Platelet aggregation tests are essential diagnostic procedures conducted within specialized hematology laboratories, particularly when there is a suspicion of platelet dysfunction. These tests are typically recommended for patients who exhibit mucocutaneous bleeding. In such cases, initial screening tests often reveal normal platelet counts, extended bleeding times, and normal results for both prothrombin time and activated partial thromboplastin time.
To perform a platelet aggregation test, platelet-rich plasma is utilized, and the process is carried out using an aggregometer. When a platelet aggregating agent is introduced to the plasma, it prompts platelets to form aggregates, leading to a decrease in optical density or an increase in light transmission. This change is recorded on a strip chart by a chart recorder. Common agents used to induce platelet aggregation include ADP (adenosine 5-diphosphate), epinephrine (adrenaline), collagen, arachidonic acid, and ristocetin. Notably, both ADP (at low doses) and epinephrine trigger primary and secondary waves of aggregation, producing a biphasic curve. The primary wave results from the direct action of the aggregating agent on the platelets, while the secondary wave is a consequence of thromboxane A2 synthesis and subsequent secretion from the platelets. On the other hand, collagen, arachidonic acid, and ristocetin induce a single wave of aggregation, known as a monophasic curve. Figure 1 displays a typical normal aggregation curve, while Figures 2 through 4, along with Table 1, illustrate the aggregation patterns observed in various platelet function disorders.
Disorder | Platelet Aggregation Pattern | Other Features |
---|---|---|
Bernard Soulier syndrome | Normal with ADP, epinephrine, collagen, and arachidonic acid; deficient with ristocetin | Autosomal recessive; severe bleeding; giant platelets |
Glanzmann's thrombasthenia | Deficient with ADP, epinephrine, collagen, and arachidonic acid; normal with ristocetin | Autosomal recessive; severe bleeding; small and discrete platelets; defective clot retraction |
Storage pool defect | Primary wave with ADP and epinephrine, normal with arachidonic acid, deficient with collagen, normal with ristocetin | Defects of platelets granules; platelet dense granules are decreased with the deficient release of ADP, ATP, calcium, and serotonin |
Aspirin-like defect | Primary wave with ADP and epinephrine, deficient with arachidonic acid, deficient with collagen, normal with ristocetin | - |
von Willebrand disease | Normal with ADP, epinephrine, collagen, and arachidonic acid; deficient with ristocetin | Autosomal dominant/recessive; abnormality in aggregation corrected with cryoprecipitate |
- Bernard-Soulier syndrome: Characterized by normal aggregation with ADP, epinephrine, collagen, and arachidonic acid, but deficient aggregation with ristocetin. It is an autosomal recessive disorder, often associated with severe bleeding and giant platelets.
- Glanzmann's thrombasthenia: Displays deficient aggregation with ADP, epinephrine, collagen, and arachidonic acid, but normal aggregation with ristocetin. It is also autosomal recessive, marked by severe bleeding, small and discrete platelets, and defective clot retraction.
- Storage pool defect: Exhibits a primary wave with ADP and epinephrine, normal aggregation with arachidonic acid, deficient aggregation with collagen, and normal aggregation with ristocetin. This condition involves defects in platelet granules, particularly a decrease in platelet dense granules leading to deficient release of ADP, ATP, calcium, and serotonin.
- Aspirin-like defect: Demonstrates a primary wave with ADP and epinephrine, deficient aggregation with arachidonic acid, deficient aggregation with collagen, and normal aggregation with ristocetin.
- von Willebrand disease: Characterized by normal aggregation with ADP, epinephrine, collagen, and arachidonic acid, but deficient aggregation with ristocetin. This disorder may be autosomal dominant or recessive, with the aggregation abnormality often corrected by cryoprecipitate.