PLATELET AGGREGATION STUDIES
Platelet aggregation tests are carried out in specialized hematology laboratories if platelet dysfunction is suspected. These tests are usually indicated in patients presenting with mucocutaneous type of bleeding and in whom screening tests reveal normal platelet count, prolonged bleeding time, normal prothrombin time, and normal activated partial thromboplastin time.
Platelet aggregation studies are carried out on platelet-rich plasma using aggregometer. When a platelet aggregating agent is added to platelet-rich plasma, platelets form aggregates and optical density falls (or light transmission increases); this is recorded by a chart recorder on a strip chart. Commonly used platelet aggregating agents are ADP (adenosine 5-diphosphate), epinephrine (adrenaline), collagen, arachidonic acid, and ristocetin. ADP (low dose) and epinephrine induce primary and secondary waves of aggregation (biphasic curve). Primary wave is due to the direct action of aggregating agent on platelets. Secondary wave is due to thromboxane A2 synthesis and secretion from platelets. Collagen, arachidonic acid and ristocetin induce a single wave of aggregation (monophasic curve) Normal aggregation curve is shown in Figure 804.1. Aggregation patterns in various platelet function defects are shown in Figures 804.2 to 804.4, and in Table 804.1.