Platelet Aggregation Test

Definition

Platelets (thrombocytes) are small anucleate (i.e. without a nucleus) disk-shaped blood cells that play a major role in the blood-clotting process. When a blood vessel wall is cut or injured, platelets adhere to the damaged site and aggregate (clump) together to form a barrier to the escape of blood. The platelet aggregation test is a measure of the platelet clumping function.

Purpose

The platelet aggregation test aids in the evaluation of bleeding disorders by measuring the rate and degree to which platelets aggregate after the addition of a chemical, an agonist, that stimulates platelet clumping. The test can be used to differentiate between several inherited and acquired disorders of platelet function.

Precautions

There are many medications that can affect the results of the platelet aggregation test. The patient should discontinue as many of these as possible beforehand. Some of the drugs that can decrease platelet aggregation include aspirin, some antibiotics, beta-blockers, dextran (Macrodex), alcohol, heparin (Lipo-Hepin), nonsteroidal anti-inflammatory drugs (NSAIDs), tricyclic antidepressants, and warfarin (Coumadin).

Description

There are many factors involved in blood clotting (coagulation). One of the first steps in the process involves small cells in the bloodstream called platelets, which are produced in the bone marrow. Platelets gather at the site of an injury, adhere to the damaged vessel wall, and aggregate together forming a plug that helps to limit the loss of blood and promote healing. Normal aggregation depends upon the release of platelet granules, normal membrane receptors on the platelets, and a normal level of plasma fibrinogen.

A defect in platelet aggregation will result in a prolonged bleeding time. Abnormal platelet aggregation may be caused by an inherited bleeding disorder (e.g., von Willebrand's disease); certain acquired bleeding disorders that occur as a consequence of another disease or condition (e.g. connective tissue or collagen disorders, kidney or liver failure, leukemia, myeloma); or by certain medications (e.g., aspirin, heparin, and NSAIDs).

The platelet aggregation test uses an instrument called an aggregometer to measure the optical density (turbidity) of platelet-rich plasma. The plasma should stand at room temperature for 30 minutes prior to the assay, but the tests should be performed within three hours of sample collection. Several different substances called agonists are used in the test. These agonists include adenosine diphosphate (ADP), epinephrine, thrombin, collagen, arachidonic acid, and ristocetin. The addition of an agonist to a plasma sample causes the platelets to aggregate, making the fluid more transparent. The aggregometer then measures the increased light transmission through the specimen. Some aggregometers measure platelet aggregation of whole blood. These instruments use two electrodes that measure impedance (resistance to current flow). When platelet aggregation occurs, the platelets collect at the electrode surface, increasing the impedance at the electrode.

Some inherited platelet disorders that can be differentiated by the aggregation response to different agonists include Glanzmann's thrombasthenia, von Willebrand's disease, and Bernard-Soulier disease. Glanzmann's thrombasthenia is an autosomal dominant condition. The platelet count is normal, but the bleeding time is prolonged. Aggregation is normal with ristocetin, but is abnormal with all the other agonists. Von Willebrand's disease is the most common inherited bleeding disorder. It is associated with an increased bleeding and clotting time and is caused by a deficiency of two coagulation factors, factor VIII and von Willebrand factor. The platelet count may be normal or low. It may be inherited as autosomal dominant or autosomal recessive forms. The aggregation profile in von Willebrand's disease is the reverse of that seen in Glanzmann's thrombasthenia. The aggregation with ADP, collagen, thrombin, epinephrine, and arachidonic acid is normal, but is abnormal with ristocetin. Bernard-Soulier disease is an autosomal recessive condition associated with large platelets and an abnormal bleeding time. The platelet count may be normal or low. It produces the same profile as von Willebrand's disease, but the abnormal aggregation with ristocetin cannot be reversed by addition of von Willebrand factor.

Preparation

The test requires a blood sample collected in sodium citrate. The patient should either avoid food and drink altogether for eight hours before the test, or eat only nonfat foods. High levels of fatty substances in the blood can affect test results.

Because the use of aspirin and/or aspirin compounds can directly affect test results, the patient should avoid

KEY TERMS

Aggregation—The blood cell clumping process that is measured in the platelet aggregation test.

Agonist—A chemical that is added to the blood sample in the platelet aggregation test to stimulate the clumping process.

Platelets—Small, round, anucleate disk-shaped blood cells that are involved in clot formation. The platelet aggregation test measures the clumping ability of platelets.

Turbidity—The cloudiness or lack of transparency of a solution.

von Willebrand's disease (vWD)—An autosomal dominant inherited lifelong bleeding disorder caused by a defective gene. The gene defect results in a decreased blood concentration of a substance called von Willebrand factor (vWF). Tests for vWF (and coagulation Factor VIII) are used along with platelet aggregation tests to diagnose this disorder.

these medications for at least one week before the test. The test should be completed within three hours of specimen collection. Specimens that sit at room temperature for four hours or more may lose the ability to aggregate.

Aftercare

Because the platelet aggregation test is ordered when some type of bleeding problem is suspected, the patient should be cautioned to watch the puncture site for signs of additional bleeding.

Complications

Risks for this test are minimal in normal individuals. Patients with bleeding disorders, however, may have prolonged bleeding from the puncture wound; or the formation of a bruise (ecchymosis) or blood clot (hematoma) in or under the skin where the blood was withdrawn.

Results

The platelet aggregation test produces a graph in which the x-axis is time and the y-axis is percent transmission. The platelet aggregation curve will vary depending upon the reaction conditions (e.g., pH of the platelet-rich plasma), agonist used, and agonist concentration. The typical aggregation curve is biphasic. It is characterized by an initial increase in light transmission (primary wave) followed by a plateau and a second steeper increase in light transmission (secondary wave) caused by irreversible platelet clumping. An exception to this pattern is aggregation with collagen, which produces a single steep increase in light transmission preceded by a lag phase. An abnormal response can be a decreased or absent primary wave, secondary wave, or both.

Abnormal platelet aggregation can be found in such inherited disorders as von Willebrand's disease, as well as in some connective tissue disorders. Abnormal aggregation can also occur in leukemia or myeloma; with medications taken during recent heart/lung bypass or kidney dialysis; and after taking certain other drugs.

Health care team roles

The physician orders the test. The specimen will be drawn by a nurse or phlebotomist, and transported to the laboratory. The clinical laboratory scientist/medical technologist will perform the test. The results are interpreted by a hematopathologist.

BOOKS

Practical Diagnosis of Hematologic Disorders, edited by Carl R. Kjeldsberg, MD. Chicago, IL: ASCP Press, 2000.

Miller, Jonathan L. "Blood Platelets." In Clinical Diagnosis and Management by Laboratory Methods, edited by John B. Henry. Philadelphia: W. B. Saunders Company, 2001.

Majerus, Philip W. "Platelets." In The Molecular Basis of Blood Diseases, edited by George Stamatoyannopoulos et al. Philadelphia: W. B. Saunders Company, 2001.

Pagana, Kathleen Deska, and Timothy James Pagana. Mosby's Diagnostic and Laboratory Test Reference. St. Louis, MO: Mosby-Year Book, Inc., 1998.

Mark A. Best