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Home :: Platelet aggregation

Platelet Aggregation

After vascular injury, platelets gather at the injury site and clump together to form an aggregate or plug that helps maintain hemostasis and promotes healing. The platelet aggregation test, an in vitro procedure, is used to measure the rate at which the platelets in a plasma sample form a clump after the addition of an aggregating reagent.


  • To assess platelet aggregation.
  • To detect congenital and acquired platelet bleeding disorders.

Patient preparation

  • Explain to the patient that this test is used to determine if blood clots properly.
  • Tell him that the test requires a blood sample. Explain who will perform the venipuncture and when.
  • Reassure him that drawing a blood sample will take less than 3 minutes.
  • Explain that he may feel slight discomfort from the tourniquet pressure and the needle puncture.
  • Instruct the patient to fast or to maintain a nonfat diet for 8 hours before the test, because lipemia can affect the test results.
  • Withhold aspirin and aspirin compounds for 14 days, and phenylbutazone, sulfmpyrazone, phenothiazines, antihistamines, tricyclic antidepressants, and anti-inflammatory drugs for 48 hours. If these medications must be continued, note this on the laboratory slip.

Procedure and posttest care

  • Perform a venipuncture, and collect the sample in a 7-ml blue-top siliconized tube.
  • Completely fill the collection tube, and invert it gently several times to mix the sample and the anticoagulant thoroughly.
  • Maintain the sample at 71.6° F (22° C) to 98.6° F (37°C) to prevent aggregation.
  • Apply pressure to the venipuncture site for 5 minutes, or until bleeding stops.
  • Resume diet and administration of medications withheld before the test.
  • If a hematoma develops at the venipuncture site, apply warm soaks.
  • Because the list of medications known to alter the results of this test is long and continually growing, the patient should be as free of drugs as possible before the test.
  • If the patient has taken aspirin within the past 14 days and the test cannot be postponed, ask the laboratory to verify the presence of aspirin in the plasma. If test results are abnormal for such a sample, the use of aspirin must be discontinued and the test repeated in 2 weeks.
  • Avoid excessive probing at the venipuncture site.
  • Remove the tourniquet promptly to avoid bruising.
  • Handle the sample gently to prevent hemolysis.

Reference values

Normal aggregation occurs in 3 to 5 minutes, but findings are temperature dependent and vary with the laboratory. Aggregation curves obtained by using different reagents help to distinguish various qualitative platelet defects

Abnormal findings

Abnormal findings may indicate von Willebrand's disease, Bernard-Soulier syndrome, storage pool disease, Glanzmann's thrombasthenia, or polycythemia vera

Interfering factors

  • Failure to observe pretest restrictions.
  • Failure to use the proper anticoagulant or to adequately mix the sample and the anticoagulant.
  • Hemolysis due to rough handling of the sample or to excessive probing at the venipuncture site.
  • Aspirin and aspirin compounds, phenylbutazone, sulfinpyrazone, phenothiazines, anti-inflammatory drugs, antihistamines, and tricyclic antidepressants (decrease).

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