Platelet Count

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Also known as: Thrombocyte Count; PLT
Formal name: Platelet Count

At a Glance

Why Get Tested?

To determine the number of platelets in a sample of your blood as part of a health exam; to screen for, diagnose, or monitor conditions that affect the number of platelets, such as a bleeding disorder, a bone marrow disease, or other underlying condition

When to Get Tested?

As part of a routine complete blood count (CBC); when you have episodes of unexplained or prolonged bleeding or other symptoms that may be due to a platelet disorder

Sample Required?

A blood sample drawn from a vein in your arm or by a fingerstick (children and adults) or heelstick (newborns)

Test Preparation Needed?

None

The Test Sample

What is being tested?

Platelets, also called thrombocytes, are tiny cytoplasm fragments of very large cells called megakaryocytes. Platelets, formed from these large cells in the bone marrow and released into the blood to circulate, are essential for normal blood clotting. The platelet count is a test that determines the number of platelets in a person's sample of blood. Normal blood smear with arrows pointing to platelets

When there is an injury to a blood vessel or tissue and bleeding begins, platelets help to stop bleeding in three ways: they adhere to the injury site, clump together (aggregate) with other platelets, and release chemical compounds that stimulate further aggregation of other platelets. These steps result in the formation of a loose platelet plug at the site of the injury in a process called primary hemostasis. At the same time, activated platelets support the coagulation cascade, a series of steps that involves the sequential activation of proteins called clotting factors. This secondary hemostasis process results in the formation of strands of fibrin that weave through the loose platelet plug, form a fibrin net, and compress to form a stable clot that remains in place until the injury has healed. When the clot is no longer needed, other factors break the clot down and remove it.

Each component of primary and secondary hemostasis must be present, activated at the right time, and functioning properly for adequate clotting. If there are insufficient platelets, or if platelets are not functioning normally, a stable clot may not form and a person may be at an increased risk of excessive bleeding.

Platelets survive in the circulation about 8 to 10 days and the bone marrow must continually produce new platelets to replace those that degrade, are used up, and/or are lost through bleeding. Determining the number of platelets in blood with a platelet count can help diagnose a range of disorders having to do with too few or too many platelets.

How is the sample collected for testing?

A blood sample is drawn though a needle placed in a vein in the arm or by a fingerstick (children and adults) or heelstick (newborns).

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, Pp 477-478, 730, 754-757.

Harmening D. Clinical Hematology and Fundamentals of Hemostasis, Fifth Edition, F.A. Davis Company, Philadelphia, Pp 578-589.

(October 1, 2010) National Heart, Lung and Blood Institute. What is aplastic anemia? Available online at http://www.nhlbi.nih.gov/health/health-topics/topics/aplastic/ through http://www.nhlbi.nih.gov. Accessed Feb 2012.

(August 1, 2010) National Heart, Lung and Blood Institute. What are thrombocythemia and thrombocytosis? Available online at http://www.nhlbi.nih.gov/health/health-topics/topics/thrm/ through http://www.nhlbi.nih.gov. Accessed Feb 2012.

(Aug 1, 2010) National Heart, Lung and Blood Institute. What is thrombocytopenia? Available online at http://www.nhlbi.nih.gov/health/health-topics/topics/thcp/ through http://www.nhlbi.nih.gov. Accessed Feb 2012.

(June 1, 2011) National Heart, Lung and Blood Institute. What is Immune Thrombocytopenic Purpura? Available online at http://www.nhlbi.nih.gov/health/health-topics/topics/itp/ through http://www.nhlbi.nih.gov. Accessed Feb 2012.

Riley R, et.al. Automated Hematologic Evaluation. Medical College of Virginia, Virginia Commonwealth University. Available online at http://www.pathology.vcu.edu/education/PathLab/pages/hematopath/pbs.html#Anchor-Automated-47857 through http://www.pathology.vcu.edu. Accessed Feb 2012.

Kasper DL, Braunwald E, Fauci AS, Hauser SL, Longo DL, Jameson JL eds, (2005). Harrison's Principles of Internal Medicine, 16th Edition, McGraw Hill, Pp 340-341, 673-675.

Pagana K, Pagana T. Mosby's Manual of Diagnostic and Laboratory Tests. 3rd Edition, St. Louis: Mosby Elsevier; 2006, Pp 409-412.

(July 17, 2010) Mayo Clinic. Diseases and Conditions, Essential Thrombocythemia. Available online at http://www.mayoclinic.com/health/thrombocythemia/DS01087/DSECTION=tests-and-diagnosis through http://www.mayoclinic.com. Accessed Feb 2012.

(July 16, 2010) Mayo Clinic. Diseases and Conditions, Thrombocytosis. Available online at http://www.mayoclinic.com/health/thrombocytosis/DS01088 through http://www.mayoclinic.com. Accessed Feb 2012.

(March 29, 2011) Thiagarajan P. Overview of Platelet Disorders. Medscape Medical Reference article. Available online at http://emedicine.medscape.com/article/201722-overview#aw2aab6c11 through http://emedicine.medscape.com. Accessed Feb 2012.

Sources Used in Previous Reviews

Henry's Clinical Diagnosis and Management by Laboratory Methods, 21st. Saunders. 2007. Pg. 1414.

Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 732-734.

George JN, Raskob GE, Shah SR. Drug-induced thrombocytopenia: A systematic review of published case reports. Ann Intern Med 129(11):886-890, 1998.

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].