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

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

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 alter test results include:

• Aspirin
• Aspirin compounds
• Nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen
• 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.

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 disorders, or even 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.

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

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)

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.

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.