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

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to swell with blood.

A needle is inserted into the vein, and the blood is collected in an airtight vial or a syringe. During the procedure, the band is removed to restore normal blood flow. Once the blood has been collected, the needle is removed. The puncture site is covered to stop any bleeding.

For an infant or young child, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a small glass tube (pipette), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if bleeding continues.

The laboratory specialist will look at how the platelets spread out in the 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 alter test results include:

• Aspirin
• Aspirin compounds
• Nonsteroidal anti-inflammatory drugs
• Tricyclic antidepressants
• Antihistamines
• Some antibiotics

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.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Getting a blood sample from some people may be harder than from others.

Your doctor may order this test if you have signs of a bleeding disorder or low-platelet count.

See also: Normal platelet count

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

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

Decreased platelet aggregation may be due to:

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

• Excessive bleeding
• Fainting or feeling light-headed
• Blood collecting under the skin (hematoma)
• Infection
• Many needle sticks to find veins

Warkentin TE, Greinacher A. The Seventh AACP Conference on Antithrombotic and Thrombolytic Therapy: Evidence-Based Guidelines. Chest. September 2004;126(suppl):311S-337S.

Napolitano LM, Warkentin TE, AlMahameed A, Nasraway SA. Heparin-induced thrombocytopenia in the critical-care setting: diagnosis and management. Crit Care Med. December 2006;34:2898-2911.