Também conhecido como
Thrombocyte count
Nome formal
Platelet Count
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Este artigo foi modificado pela última vez em 10 de Julho de 2017.
At a Glance
Why Get Tested?
When To Get Tested?
As part of a routine complete blood count (CBC), during episodes of unexplained or prolonged bleeding, or to diagnose/monitor a bone marrow/blood disease
Sample Required?
A blood sample drawn from a vein in your arm
Test Preparation Needed?
What is being tested?
Platelets (thrombocytes) are tiny fragments of cells called megakaryocytes that are made in the bone marrow. These fragments (about 2-3 microns in diameter) are released from the bone marrow to circulate in the blood. They are the first components to be activated when there has been an injury to a blood vessel or tissue. Because they are very "sticky," they begin the formation of a blood clot. The platelet count is a test that determines the number of platelets in your blood.

How is the sample collected for testing?

A blood sample is drawn though a needle placed in a vein in the arm.

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

No test preparation is needed.
Accordion Title
Common Questions
  • How is it used?
    Bleeding disorders or bone marrow diseases, such as leukemia, require the determination of the number of platelets present and/or their ability to function correctly.
  • When is it ordered?
    A platelet count is often ordered as a part of a complete blood count, which may be done at an annual physical examination. It is almost always ordered when a patient has unexplained bruises or takes what appears to be an unusually long time to stop bleeding from a small cut or wound.
  • What does the test result mean?
    In an adult, a normal count is about 150,000 to 450,000 platelets per microliter (x 10–6/Liter) of blood.

    If platelet levels fall below 20,000 per microliter, spontaneous bleeding may occur and is considered a life-threatening risk. Patients who have a bone marrow disease, such as leukemia or another cancer in the bone marrow, often experience excessive bleeding due to a significantly decreased number of platelets (thrombocytopenia). As the number of cancer cells increases in the bone marrow, normal bone marrow cells are crowded out, resulting in fewer platelet-producing cells.

    Low number of platelets may be seen in some patients with long-term bleeding problems (e.g., chronic bleeding stomach ulcers), thus reducing the supply of platelets. Decreased platelet counts may also be seen in patients with Gram-negative sepsis.

    Individuals with an autoimmune disorder (such as lupus or idiopathic thrombocytopenia purpura (ITP), where the body’s immune system creates antibodies that attack its own organs) can cause the destruction of platelets.

    Certain drugs, such as acetaminophen, quinidine, sulfa drugs, digoxin, vancomycin, valium, and nitroglycerine, are just a few that have been associated with drug-induced decreased platelet counts. Patients undergoing chemotherapy or radiation therapy may also have a decreased platelet count. Up to 5% of pregnant women may experience thrombocytopenia at term.

    Platelet consumption may be observed in renal diseases. Thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) are seen in renal failure and can result in fewer circulating platelets in the blood. Similarly, a condition known as splenic sequestration, where platelets pool within the spleen, can also cause a platelet decrease.

    More commonly (up to 1% of the population), easy bruising or bleeding may be due to an inherited disease called von Willebrand’s disease. While the platelets may be normal in number, their ability to stick together is impaired due to a decrease in von Willebrand’s factor, a protein needed to initiate the clotting process. Many cases may go undiagnosed due to the mild nature of the disease. Many cases are discovered when a patient has to have surgery or a tooth extraction or when delivering a baby. However, some cases are more severe and can be aggravated by use of certain drugs, resulting in a life-threatening situation.

    Increased platelet counts (thrombocytosis) may be seen in individuals who show no significant medical problems, while others may have a more significant blood problem called myeloproliferative disorder. Some, although they have an increased number of platelets, may have a tendency to bleed due to the lack of stickiness of the platelets; in others, the platelets retain their stickiness but, because they are increased in number, tend to stick to each other, forming clumps that can block a blood vessel and cause damage, including death (thromboembolism).

  • Is there anything else I should know?
    Living in high altitudes, strenuous exercise, and being post partum may cause increased platelet levels. Drugs that may cause increased platelet levels include estrogen and oral contraceptives.

    Decreased levels may be seen in women before menstruation.

    Other inherited disorders caused by defective platelets or decreased/absent proteins that activate the platelets include Glanzmann’s Thrombasthenia, Bernard-Soulier disease, Chediak-Higashi syndrome, Wiskott-Aldrich syndrome, May-Hegglin syndrome, and Down syndrome. The occurrence of these genetic abnormalities, however, is relatively rare.

  • What do I need to be aware of concerning my platelets if I am having surgery?
    Platelets survive about 8-10 days. If you ingest any aspirin products within this period before surgery, platelet function may be diminished, resulting in excessive bleeding. Consequently, surgery may be canceled.
  • Are there signs or symptoms of high or low platelet levels that I should pay attention to?
    Bruising for no apparent reason, bleeding from the nose, mouth, or rectum also without obvious injury, excessive or prolonged menstrual periods, or the inability to stop a small wound from bleeding within a reasonable period of time may indicate a platelet deficiency.
  • How stable are platelet counts?
    Platelet counts can vary throughout the course of the day; however, counts are highest at midday. Under certain conditions, platelets may clump and falsely appear to be low in number.
View Sources
Henry's Clinical Diagnosis and Management by Laboratory Methods, 21st. Saunders. 2007. p. 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].