Platelet Aggregation test
How the test is performed:
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet 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.
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 form clumps after the addition of a material that normally stimulates aggregation. This may include ADP (adenosine diphosphate), epinephrine, thrombin, 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:
Fast for 8 hours (sometimes a nonfat diet may be allowed).
Consult with the health care provider if you are taking any medications that may alter test results (see "special considerations"). 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.
Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
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, 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 such things as another disorder or medication).
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