To help determine the cause of, or potential for, excessive bleeding and/or to diagnose a platelet function disorder; to monitor and evaluate platelet function; to monitor the presence and effectiveness of anti-platelet medications
Platelet Function Tests
When you bruise easily or experience excessive or prolonged bleeding; when you are taking medications that can alter platelet function; prior to or during certain surgeries.
Platelet function testing is performed on a sample of blood obtained from a vein in the arm using a needle. This is a process which may be referred to as ‘venepuncture’.
You may be instructed to refrain from taking drugs that can affect the function of normal platelets and hence 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 cyclo-oxygenase-2 (COX-2) inhibitors. However, do not stop taking your medications unless instructed to do so by your health care provider.
Platelets are small cell fragments and are found in the blood along with red cells and white cells. Platelets are produced in the bone marrow and released into the blood where they play an important role in coagulation (blood clotting), helping to stop bleeding when blood vessels are injured. These small, disc-shaped cells usually live for around 5-10 days in the blood before they are destroyed.
Platelets are very important for efficient blood coagulation and preventing unnecessary or excessive blood loss. If there are too few platelets, or if the platelets that are present don’t function properly, then there may be problems with blood clot formation. Platelet function tests are used to indirectly evaluate how well a person's platelets work in helping to stop bleeding within the body.
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 haemostasis. 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 haemostasis 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 platelet count simply provides information on the number of these cells that are present in blood; it does not reveal if these cells are functioning properly. Specialised tests, called the platelet function tests, are used when there is suspicion of a platelet function defect.
Platelet function tests involve testing the response of an individual's platelets to a variety of stimuli (platelet agonists) in the laboratory, using the timing and magnitude of platelet aggregation to the platelet agonists to measure the ability of platelets to promote clotting in a sample of blood. There are a variety of tests available but no single 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.
Closure time assays
This test measures the time required for the platelets in a sample of blood to form a platelet plug that is big and strong enough to seal 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, however it relies on the use of specialist equipment which may not be available in all 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. Whole blood from a patient is incubated with a variety of agonists that can stimulate clot formation and the time to clot and the strength of the clot that is formed are measured to assess the clotting potential of the individual. It is most often performed in larger hospitals, either in the operating theatre 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.
In the past, the primary screen for platelet dysfunction was the bleeding time test. This is the only test that directly measures platelet function within the body, the other tests being ‘indirect’ tests involving removing platelets from an individual (in a blood collection tube) and testing their response in vitro. The bleeding time test involves making two small, shallow, standardised cuts on the inner forearm and measuring the amount of time for bleeding to stop. The bleeding time procedure has fallen from favour in recent years. Many hospitals no longer offer it, and several national organisations 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 testing is carried out in specialised, larger, hospital laboratories only, due to the complexity of performing the test in the laboratory and the need for specialist staff to perform and interpret the tests. Platelet aggregometry testing involves collection of around 30mL of blood from a patient. This whole blood is then centrifuged (spun down) to obtain platelet-rich plasma (PRP). The PRP is then mixed with 4 to 8 different agonists to activate the platelets and the responses of the platelets to these agonists is measured. The results of this panel of testing is reviewed, along with the patient bleeding history and information on platelet number and appearance under the microscope to ascertain if there is a platelet abnormality present. Platelet aggregation testing can diagnose a variety of inherited and acquired platelet function disorders.
Platelets can also be evaluated for functional defects using flow cytometry. This test uses a specialised instrument that is capable of detecting fluorescence emitted by stained platelets 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 specialised procedure available only in few reference laboratories to diagnose inherited platelet function disorders.
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 in which 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 disorder that is associated with platelet dysfunction. Decreased production or dysfunction of von Willebrand factor (VWF) results in reduced platelet adherence to the injured blood vessel and increased blood loss.
- To monitor platelet function during complex surgical procedures, including cardiopulmonary bypass surgery, cardiac catheterisation, liver transplantation, and trauma surgery. For example, those undergoing cardiopulmonary bypass surgery are given anticoagulants to reduce blood 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. Monitoring the number of platelets in blood (platelet count) during cardiac surgery also helps the health care team 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). Healthcare professionals currently evaluate a person for known risk factors and rely on the person's clinical history and pre-operative testing 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.
- To monitor anti-platelet therapy given to some people 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 a heart attack or stroke. Some people on this therapy who have another heart attack are thought to have aspirin resistance. At present, aspirin resistance is a somewhat vague term, with no consensus on its definition, how many people are affected by it, or on how to measure it. There are questions as to whether testing can predict what will happen in an individual person, whether the resistance will persist or be transient, and whether it is also associated with resistance to other anti-platelet therapies such clopidogrel. There is also lack of agreement 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.
Platelet function testing may include one or more of the following:
- Closure time assay
- Bleeding time
- Platelet aggregometry
- Flow cytometry
For a more detailed explanation of these tests, read the "What is being tested?" section.
Some other tests that may be done in conjunction with or as follow up to platelet function tests to evaluate platelet disorders include full blood count (FBC), platelet count, PT, APTT, and von Willebrand factor.
When is it requested?
One or more platelet function tests are requested whenever a healthcare profesdsional wants to evaluate platelet function. This may be:
- Prior to surgery or other invasive procedure
- When a person is experiencing symptoms of platelet dysfunction, such as excessive bleeding or bruising; unexpected or prolonged bleeding or bruising, investigation of petechiae or purpura (small purple/red dots in the skin caused by bleeding from small vessels); frequent nosebleeds, heavy menstrual bleeding, bleeding gums, excessive bleeding during dental procedures, etc.
- During surgery, especially prolonged procedures
- 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 tests were performed. Many platelet disorders are very rare and require specialist interpretation.
In the investigation of excessive bleeding or the potential for bleeding during surgery, abnormal results may indicate the presence of a platelet disorder. 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 – decreased production or dysfunction of von Willebrand factor results in reduced platelet adherence to the injured blood vessel and increased blood loss
- Glanzmann thrombasthenia – affects platelets' ability to aggregate, usually apparent early in infancy with excessive bruising
- Bernard-Soulier syndrome – characterised by reduced platelet adhesion
- Storage pool disease – can affect platelets' ability to release substances that promote aggregation
Acquired platelet dysfunction – those that are not inherited – may be due to chronic conditions such as:
- Kidney failure (uraemia)
- Myelodysplastic syndrome (MDS)
Some acquired platelet disorders that are temporary include:
- Decreased function due to medications like aspirin and non-steroidal anti-inflammatory drugs
- Abnormal function after prolonged cardiac bypass surgery
When a person is on an anti-platelet medication, such as aspirin, 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 (salicylates)
- Non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and any over-the-counter medications that contain NSAIDs
- Tricyclic antidepressants
- Some antibiotics
If abnormal platelet function is detected then the results of the platelet function tests and any other tests that have been performed will be taken into consideration and further investigations (blood tests) may be requested. This information, along with any clinical signs and symptoms will be considered by your healthcare professional who will be able to provide you with more information regarding the significance of the results.
Should everyone have platelet function tests?
Can my doctor choose from a variety of platelet function tests?
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.
On This Site
Tests: Full Blood Count; Blood Smear; Platelet Count; PT and INR; PTT; Bone Marrow Aspiration and Biopsy; Coagulation Factors; von Willebrand Factor; Clopidogrel (CYP2C19 Genotyping)
Conditions: Bleeding Disorders, Excessive Clotting Disorders
Elsewhere On The Web
Patient: Thrombocytopenia and Platelet Function Disorders