Health Encyclopedia

Platelet count

Definition

A platelet count is a test to measure how many blood cells, called platelets, you have in your blood. Platelets help the blood clot.

Alternative Names

Thrombocyte count

How the test is performed

Blood is drawn from a vein, often on the inside of the elbow. 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 a vial or a syringe. The band and needle are removed, and the puncture site is covered to stop any bleeding.

For infants and young children, 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

No preparation is necessary.

How the test will feel

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

The number of platelets in your blood can be affected by many diseases. Platelets may be counted to monitor or diagnose diseases, or identify the cause of excess bleeding.

Normal Values

150,000 to 400,000/mm3

What abnormal results mean

If the number of platelets is below normal (thrombocytopenia), the cause may be:

Cancer chemotherapy
Disseminated intravascular coagulation (DIC)
Hemolytic anemia
Hypersplenism
Idiopathic thrombocytopenic purpura (ITP)
Leukemia
Massive blood transfusion
Prosthetic heart valve

If the number is higher than normal (thrombocytosis), the cause may be:

Anemia
Certain malignancies
Early CML
Polycythemia vera
Post-splenectomy syndrome
Primary thrombocytosis

A platelet count may be performed under many conditions and to assess many diseases.

What the risks are

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

Drugs that can lower platelet counts include chemotherapy drugs, 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.

References

McPherson RA and Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: WB Saunders; 2007:477-8.

Hoffman R, Benz Jr. EJ, Shattil SJ, et al., eds. Hematology: Basic Principles and Practice. 4th ed. Philadelphia, Pa: Churchill Livingston; 2005:2001.

Review Date:3/8/2007
Reviewed By:Mark Levin, MD, Hematologist and Oncologist, Newark, NJ. Review provided by VeriMed Healthcare Network.

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