing, deficient, or defective, clotting time may be delayed. Blood that takes too long to clot in a PT test may be a sign of:
- a hereditary deficiency in clotting factors (such as in the bleeding disorders hemophilia and von Willebrand disease)
- liver disease (because most clotting factors are made in the liver)
- vitamin K deficiency (because vitamin K is a necessary component of several clotting factors)
- therapy using warfarin, a blood-thinning medication
- certain medical conditions in which the body uses up or destroys clotting factors too rapidly
Why It's Done
Doctors may order a PT test as part of an evaluation for a bleeding disorder. Symptoms 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 often use the PT test to ensure clotting ability is normal before a patient undergoes a major procedure such as surgery.
The test also may be ordered to monitor the clotting ability of people with liver disease or vitamin K deficiency.
Prothrombin time is also useful in monitoring the effects of the blood-thinning medication warfarin. 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 — careful monitoring is necessary.
In many cases, the PT test is performed with the partial thromboplastin time (PTT) test to give doctors a more complete picture of clotting factor function.
No special preparations are needed for the PT test. If your child takes a daily dose of blood-thinning medication, ask the doctor if the timing of the dose needs to be changed in preparation for the test.
On the day of the test, it may help to have your child wear a T-shirt or short-sleeved shirt to allow easier access for the technician who will be drawing the blood.
A health professional will usually draw the blood from a vein. The skin surface is cleaned with antiseptic and an elastic band (tourniquet) is placed around the upper arm to apply pressure and cause the veins to swell with blood. A needle is inserted into a vein (usually in the arm inside the elbow or on the back of the hand) and blood is withdrawn and collected in a vial or syringe. A chemical in the vial keeps the blood from clotting before the test begins.
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 blood for this test will only take a few minutes.
What to Expect
Collecting a blood sample 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.
Getting the Results
Prothrombin time is measured in seconds. The results are compared with the average clotting time of healthy people.
This time will be 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 PT 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 children are afraid of needles. Explaining the test in terms your child can understand might help ease any 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 for your child to look away when the needle is being inserted into the skin.
If You Have Questions
If you have questions about the PT test, speak with your doctor.
- Blood Test: von Willebrand Factor (vWF) Antigen
- Blood Test: Partial Thromboplastin Time (PTT)
- von Willebrand Disease
- Blood Test: Factor VIII Activity
Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor.
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