Analysers used for function testing
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 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, 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.
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 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.