What is platelet function?
Platelets are vital for normal blood clotting. Produced in the bone marrow, they circulate in the blood until they are needed. When there is an injury to a blood vessel, platelets adhere to the collagen in the injured vessel wall (with the help of von Willebrand factor, which acts as part of the “glue”), aggregate with other platelets, release compounds that stimulate further aggregation, and form a loose platelet plug in a process called primary haemostasis. At the same time, platelets support the coagulation cascade, a series of steps that involves the sequential activation of clotting factors. This secondary haemostasis 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.
If there are insufficient platelets, or if they are not functioning normally, a patient may be at an increased risk of bleeding. The number of platelets can be easily determined with a platelet count, but the overall platelet function is more difficult to measure. Unfortunately, there is 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 the past, the primary screen for platelet dysfunction was the bleeding time. However, the bleeding time procedure has fallen from favour in recent years. Many centres are no longer offering it, and several national organisations have issued position statements against its routine use as a pre-surgical screen. 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 leaves small thin scars on the forearm.
Analysers used for function testing
The PFA-100® (Platelet Function Analyzer – 100) is a testing device that many laboratories are using as a platelet function screen, in place of the bleeding time, to approximate the clotting process. To do the test, a tube of blood is drawn from a vein in the arm and then a portion of the blood is put into a test cartridge. Vacuum is then used to draw blood through a very thin glass tube that has been coated with collagen and with either adrenaline or ADP. This coating activates the platelets in the moving sample and promotes platelet adherence and aggregation. The time it takes for a clot to form inside the glass tube and prevent further blood flow is measured as a closure time (CT). An initial screen is done with collagen/adrenaline. If the CT is normal, it is unlikely that a platelet dysfunction exists. The collagen/ADP test is run to confirm an abnormal collagen/adrenaline test. If both tests are abnormal, the patient may have a platelet function disorder and further testing for inherited or acquired bleeding disorders is indicated. If the collagen/ADP test is normal, then the abnormal collagen/adrenaline test may be due to aspirin ingestion. This is the most frequently encountered abnormal collagen/adrenaline result as a single dose of aspirin can affect platelet function for up to 10 days.
While the PFA-100® test has gained acceptance as a useful screen for platelet dysfunction it has not been shown to be able to predict the likelihood that a patient will bleed excessively during surgery and its full clinical utility has yet to be established.
Light transmission platelet aggregometry is a test of platelet function widely used in academic centres and large hospitals. Blood is drawn from a vein in the arm, and the response of either whole blood or platelet-rich plasma to specific agents known to induce aggregation of platelets is studied. This test measures the change in light transmission as the platelets aggregate and is used to diagnose inherited and acquired platelet function disorders. It is affected by aspirin and a variety of other drugs that alter platelet function.
There are many other platelet function tests that measure particular aspects of platelet aggregation or clot formation. Some are still only being used for research, while others are being used by some doctors for specific purposes. The VerifyNow® Aspirin Assay (formerly Ultegra RPFA®-ASA), for instance, is a test that may be ordered to help detect platelet aspirin effect; VerifyNow® IIb/IIIa Assay is a test that may be used to monitor abciximab (an anti-platelet therapy); and Plateletworks® is a testing method used to monitor changes in platelet function by measuring aggregation ability. An older test that is staging a comeback is thromboelastography or TEG, which measures clot strength and has been used to monitor platelet function and coagulation during cardiovascular surgery and to predict bleeding and monitor blood transfusion effectiveness during cardiopulmonary surgery. It should be noted that since 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.
Another test, the calibrated automated thrombogram (CAT test), measures thrombin generation and can be done on stored blood. Thrombin is a critical enzyme leading to the formation of a blood clot. This test can show the rate of thrombin generation and is affected by coagulation and platelet disorders. Neither the CAT nor the TEG are in routine use outside of large hospitals.
Why are platelet function tests done?
Screen at-risk pre-surgical patients to determine whether they are likely to bleed excessively during an invasive procedure. Doctors currently evaluate a patient for known risk factors and rely on a patient’s clinical history and secondary haemostasis (PT and aPTT) to determine their overall risk of excessive bleeding. There is no good platelet function screening test to predict which patients are likely to bleed during surgery.
Identify and help diagnose platelet dysfunction in those with a history of excessive bleeding. This may be someone with nosebleeds, heavy menstrual bleeding, excessive bleeding during dental procedures, etc. 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’s disease, for instance, is the most common inherited platelet dysfunction. It is associated with decreased production or dysfunction of von Willebrand factor and results in reduced adherence to the injured blood vessel and increased blood loss (most patients do clot eventually but it is a much slower process). Other inherited disorders cause decreased platelet aggregation and may include: Glanzmann’s thrombasthenia, Bernard-Soulier syndrome, and storage pool disease. Acquired platelet dysfunction may be chronic – such as can be seen with kidney failure (uraemia) and myeloproliferative disorders – or temporary, such as due to medications like aspirin and other nonsteroidal anti-inflammatory drugs.
Monitor anti-platelet therapy given to some patients after a stroke or heart attack to help inhibit blood clotting. Currently, most anti-platelet therapies cannot be routinely monitored with platelet function testing. An exception to this is abciximab, which can be monitored with the VerifyNow® IIb/IIa assay. As more potent anti-platelet therapies are created, abcixumab use has declined.
Detect aspirin resistance. Low dose aspirin is being prescribed to many patients who have had a cardiovascular incident, as an anti-platelet therapy. Some of the patients on this therapy who do have another heart attack are thought to have aspirin resistance. Currently aspirin resistance is a poorly defined entity with no consensus on the best way to measure it or to treat patients affected by it. It is also not entirely clear 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 of a variety of platelet aggregation testing methods.
Monitor platelet function during cardiopulmonary bypass surgery and cardiac catheterisation. 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. Plateletworks® is one method that is being used by some hospitals to monitor platelet function during cardiac procedures.
Common platelet function tests
Below are some of the tests that are being used to determine platelet function. In the coming years, these testing methods are expected to change and evolve as researchers continue to search for procedures that better describe and evaluate the complex clotting process.
|Platelet function analyser
|Screening, mimics clotting process, evaluates adhesion and aggregation
||Rapid, simple screening tool, identifies aspirin ingestion, can screen for von Willebrand and may monitor von Willebrand and anti-platelet therapy
||Sensitive to platelet count and hematocrit, full clinical use yet to be established
||Follow-up to abnormal Bleeding Time or PFA-100, detects responsiveness to panel of platelet activators (collagen, ADP, epinephrine, ristocetin, Arachidonic acid)
||Can diagnose inherited and acquired platelet dysfunction, measures aggregation
||Labour intensive and relatively slow
||Measure platelet glycoproteins, activation markers, platelet function and turnover, platelet counting, reticulated platelets
||Flexible and powerful, range of tests, can diagnose disorders
||Requires specialised operator, expensive equipment, mostly research tool at this point
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RCPA manual - Platelet function analyser
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Last Review Date: May 1, 2018