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 injury site (with the help of von Willebrand factor, which acts as the “glue”), aggregate with other platelets, release compounds that stimulate further aggregation, and form 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 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.
If there are insufficient platelets, or if they are not functioning normally, a patient may be at an increased risk of excessive 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 favor in recent years. Many hospitals are no longer offering it, and several national organizations 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 frequently leaves small thin scars on the forearm.
The PFA-100® (Platelet Function Analyzer – 100) is a testing device that many hospitals are using as a platelet function screen, in place of the bleeding time, to mimic 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 epinephrine (EPI) 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/EPI. If the CT is normal, it is unlikely that a platelet dysfunction exists. The collagen/ADP test is run to confirm an abnormal collagen/EPI test. If both tests are abnormal, it is likely that the patient has a platelet dysfunction and further testing for inherited or acquired bleeding disorders is indicated. If the collagen/ADP test is normal, then the abnormal collagen/EPI test may be due to aspirin ingestion. This is the most frequently encountered abnormal collagen/EPI result as a single dose of aspirin can affect platelet function for about 10 days.
While the PFA-100® test has gained acceptance as a useful screen for platelet dysfunction, there is no consensus that it is THE replacement test for the bleeding time. The PFA-100 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.
Platelet aggregometry is a test of platelet function widely used in academic centers and large hospitals. One or two tubes of blood are 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 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 dysfunction due to aspirin ingestion; 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.
