What are platelet tests?

Platelets, also known as thrombocytes, are small blood cells. They form in your bone marrow, the sponge-like tissue in your bones. Platelets are essential for blood clotting. A blood clot is a mass of blood that forms when platelets, proteins, and cells in the blood stick together. When you get hurt, your body forms a blood clot to stop the bleeding.

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 may include one or more of the following:

• 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 your bleeding.
• Platelet aggregometry. This group of tests is 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 test uses lasers to look for proteins on the surface of platelets. It can help diagnose inherited platelet disorders. This specialized test is only available at certain hospitals and laboratories.
• Bleeding time. This test measures how long it takes for bleeding to stop after small cuts are made in your forearm. It was once commonly used to screen for various platelet disorders but is not used much anymore.

Other names: platelet count, PLT, PLT count thrombocyte count, platelet function tests, PFT, platelet function assay, PFA, 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 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 coronary artery bypass surgery 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 lower-than-normal or higher-than-normal platelets.

Symptoms of lower-than-normal platelets include:

• Bleeding for a long time after a minor cut or injury.
• Nosebleeds.
• Unexplained bruising.
• Pinpoint sized red spots on the skin, known as petechiae.
• Purplish, red, or brown spots on your skin, known as purpura. These may be caused by bleeding under the skin.
• Menstrual periods that have a heavy flow or last a long time.
• Blood in your urine, stool (poop), or from your rectum (where stool passes out of your body).

Symptoms of higher-than-normal platelets include:

• Numbness of hands and feet
• Headache
• Dizziness
• Weakness
• Pain, swelling, and warmth in your lower legs

You may also need platelet function testing if you are:

• Having a complicated surgery
• Taking medicines to lower blood 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. Tell your provider about everything you take. But don't stop taking any medicines unless your provider tells you to. They 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 be a sign of:

• 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 type of disease that causes your body to attack its own healthy tissues, which can include platelets.
• Infection or damage to your bone marrow.
• Cirrhosis.
• Vitamin B12 deficiency (not enough).
• Gestational thrombocytopenia, a common, but mild, low-platelet condition affecting pregnant people. It is not known to cause any harm to a pregnant person or their developing 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 be a sign of:

• Certain types of cancer, such as lung cancer or breast cancer
• Anemia
• Inflammatory bowel disease, such as Crohn's disease and ulcerative colitis
• Rheumatoid arthritis
• A viral or bacterial infection
• Severe blood loss

If your platelet function test results were not normal, it may be a sign of an inherited or acquired platelet disorder:

• Inherited disorders are passed down from your family. These 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 environmental exposure. 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 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

Your provider may consider your symptoms, medical history, and the results of other blood tests to understand the results of your platelet tests.

Learn more about laboratory tests, reference ranges, and understanding results.

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 your blood to clot
• Prothrombin time and INR test, which measures how many seconds it takes for a clot to form in your blood sample

If you are taking anti-platelet medicine, like aspirin, your results show how well your medicine is working.