Platelet Function Tests

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Also known as: Platelet Aggregation Studies; PFT; Platelet Function Assay; PFA
Formal name: Platelet Function Tests
Related 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)

At a Glance

Why Get Tested?

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

When to Get Tested?

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.

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?

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.

The Test Sample

What is being tested?

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 individuals 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 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 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.

Bleeding time
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.

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 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 platlets 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.

Flow cytometry
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 the sample collected for testing?

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’.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

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.

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.