Platelet aggregation is the clustering together of thrombocytes, which are the small, cell-like structures made in the bone marrow that help prevent bleeding. This clumping can occur in response to many different agents, including the nucleotide adenosine diphosphate (ADP), and the proteins thrombin and collagen. Platelet aggregation is a crucial step in the process of clot formation, which stops bleeding.
A platelet aggregation test can be performed to determine how well platelets are sticking together. The test measures this by using a platelet antagonist to initiate clumping in the patient's blood sample. The most common platelet antagonists are ADP, thrombin, and ristocetin. After the antagonist is added to the blood sample, a special machine called an aggregometer measures the turbidity, or cloudiness, in the sample. The aggregometer records how quickly the platelets clump together by measuring the increased light and clarity in the fluid.
A physician may order the test if the results of a patient's complete blood count (CBC) indicates thrombocytopenia, which is a lower than normal level of platelets. The test may also be ordered if the patient has a family member that has been diagnosed with a bleeding disorder involving platelet dysfunction, or if he or she exhibits signs or symptoms of a bleeding disorder. These may include frequent nosebleeds, bleeding gums and easy bruising. Testing can help determine if these symptoms are due to a genetic problem, a different blood disorder, or if they are a side effect of a medication.
A decrease in platelet aggregation may suggest either a congenital disorder that has been present since birth, or an acquired one. Examples of congenital disorders that can result in decreased aggregation include Von Willebrand's disease, which is a lack of Von Willebrand factor or ristocetin co-factor in the blood, and Bernard-Soulier syndrome, a rare disease in which platelets cannot respond to Von Willebrand factor. Storage pool disease, a group of disorders that are characterized by defects in the materials that trigger aggregation, may also be detected using a platelet aggregation test.
Acquired conditions that may cause abnormal test results include autoimmune disorders that create antibodies against platelets, such as AIDS. Myeloproliferative diseases that cause the bone marrow to produce too many red blood cells, white blood cells and platelets, may also cause problems with the ability of platelets of aggregate. Storage pool disease can be an acquired cause of platelet aggregation in certain situations, especially in response to cardiopulmonary bypass — a procedure that temporarily takes control of heart and lung function during surgery — or vasculitis, an inflammation of the blood vessels.