MedlinePlus Medical Encyclopedia: Platelet count

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Platelet count

Contents of this page:
Alternative names Definition How the test is performed How to prepare for the test How the test will feel	Why the test is performed Normal Values What abnormal results mean What the risks are Special considerations

Alternative names

Thrombocyte count

Definition Return to top

This is a test to measure the number of platelets in blood.

Platelets are necessary for normal blood clotting (hemostasis). Most important, they aggregate (clump together) to plug small holes in damaged blood vessels. They also activate factor VIII (a component of the coagulation cascade) and release phospholipids necessary for coagulation.

How the test is performed Return to top

Blood is drawn from a vein, usually on inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test Return to top

No special preparation is necessary.

Infants and children:
The preparation you can provide for this test depends on your child's age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following:

infant test or procedure preparation (birth to 1 year)
toddler test or procedure preparation (1 to 3 years)
preschooler test or procedure preparation (3 to 6 years)
schoolage test or procedure preparation (6 to 12 years)
adolescent test or procedure preparation (12 to 18 years)

How the test will feel Return to top

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed Return to top

The platelet count can be affected by many disease states. It may also be measured to assess the cause of excess bleeding.

Normal Values Return to top

150,000-400,000/mm3 (millimeters cubed)

What abnormal results mean Return to top

A diminished number of platelets (below the lower limit of normal) is called thrombocytopenia and an elevated number (above the upper limit of normal) is called thrombocytosis.

A decreased numbers of platelets may be associated with:

cancer chemotherapy
disseminated intravascular coagulation (DIC)
hemolytic anemia
hypersplenism
idiopathic thrombocytopenic purpura (ITP)
leukemia
prosthetic heart valve
massive blood transfusion

Increased numbers of platelets may be associated with:

polycythemia vera
post-splenectomy syndrome
primary thrombocytosis
certain malignancies
early CML

This test may be performed under many conditions and in the assessment of many disease states.

What the risks are Return to top

excessive bleeding
fainting or feeling light-headed
hematoma (blood accumulating under the skin)
infection (a slight risk any time the skin is broken)
multiple punctures to locate veins

Special considerations Return to top

Drugs that can decrease platelet counts include chemotherapeutic agents, chloramphenicol, colchicine, H2 blocking agents, heparin, hydralazine, indomethacin, isoniazid, quinidine, streptomycin, sulfonamide, thiazide diuretic, and tolbutamide.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Update Date: 6/1/2003

Updated by: Marcia S. Brose, M.D., Ph.D., Division of Hematology/Oncology, Hospital of the University of Pennsylvania, Philadelphia, PA. Review provided by VeriMed Healthcare Network.

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