Platelet Function Testing

Article excerpt


Platelets or thrombocytes are small cells that are vital in the blood clotting process. Platelets are produced in the bone marrow and circulate in the blood until they are activated by collagen, thrombin, ADP receptors (P2Y1 and P2Y12) expressed on platelets, a negatively charged surface as well as many other activating factors." They are rapidly deployed to injury or infection sites and potentially modulate inflammatory processes by interacting with leukocytes and secreting cytokines and chemokines. (3)

Platelet Function

When there is an injury to a blood vessel, the circulating platelets will adhere to the injury site, aggregate with other platelets and form a loose platelet plug. The high concentration of myosin and actin filaments in platelets are stimulated to contract during aggregation, which help to reinforce the plug. During this time, they will also release a number of different coagulation factors and platelet activating factors to ensure further reinforcing of the platelet plug. This process will end in the formation of fibrin strands woven through the plug and cross linked to form a fibrin net. The fibrin net will remain in place until the injury has healed. Upon healing, other factors break down the clot and remove it. Normal platelet count in a healthy adult is between 150,000400,000 per mm of blood. (3) However, an adequate number of platelets will not guarantee adequate function. (2,5)

The discovery of platelet dysfunction in a patient may not always be associated with venous injury. Many times the dysfunction is linked to an inherited or acquired bleeding disorder or may be discovered during a routine workup. (5) Suggested indications for platelet function testing include the following:

--Patient who experiences easy bruising

--Women experiencing menorrhagia

--Hospitalized patients experiencing bleeding

--Pre-op assessment

--Anti-platelet therapy monitoring

--Uremia (circulating substances impair platelet interactions with the vessel wall) (1)

--Cirrhosis (thrombocytopenia, coagulopathy and vascular abnormality) (1)

Platelet Function Tests

In years past, the primary test used to evaluate platelet function was the bleeding time (BT). It is performed by making a standard incision on the forearm and measuring the time required for bleeding to stop. The BT is currently being phased out of most hospitals nationwide due to the fact that is it not sensitive or specific and does not reflect the risk or potential severity of a surgical bleed. The BT is rarely reproducible and varies by the technologist performing the test. It can be affected by ingestion of aspirin or other drugs containing aspirin and leaves a small scar on the forearm. (5)

Continuous research and emerging technology have created new instrumentation in the platelet function testing. Microprocessor controlled instrumentation and disposable test cartridges have enabled medical technologist to perform platelet function testing on whole, citrated blood obtained from a single venipuncture. Improved diagnostic value, patient acceptance and cost are reason enough to evaluate platelet function on new instrumentation over the use of the BT.

Platelet Function Analyzers

The PFA-100[R] analyzer was the first commercially available in-vitro test that incorporated high-shear flow to provide a realistic hemodynamic environment to measure platelet function. Blood is aspirated into the instrument under a constant pressure through a capillary in a membrane that is coated with either collagen and epinephrine or collagen and ADP. This being the stimuli for platelet aggregation and adherence, activation and aggregation will occur in the instrument to result in the formation of a platelet plug. This plug will occlude the flow in the instrument and the time taken to occlude the flow is called the "closure time" (CT). …