Platelet function tests

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 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 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 favour in recent years. Many hospitals 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 frequently leaves small thin scars on the forearm.