Platelet Aggregation [CO003900]

Related Information

• Activated Partial Thromboplastin Time
• Platelet Count
• Platelet Hyperaggregation
• Prothrombin Time
• von Willebrand Factor

Synonyms Aggregometer Test; Platelet Function Studies

Applies to ATP:ADP Ratio; Beta-Thromboglobulin

Test Includes Response to adenosine diphosphate (ADP), epinephrine, collagen, ristocetin, and arachidonic acid

Abstract Platelet aggregation tests are used to assess platelet function.

Patient Preparation Patients should not have aspirin (or any medication containing aspirin) for at least 7 days prior to testing. Nonsteroidal anti-inflammatory drugs or other platelet-inhibiting agents should also be avoided.

Specimen Platelet-rich plasma

Container Three blue top or plastic (sodium citrate) tubes

Collection Routine venipuncture. Immediately invert tubes gently at least 4 times to mix. Deliver tubes immediately to the laboratory at room temperature (platelets are activated at cold temperatures).

Storage Instructions Keep specimen at room temperature and perform test immediately (or within 2 hours, if transportation to a reference laboratory is required). Do not refrigerate or freeze specimen.

Causes for Rejection Specimen received more than 2 hours after collection, specimen clotted, specimen received on ice.

Turnaround Time Less than 1 day

Special Instructions Usually must be scheduled in advance with the laboratory.

Reference Interval >60% of platelets aggregate with each agonist tested. Normally no significant spontaneous aggregation. Normal newborns can have decreased aggregation compared to adults.¹

Use Assess platelet function. When a familial bleeding disorder is suspected, this test is usually not performed unless routine tests are normal (PT, PTT, and platelet count) and von Willebrand tests are normal, because von Willebrand disease is much more common than hereditary platelet dysfunction.

Methodology Citrated plasma is centrifuged at a gentle speed, to draw red and white blood cells into a pellet, leaving platelets suspended in the plasma. Various platelet aggregating agents (agonists) are added to aliquots of the platelet-rich plasma, and the resulting platelet aggregation is measured in an aggregometer.² The aggregometer measures platelet aggregation by monitoring optical density. As platelets aggregate, more light can pass through the specimen. The platelet agonists commonly include arachidonate, ADP, collagen, epinephrine, and ristocetin. One aliquot usually has no platelet agonist added, to assess for spontaneous platelet aggregation.

A rapid, whole blood point-of-care device has been compared to platelet aggregation in monitoring platelet function during antiplatelet therapy.³ Another rapid whole blood platelet function analyzer has been studied in small numbers of patients with various platelet function abnormalities.⁴

Additional Information The most common cause of platelet dysfunction detected in this assay is medications. With aspirin and related compounds, arachidonate aggregation is markedly decreased or absent, and other aggregation tracings may be variably impaired. A variety of other platelet-inhibiting agents, such as ticlopidine, clopidogrel, and abciximab, are known to impair platelet aggregation. A vast number of other medications have been implicated in impaired in vitro platelet aggregation, and the clinical significance, if any, is usually uncertain. If a patient is found to have impaired platelet aggregation in this assay, a careful review of prescribed, as well as over-the-counter medications, is indicated. An on-line literature search for each medication is often informative. Other acquired causes of impaired platelet aggregation include uremia and paraproteinemia (monoclonal gammopathy). Myeloproliferative disorders can impair platelet aggregation, by epinephrine in particular. Hyperaggregation has also been reported with myeloproliferative disorders.

Hereditary platelet dysfunction is far less common than acquired dysfunction. A hereditary disorder may be considered in patients with bleeding histories and no obvious acquired etiology to account for an abnormal platelet aggregation study. Ideally, the aggregation study is repeated on a fresh specimen to determine if the abnormality is reproducible. The presence of the same abnormality in family members supports the diagnosis of a hereditary defect. Platelet storage pool disorders may variably decrease responses to epinephrine, ADP, and occasionally other agonists. Platelet storage pool disorders are characterized by deficiencies in alpha or dense platelet granules. Alpha granules normally store platelet factor 4 (PF4), beta-thromboglobulin, and other substances. Dense granules normally contain ADP, serotonin, and other compounds. Alpha granule deficiency is a rare disorder called "gray platelet syndrome", because platelets appear gray with light microscopy due to a lack of alpha granules. Alpha granules give normal platelets their purple granular appearance. In gray platelet syndrome, platelets are large; thrombocytopenia may be present; and beta-thromboglobulin (a research test) is decreased in platelets but may be elevated in plasma. A research test for dense granule deficiency is the platelet ATP:ADP ratio, which is increased with dense granule deficiency. Uncommonly, patients are deficient in both alpha and dense granules. Rare genetic disorders may underlie some cases of storage pool deficiency, including Hermansky-Pudlak syndrome (dense granule granule deficiency with pulmonary fibrosis and albinism), Chédiak-Higashi syndrome, Wiskott-Aldrich syndrome, or thrombocytopenia with absent radius syndrome.

Glanzmann thrombasthenia is a rare inherited condition in which platelet glycoprotein IIb/IIIa (GPIIb/IIIa) is deficient. GPIIb/IIIa mediates platelet aggregation using fibrinogen to link platelets to each other. Therefore, in Glanzmann thrombasthenia, aggregation is decreased with all agonists (ADP, collagen, epinephrine, arachidonate) except ristocetin. Ristocetin agglutinates platelets using von Willebrand factor and platelet glycoprotein Ib (GPIb). Bernard-Soulier disease is a rare inherited disorder characterized by GPIb deficiency and therefore decreased ristocetin-induced aggregation. Giant platelets and often thrombocytopenia are also present. With severe von Willebrand disease, ristocetin aggregation can be decreased, but most cases of von Willebrand disease are mild and ristocetin aggregation is most often normal. For that reason, platelet aggregation is not used to screen for von Willebrand disease.

Note: The term "agglutination" is often used to describe ristocetin-induced platelet aggregation, because true platelet aggregation links platelets through fibrinogen and GPIIb/IIIa, whereas ristocetin links platelets through von Willebrand factor and GPIb.

Footnotes

1. Michelson AD, "Platelet Function in the Newborn,"Semin Thromb Hemost, 1998, 24(6):507-12.

2. Brown BA, Hematology: Principles and Procedures, 6th ed, Philadelphia, PA: Lea & Febiger, 1993, 271-4.

3. Kereiakes DJ, Broderick TM, Roth EM, et al, "Time Course, Magnitude, and Consistency of Platelet Inhibition by Abciximab, Tirofiban, or Eptifibatide in Patients With Unstable Angina Pectoris Undergoing Percutaneous Coronary Intervention,"Am J Cardiol, 1999, 84(4):391-5.

4. Fressinaud E, Veyradier A, Truchaud F, et al, "Screening for von Willebrand Disease With a New Analyzer Using High Shear Stress: A Study of 60 Cases,"Blood, 1998, 91(4):1325-31.

References

Gahl WA, Brantly M, Kaiser-Kupfer MI, et al, "Genetic Defects and Clinical Characteristics of Patients With a Form of Oculocutaneous Albinism (Hermansky-Pudlak syndrome),"N Engl J Med, 1998, 338(18):1258-64.

Nurden AT, "Inherited Abnormalities of Platelets,"Thromb Haemost, 1999, 82(2):468-80.