Platelets (also known as thrombocytes) are small, round cell fragments that are vital for normal blood clotting. Platelet function tests indirectly evaluate how well a person's platelets work in helping to stop bleeding within the body.

Platelets are produced in the bone marrow and circulate in the blood. When there is an injury to a blood vessel and bleeding begins, platelets help to stop bleeding in three ways. They:

• Adhere to the injury site
• Clump together (aggregate) with other platelets
• Release compounds that stimulate further aggregation

These reactions result in the formation of a loose platelet plug in a process called primary hemostasis. At the same time, activated platelets support the coagulation cascade, a series of steps that involves the sequential activation of proteins called clotting factors. This is called secondary hemostasis and the two processes result in the formation of a stable clot that remains in place until the injury has healed.

If there are insufficient platelets or if they are not functioning normally in any of the three main ways, a stable clot may not form and a person may be at an increased risk of excessive bleeding. The number of platelets in blood can be determined with a platelet count and can help diagnose disorders having to do with too many or too few platelets. However, the overall ability of platelets to function properly in the body is more difficult to measure.

Platelet function tests are a group of assays that use specialized equipment to measure the ability of platelets to aggregate and promote clotting in a sample of blood. There are a variety of tests available but no one test that identifies all problems with platelet function. Also, there is no widespread agreement on which test(s) is best for each circumstance.

In addition to evaluating people for excessive bleeding, platelet function tests may be used in other situations. There are situations in which it is desirable to decrease the ability of platelets to aggregate, as in for people who are at an increased risk of developing a dangerous blood clot or at increased risk for heart attacks. These people may be prescribed medications that reduce platelet activation or reduce their ability to aggregate. People on these types of anti-platelet medications, such as low-dose aspirin or clopidogrel, may have platelet function tests done as a way of monitoring their treatment. However, there is currently no consensus among medical experts on the usefulness of platelet function tests in anti-platelet therapy.

Screening Tests

Closure time assays
This test measures the time required for the platelets in a sample of blood to plug a small hole in a tiny tube after being exposed to various activating substances. This is called the closure time. Prolonged closure times indicate lower platelet function but do not identify the cause. This test may be abnormal if the platelet count is low, if platelet function is reduced, if other proteins needed for platelet function are reduced, or if anti-platelet medications are present. This type of assay can be used to screen for von Willebrand disease and some platelet function disorders, but it will not detect all platelet function disorders, particularly the milder forms. This test is relatively simple to perform and is available in many health care facilities.

Viscoelastometry (or Thromboelastometry)
Blood clots have to be strong to stop bleeding and prevent new bleeding until healing can occur. This type of testing is designed to determine the strength of a blood clot as it forms. It is most often performed in larger hospitals, either in the operating room as a point-of-care test or in the clinical laboratory.

Endpoint bead or endpoint platelet aggregation assays
These assays determine the number of coated beads or platelets that aggregate after substances are added to activate platelets in a sample of blood. They provide a single measure of aggregation (an endpoint) rather than a measure of aggregation over time. More platelets aggregating or sticking to beads indicates better platelet function. These tests may be abnormal if the platelet count is low, if platelet function is reduced, or if anti-platelet medications are present.

Bleeding time
In the past, the primary screen for platelet dysfunction was the bleeding time. This is the only test that directly measures platelet function within the body. It involves making two small, shallow, standardized cuts on the inner forearm and measuring the amount of time for bleeding to stop. The bleeding time procedure has fallen from favor in recent years. Many hospitals no longer offer it, and several national organizations have issued position statements against its routine use. The bleeding time is not sensitive or specific , and it does not necessarily reflect the risk or severity of surgical bleeding. It is poorly reproducible, can be affected by aspirin ingestion and by the skill of the person performing the test, and frequently leaves small, thin scars on the forearm.

Diagnostic Tests

Platelet aggregometry
Many different substances can activate a platelet, including proteins in the wound, factors released from other activated platelets, and factors produced by the coagulation system that aids platelets in forming a strong plug to stop bleeding. Many different platelet abnormalities have been described due to problems with one or more of these activating systems. Platelet aggregometry consists of 4 to 8 separate tests. In each test, a different platelet activating substance is added to blood, followed by measurement of platelet aggregation over several minutes. When complete, a physician reviews and interprets the entire panel of tests to determine if there is any evidence of abnormal platelet function. Platelet aggregation testing can diagnose a variety of inherited and acquired platelet function disorders. It is typically performed at academic medical centers or large hospitals due to the complexity of the testing and interpretation.

Flow cytometry
Platelets can be evaluated for functional defects using flow cytometry. This test uses lasers to determine proteins that are present on the platelet surface and how they change when the platelet is activated. Platelet flow cytometry is a highly specialized procedure available only in few reference laboratories to diagnose inherited platelet function disorders.

A blood sample is drawn though a needle from a vein in the arm.

In general, no test preparation is needed. However, you may be instructed to refrain from taking drugs that can affect the results of these tests, such as aspirin, non-steroidal anti-inflammatory drugs (NSAIDs), or any over-the-counter medications that contain drugs such as these. The most common NSAIDs include ibuprofen, naproxen, and COX-2 inhibitors. (See MedlinePlus Drugs & Supplements for more information on drugs, drug ingredients, and brand names.) However, do not stop taking your medications unless instructed to do so by your health care provider.