Platelet aggregation test

Definition 
This is a test of the ability of platelets, a component of blood, to clump together and cause blood clotting.

How the test is performed 

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, and 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 circulation. Once the blood has been collected, the needle is removed, and 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 pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

The platelet aggregation test measures the rate and degree to which dispersed platelets in a sample of plasma (the liquid portion of blood) form clumps after the addition of a material that normally stimulates aggregation. These materials may include ADP (adenosine diphosphate), epinephrine, arachidonic acid, collagen, or ristocetin.

Clumping of platelets causes the sample to be more clear (less turbid). A machine measures the changes in turbidity (cloudiness) and prints a graphic recording of the results.

How to prepare for the test 
The list of medications that may affect test results is increasing rapidly; notify your health care provider of any medications (including over-the-counter preparations) that you have taken within the preceding 2 weeks.

For infants and children, the preparation you can provide for this test depends on your child’s age and previous experience. For general information regarding how you can prepare your child, see the following topics:

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

Why the test is performed 
This test is most often indicated when there is a bleeding tendency qualitatively similar to low-platelet bleeding despite normal platelet count (normal number of platelets).

Platelets (thrombocytes) are small cells in the bloodstream that are part of the blood clotting process. They are manufactured in the bone marrow. When bleeding occurs, platelets clump together at the site of injury, swell, and stick to the injured area, acting as a plug to reduce the bleeding.

Platelet aggregation tests help to diagnose diseases of platelet dysfunction and distinguish between inherited bleeding problems (such as hemophilia or von Willebrand disease) and acquired bleeding problems (those that occur because of another disorder or as a side-effect of medication).

Normal Values
 
The normal time it takes to cause clumping is temperature dependent and may vary from laboratory to laboratory. The shape of the vial is also important.

Response to the different “aggregation stimulators” can help to distinguish various platelet defects. In particular, the difference between the response to ristocetin and the other products is important, because ristocetin triggers aggregation through a different mechanism than the other products. The time and pattern of platelet aggregation also varies between stimulators.

What abnormal results mean
 
Congenital disorders that cause decreased platelet aggregation:

     
  • Glanzmann’s thrombasthenia (a rare inherited platelet disorder) - platelets fail to aggregate in response to normal stimulation but do clump normally in response to ristocetin.  
  • Bernard-Soulier syndrome (a rare inherited disorder in which platelets are unable to respond to von Willebrand factor) - platelets aggregate normally in response to most stimulation but do not aggregate in response to ristocetin. The response to collagen is blunted.  
  • Storage pool disease (a group of disorders characterized by defective stimulation by materials, especially ADP, which trigger platelet aggregation)- variable abnormalities of platelet aggregation  
  • Von Willebrand’s disease (inherited lack of von Willebrand factor or ristocetin cofactor) - aggregation with ristocetin is usually abnormal in type I disease and increased in type II.

Acquired conditions that cause decreased platelet aggregation:

     
  • use of platelet-inhibiting medications (see special considerations)  
  • autoimmune disorders that produce antibodies against platelets  
  • myeloproliferative disorders  
  • uremia (a result of Kidney failure)  
  • acquired storage pool disorder (platelets are no longer effective because of temporary damage, such as in response to cardiopulmonary bypass or vasculitis)  
  • fibrin degradation products

What the risks are
 

     
  • excessive bleeding  
  • fainting or feeling light-headed  
  • hematoma (blood accumulating under the skin)  
  • infection (a slight risk any time the skin is broken)  
  • multiple punctures to locate veins

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.

Special considerations
 
Medications that may alter test results include:

     
  • aspirin  
  • aspirin compounds  
  • nonsteroidal anti-inflammatory drugs  
  • tricyclic antidepressants  
  • antihistamines  
  • some antibiotics

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

Johns Hopkins patient information

Last revised: December 8, 2012
by Armen E. Martirosyan, M.D.

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