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.

No preparation is necessary.

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.

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.

150,000 to 400,000/mm3

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

• 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.

• 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

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.

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.