Platelet aggregation can be measured by bringing platelet-rich plasma into contact with known inducers of platelet aggregation. Most inducers, such as collagen, epinephrine, and thrombin, act through the effects of ADP, which is released by the platelets themselves. Adding exogenous ADP causes platelet aggregation directly. Ristocetin, an antibiotic, may also be used for this test.²⁷

Platelet aggregation is quantified by determining whether platelet-rich plasma becomes clear as evenly suspended platelets aggregate and fall to the bottom of a test tube. Normally, platelet aggregates should be visible in less than 5 minutes.

Platelet aggregation in response to specific inducing agents is diagnostic for specific disorders. Aspirin, other anti-inflammatory agents, and many phenothiazines markedly inhibit the aggregating effect of collagen and epinephrine but do not interfere with the direct action of added ADP. Also, conditions that depress the release-inducing effects of collagen and epinephrine and of directly added ADP affect platelet aggregation.

Individuals with von Willebrand's disease have platelets that respond normally to epinephrine, collagen, and ADP. Without vWF in their plasma, however, the platelets will not be aggregated by ristocetin.²⁸

Other disorders that may impair platelet aggregation include Glanzmann's thrombasthenia, Bernard-Soulier syndrome (hereditary giant platelet syndrome), idiopathic thrombocytopenic purpura, and infectious mononucleosis. Drugs that interfere with platelet aggregation are listed in Table 2-5.

• Platelet aggregates should be visible in less than 5 minutes.

• Ingestion of aspirin and other drugs known to interfere with platelet aggregation within 5 to 7 days of the test (see Table 2-5).
• Delay in processing the sample or excessive agitation of the sample may alter test results.

• Suspected von Willebrand's disease or other inherited platelet disorder
• Evaluation of platelet aggregation in clients with disorders known to cause alterations (e.g., uremia, severe liver disease, myeloproliferative disorders, dysproteinemias)
• Therapy with drugs known to alter platelet aggregation (see Table 2-5)

Nursing Care Before the Procedure

Client preparation is essentially the same as that for any study involving the collection of a peripheral blood sample (see Appendix I). It is generally recommended that the person abstain from food for 8 hours before the test and, if possible, from drugs that may impair platelet aggregation for 5 to 7 days before the test.

A venipuncture is performed and the sample collected in a light-blue-topped tube.

Nursing Care After the Procedure

Care and assessment after the procedure are essentially the same as for any study involving the collection of a peripheral blood sample. Because the client may have platelet deficiency, maintain digital pressure directly on the puncture site for 3 to 5 minutes after the needle is withdrawn. Also, inspect the site for excessive bruising after the procedure.

• Complications and precautions: Note and report drugs that alter platelet aggregation and discontinue if test results indicate prolonged aggregation. Report any abnormal test results to the physician.