Platelets, also known as thrombocytes, are small blood cells that are essential for blood clotting. Clotting is the process that helps you stop bleeding after an injury. There are two types of platelet tests: a platelet count test and platelet function tests.
A platelet count test measures the number of platelets in your blood. A lower than normal platelet count is called thrombocytopenia. This condition can cause you to bleed too much after a cut or other injury that causes bleeding. A higher than normal platelet count is called thrombocytosis. This can make your blood clot more than you need it to. Blood clots can be dangerous because they can block blood flow.
Platelet function tests check your platelets’ ability to form clots. Platelet function tests include:
- Closure time. This test measures the time it takes for platelets in a blood sample to plug a small hole in a tiny tube. It helps screen for different platelet disorders.
- Viscoelastometry. This test measures the strength of a blood clot as it forms. A blood clot has to be strong to stop bleeding.
- Platelet aggregometry. This is a group of tests that are used to measure how well platelets clump together (aggregate).
- Lumiaggregometry. This test measures the amount of light produced when certain substances are added to a blood sample. It can help show if there are defects in the platelets.
- Flow cytometry. This is a test that uses lasers to look for proteins on the surface of platelets. It can help diagnose inherited platelet disorders. This is a specialized test. It is only available at certain hospitals and laboratories.
- Bleeding time. This test measures the amount of time for bleeding to stop after small cuts are made in the forearm. It was once commonly used to screen for a variety of platelet disorders. Now, other platelet function tests are used more often. The newer tests provide more reliable results.
Other names: platelet count, thrombocyte count, platelet function tests, platelet function assay, platelet aggregation studies
What are they used for?
A platelet count is most often used to monitor or diagnose conditions that cause too much bleeding or too much clotting. A platelet count may be included in a complete blood count, a test that is often done as part of a regular checkup.
Platelet function tests may be used to:
- Help diagnose certain platelet diseases
- Check platelet function during complex surgical procedures, such as cardiac bypass and trauma surgery. These types of procedures have an increased risk of bleeding.
- Check patients before surgery, if they have a personal or family history of bleeding disorders
- Monitor people who are taking blood thinners. These medicines may be given to reduce clotting in people at risk for heart attack or stroke.
Why do I need a platelet test?
You may need platelet count and/or platelet function testing if you have symptoms of having too few or too many platelets.
Symptoms of too few platelets include:
- Prolonged bleeding after a minor cut or injury
- Unexplained bruising
- Pinpoint sized red spots on the skin, known as petechiae
- Purplish spots on the skin, known as purpura. These may be caused by bleeding under the skin.
- Heavy and/or prolonged menstrual periods
Symptoms of too many platelets include:
- Numbness of hands and feet
You may also need platelet function testing if you are:
- Undergoing a complex surgery
- Taking medicines to reduce clotting
What happens during a platelet test?
Most platelet tests are done on a blood sample.
During the test, a health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.
Will I need to do anything to prepare for the test?
You don’t need any special preparations for a platelet count test
If you are getting a platelet function test, you may need to stop taking certain medicines, such as aspirin and ibuprofen, before your test. Your health care provider will let you know if there are any special instructions to follow.
Are there any risks to the test?
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
What do the results mean?
If your results show a lower than normal platelet count (thrombocytopenia), it may indicate:
- A cancer that affects the blood, such as leukemia or lymphoma
- A viral infection, such as mononucleosis, hepatitis, or measles
- An autoimmune disease. This is a disorder that causes the body to attack its own healthy tissues, which can include platelets.
- Infection or damage to the bone marrow
- Vitamin B12 deficiency
- Gestational thrombocytopenia, a common, but mild, low-platelet condition affecting pregnant women. It is not known to cause any harm to a mother or her unborn baby. It usually gets better on its own during pregnancy or after birth.
If your results show a higher than normal platelet count (thrombocytosis), it may indicate:
- Certain types of cancer, such as lung cancer or breast cancer
- Inflammatory bowel disease
- Rheumatoid arthritis
- A viral or bacterial infection
If your platelet function test results were not normal, it may mean you have an inherited or acquired platelet disorder. Inherited disorders are passed down from your family. The conditions are present at birth, but you may not have symptoms until you are older. Acquired disorders are not present at birth. They may be caused by other diseases, medicines, or exposure in the environment. Sometimes the cause is unknown.
Inherited platelet disorders include:
- Von Willebrand disease, a genetic disorder that reduces the production of platelets or causes the platelets to work less effectively. It can cause excess bleeding.
- Glanzmann’s thrombasthenia, a disorder that affects platelets’ ability to clump together
- Bernard-Soulier syndrome, another disorder that affects platelets’ ability to clump together
- Storage pool disease, a condition that affects platelets’ ability to release substances that help platelets clump together
Acquired platelet disorders may be due to chronic diseases such as:
- Kidney failure
- Certain types of leukemia
- Myelodysplastic syndrome (MDS), a disease of the bone marrow
Is there anything else I need to know about platelet function tests?
Platelet tests are sometimes done along with one or more of the following blood tests:
- MPV blood test, which measures the size of your platelets
- Partial thromboplastin time (PTT) test, which measures the time it takes for blood to clot
- Prothrombin time and INR test, which checks the body’s ability to form blood clots