The platelet aggregation test checks to see how well platelets, a part of blood, clump together and cause blood clotting.

A blood sample is needed. For information on how this is done, see: Venipuncture

The laboratory specialist will look at how the platelets spread out in the liquid part of the blood (plasma) and whether they form clumps after a certain chemical or drug is added. When platelets clump together, the blood sample is more clear. A machine measures the changes in cloudiness and prints a record of the results.

Many medications may affect test results. Tell your health care provider about any medications (including over-the-counter drugs) you have taken within 2 weeks before the test.

Medications that may affect test results include:

• Antibiotics: penicillins, cephalosporins, nitrofurantoin
• Antihistamines
• Aspirin
• Aspirin-containing compounds
• Clopidogrel
• Dipyridamole
• Nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and naproxen
• Theophylline
• Ticlopidine
• Tricyclic antidepressants

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Your doctor may order this test if you have signs of a bleeding disorder or low platelet count, or if a member of your family has a known bleeding disorder due to platelet dysfunction.

The test can help diagnose problems with platelet function and determine whether the problem is due to your genes, another disorder, or a side effect of medicine.

See also: Platelet count

The normal time it takes platelets to clump depends on temperature and may vary from laboratory to laboratory.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

Decreased platelet aggregation may be due to:

• Autoimmune disorders that produce antibodies against platelets
• Bernard-Soulier syndrome
• Fibrin degradation products
• Glanzmann's thrombasthenia
• Medicines that block platelet formation
• Myeloproliferative disorders
• Storage pool disease
• Uremia (a result of kidney failure)
• Von Willebrand's disease

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Note: This test is often performed because the patient has a bleeding problem. Bleeding may be more of a risk for this person than for people without bleeding problems.

McMillan R. Hemorrhagic disorders: Abnormalities of platelet and vascular function. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 179.

Miller JL, Rao AK. Blood platelets and von Willebrand disease. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis by Laboratory Methods. 21st ed. Philadelphia, Pa: Saunders Elsevier; 2006:chap 39.

Schafer A. Hemorrhagic disorders: Approach to the patient with bleeding and thrombosis. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 178.

Schmaier AH. Laboratory evaluation of hemostatic and thrombotic disorders. In: Hoffman R, Benz EJ Jr., Shattil SJ, et al, eds. Hoffman Hematology: Basic Principles and Practice. 5th ed. Philadelphia, Pa: Churchill Livingstone Elsevier; 2008:chap 122.