platelet aggregation test

Platelet Aggregation Test 

Definition

Platelets are disk-shaped blood cells that are also called thrombocytes. They play a major role in the blood-clotting process. The platelet aggregation test is a measure of platelet function.

Purpose

The platelet aggregation test aids in the evaluation of bleeding disorders by measuring the rate and degree to which platelets form a clump (aggregate) after the addition of a chemical that stimulates clumping (aggregation).

Precautions

There are many medications that can affect the results of the platelet aggregation test. The patient should discontinue as many 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 and clump together to form a plug, or aggregate, that helps to limit the loss of blood and promote healing.

Inherited bleeding disorders (e.g., hemophilia or von Willebrand's disease) and acquired bleeding problems that occur because of another disorder or a medication can affect the number of platelets and their level of function. When these problems are present, the result is a drop in platelet aggregation and a lengthened bleeding time.

The platelet aggregation test uses a machine called an aggregometer to measure the cloudiness (turbidity) of blood plasma. Several different substances called agonists are used in the test. These agonists include adenosine diphosphate, epinephrine, thrombin, collagen, and ristocetin. The addition of an agonist to a plasma sample causes the platelets to clump together, making the fluid more transparent. The aggregometer then measures the increased light transmission through the specimen.

Preparation

The test requires a blood sample. 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 these medications for two weeks before the test. If the patient must take aspirin and the test cannot be postponed, the laboratory should be notified and asked to verify the presence of aspirin in the blood plasma. If the results are abnormal, aspirin use must be discontinued and the test repeated in two weeks.

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.

Risks

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 (hematoma) under the skin where the blood was withdrawn.

Normal results

The normal time for platelet aggregation varies somewhat depending on the laboratory, the temperature, the shape of the vial in which the test is performed, and the patient's response to different agonists. For example, the difference between the response to ristocetin and other products should be noted because ristocetin triggers aggregation through a different mechanism than other agonists.

Abnormal results

Prolonged platelet aggregation time can be found in such congenital disorders as hemophilia and von Willebrand's disease, as well as in some connective tissue disorders. Prolonged aggregation times can also occur in leukemia or myeloma; after recent heart/lung bypass or kidney dialysis; and after taking certain drugs.

Resources

Books

Pagana, Kathleen Deska. Mosby's Manual of Diagnosticand Laboratory Tests. St. Louis: Mosby, Inc., 1998.

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.

Hemophilia — An inherited bleeding disorder caused by a deficiency of factor VIII, one of a series of blood proteins essential for blood clotting.

Platelets — Small, round, 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 — An inherited lifelong bleeding disorder caused by an abnormal gene, similar to hemophilia. The gene defect results in a decreased blood concentration of a substance called von Willebrand's factor (vWF).

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platelet aggregation test

n.

A test of the ability of platelets to adhere to one another and form a hemostatic plug.

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platelet aggregation test,

a blood test that can detect diseases that affect either platelet number or function, thereby prolonging bleeding time. This test can provide information about prolonged platelet aggregation times, connective tissue disorders such as lupus erythematosus, recent cardiopulmonary or dialysis bypass, various myeloproliferative diseases, primary protein disease, von Willebrand's disease, uremia, and congenital disorders such as Wiskott-Aldrich syndrome, Bernard-Soulier syndrome, and glycogen storage disorders.

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platelet

a small disk or platelike structure, the smallest of the formed elements in blood. Blood platelets (called also thrombocytes) are disk-shaped, non-nucleated blood elements with a very fragile membrane; they tend to adhere to uneven or damaged surfaces. They average about 250,000 per cubic millimeter of blood and are formed in the red bone marrow by fragmentation of megakaryocytes, the largest of the bone marrow cells. Platelet production is controlled by a hormone, thrombopoietin, and regulatory lymphocytes acting at the stem cell level. At any given time about one-third of the total blood platelets can be found in the spleen; the remaining two-thirds are in the circulating blood.

The functions of platelets are related to the clotting of blood. Because of their adhesion and aggregation capabilities platelets can occlude small breaks in blood vessels and prevent the escape of blood. Platelets which have adhered to exposed collagen in damaged vessels release ADP in milliseconds which in turn initiates the synthesis of thromboxane A₂, a very potent prostaglandin which causes platelet aggregation and localized vasoconstriction. Fibrinogen, factors V and VIII, calcium ions, platelet phospholipid (PF-3), associated with the platelet membrane are also released. Substances contained within the platelet granules such as thromboglobulin, heparin neutralizing activity (PF-4) mitogens such as platelet derived growth factor, thrombospondin, ADP, serotonin and calcium ions are also released by aggregated platelets.

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platelet-activating factor (PAF)

see platelet-activating factor.

platelet adhesion

the adherence of platelets to any area with damaged blood vessels; an important component of hemostasis.

platelet aggregation

the progressive accumulation of platelets, attracted by other platelets once adhesion begins. Thromboxane A₂ causes irreversible platelet aggregation.

platelet aggregation test

a known platelet aggregating factor such as collagen, ADP or thrombin is added to a suspension of the platelets under test and the degree of aggregation measured by decrease in turbidity of the suspension.

platelet count

may be performed directly (in a hemocytometer chamber) or indirectly (estimating from the stained blood smear by number per field or in comparison to the number of white blood cells), expressed as number of cells per liter of blood.

platelet-derived growth factor

one of three growth factors released by platelets which undergo the release reaction; the growth factors stimulate endothelial cell proliferation.

platelet distribution width (PDW)

an indication of variation in platelet size which can be a sign of active platelet release.

platelet factor 3 (PF-3) test, platelet release test

test the antiplatelet activity of serum; used to detect circulating antiplatelet antibodies. Antibody-antigen reactions involving platelets cause the release of PF-3 from platelets which in turn shortens the contact-activated clotting time of platelet-rich plasma (PRP).

platelet factors

factors important in hemostasis which are contained in or attached to the platelets: platelet factor 1 is adsorbed clotting factor V from the plasma; platelet factor 2 is an accelerator of the thrombin-fibrinogen reaction; platelet factor 3 is a phospholipid with potent procoagulant activity; platelet factor 4 is capable of inhibiting the activity of heparin (heparin neutralizing activity).

mean platelet volume (MPV)

elevated level is an indication of increased megakaryocyte shedding of platelets and decreased level is seen in thrombocytopenia.

platelet plug formation

see platelet aggregation (above).

platelet release reaction

measured by the degree of secondary ADP-mediated aggregation that occurs. This is assessed by the amount of PF-4, PF-3 or serotonin, etc. released.

platelet retention

tested by testing the adhesiveness of a suspension of the subject platelets to a glass bead column or standard size filter.

platelet rich plasma

plasma prepared by centrifugation to separate out red blood cells but not platelets for transfusion.

platelet storage-pool disease

an inherited autosomal thrombopathia in American foxhounds and cats characterized by a deficiency of platelet storage granules.

platelet transfusion

transfusion of fresh, nonchilled whole blood is the usual method of transfusing platelets to an animal with thrombocytopenia.