Platelet Function Tests
- 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, platelets support the coagulation cascade, a series of steps that involves the sequential activation of proteins called clotting factors. This secondary hemostasis process culminates in the formation of strands of fibrin that are woven through the loose platelet plug, cross-linked to form a fibrin net, and compressed to form a stable clot that remains in place until the injury has healed. When the clot is no longer needed, other factors break the clot down and remove it.
Each component of primary and secondary hemostasis must be present, activated at the right time, and functioning properly for adequate clotting. 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. The goal of these tests is to evaluate how well platelets are doing their job in the body. There are a variety of tests available, but no one test that identifies all problems with platelet function, nor is there widespread agreement on which test(s) are best for each circumstance.
In addition to evaluating people for excessive bleeding, platelet function tests may be used in other situations. There are conditions in which it is desirable to decrease the ability of platelets to aggregate, as in 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 medications may have platelet function tests done as a way of monitoring their treatment.
Closure time assays
In this test, blood is exposed to various substances that activate platelets. The blood is then drawn through a simulated wound, a small hole in a tiny tube that is coated with collagen, a protein that promotes platelet binding to wounds. In normal blood, activated platelets will bind to the coated hole, eventually plugging it. The time required to plug the hole is measured. This is called the closure time. The longer the closure time, the lower the platelet function. 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.
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. Substances are added to blood to start clotting while clot strength is being measured over time. Measurements are made of total clot strength, time to reach maximum strength, and loss of strength over time. These tests may be abnormal if the platelet count is low, if platelet function is reduced, or if anti-platelet medications are present. This type of testing 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. 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.
In the past, the primary screen for platelet dysfunction was the bleeding time – a test that involved making two small, shallow, standardized cuts on the inner forearm and measuring the amount of time that they took to stop bleeding. The bleeding time procedure has fallen from favor in recent years. Many hospitals are no longer offering 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.
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.
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.
How is the sample collected for testing?A blood sample is drawn though a needle from a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?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 the MedlinePlus Drugs and Supplements webpage for more information on drugs, drug ingredients, and brand names.)
How is it used?Various platelet function tests are used to evaluate the ability of platelets to clump together and begin to form a clot. They may be used for a variety of reasons. Examples of some of the situations they may be used include:
- To identify and help diagnose platelet dysfunction in those with a history of excessive bleeding. It is in this area that platelet function tests are of the most use. They can be used to screen for dysfunction and, along with other bleeding disorder tests such as platelet aggregometry, to help diagnose inherited and acquired platelet dysfunctions. Von Willebrand disease, for instance, is the most common inherited platelet dysfunction. It is associated with decreased production or dysfunction of Von Willebrand factor (VWF) and results in reduced platelet adherence to the injured blood vessel and increased blood loss.
- To monitor anti-platelet therapy given to some patients after a stroke or heart attack to help inhibit blood clotting. Currently, most anti-platelet therapies are not routinely monitored with platelet function testing. As more anti-platelet therapies are created, it is anticipated that additional methods will be developed to monitor them.
- To detect aspirin resistance. Low dose aspirin is being prescribed as an anti-platelet therapy to many people who have had a cardiovascular incident, such as heart attack or stroke. Some of those on this therapy who do have another heart attack are thought to have aspirin resistance. At the moment aspirin resistance is a somewhat nebulous term, with no consensus on its definition, how many people are affected by it, on how best to measure it, whether testing can predict what will happen in an individual person, whether the resistance will persist or be transient, whether it is also associated with resistance to other anti-platelet therapies such clopidogrel (Plavix), and with no consensus on how to alter therapy to address it. Many do not recommend testing for aspirin resistance at the moment and/or see it primarily as a research tool. Most agree that there is still much work to be done on determining its clinical relevance. A few doctors are attempting to identify aspirin resistance in their patients by ordering one or more platelet function tests.
- To monitor platelet function during complex surgical procedures including cardiopulmonary bypass surgery, cardiac catheterization, liver transplantation, and trauma surgery. Those undergoing cardiopulmonary bypass surgery are given anticoagulants to keep them from clotting, resulting in an increased risk of excessive bleeding. At the same time, bypassing the heart and mechanically circulating the blood activates large numbers of platelets and causes them to become dysfunctional. Counting the number of platelets in blood during cardiac surgery also helps the doctors maintain a delicate balance between bleeding and clotting.
- To screen at-risk pre-surgical patients to determine whether they are likely to bleed excessively during an invasive procedure. These include, for example, people with a prior history of bleeding problems or those on drugs that affect the ability of blood to clot, such as aspirin and non-steroidal anti-inflammatory drugs (NSAIDs). Doctors currently evaluate a person for known risk factors and rely on the person's clinical history and secondary hemostasis tests (PT and PTT) to determine the overall risk of excessive bleeding. There is no single platelet function screening test that will definitively predict which people are likely to bleed during surgery.
When is it ordered?One or more platelet function tests are ordered whenever a doctor wants to evaluate platelet function. This may be:
- Prior to surgery or other invasive procedure
- During surgery, especially prolonged procedures
- When a person is experiencing symptoms of platelet dysfunction, such as excessive bleeding; these include numerous nosebleeds, heavy menstrual bleeding, excessive bleeding during dental procedures, etc.
- When a person is taking a medication that can have an effect on platelet function.
What does the test result mean?The interpretation of results of the various types of platelet function tests depends on why the test(s) were performed.
In the investigation of excessive bleeding or the potential for bleeding during surgery, abnormal results may indicate the presence of platelet dysfunction. Further testing, such as specific bleeding disorder tests or clinical evaluation is often necessary to identify an inherited disorder or acquired condition as the cause of the dysfunction.
Examples of inherited platelet function disorders include:
- Von Willebrand disease — associated with decreased production or dysfunction of Von Willebrand factor and results in reduced platelet adherence to the injured blood vessel and increased blood loss
- Glanzmann's thrombasthenia — affects platelets ability to aggregate
- Bernard-Soulier syndrome — characterized by reduced platelet adhesion
- Storage pool disease — can affect platelet ability to release substances that promote aggregation
Acquired platelet dysfunction — those that are not inherited — may be due to chronic conditions such as:
Some acquired platelet disorders that are temporary include:
- Decreased function due to medications like aspirin and nonsteroidal anti-inflammatory drugs
- Abnormal function after prolonged cardiac bypass surgery
When a person is taking a medication that can affect platelet function, such as aspirin, then the results of testing reflect the platelet response to the medication.
Is there anything else I should know?Platelet function testing is not a perfect reflection of the clotting process in the body (in vivo). A person with normal platelet function test results may still experience excessive bleeding or inappropriate clotting during and after a surgery.
Most samples for platelet function testing are only stable for a very short period of time. Testing choices are often limited to what is locally available.
There are several drugs that can affect the results of platelet function tests. Some of these include:
- Aspirin and aspirin-containing compounds
- Nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and any over-the-counter medications that contain NSAIDs
- Tricyclic antidepressants
- Some antibiotics
Should everyone have platelet function tests?
Can my doctor choose from a variety of platelet function tests?Typically, a hospital or laboratory will offer one or more tests but not a wide variety. These tests evaluate the same thing – platelet function – in different ways. Since the sample must be tested promptly, the doctor will choose from what is available. Rarely, if a doctor wanted a particular type of test done, then it might be necessary for you to go to a clinic, hospital, or another city where that test is performed.
Will my platelet function change over time?It could. While some conditions associated with platelet dysfunction are inherited, others are acquired and may occur at any point in your life. Platelet dysfunction that is due to a chronic disease may persist but can generally be managed. Dysfunction due to medication will typically resolve once the medication is discontinued.