Platelet count

Alternative names 
Thrombocyte count

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

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
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:

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

Normal Values

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

What abnormal results mean

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:

  • cancerChemotherapy  
  • 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

  • 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 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.

Johns Hopkins patient information

Last revised: December 3, 2012
by Martin A. Harms, M.D.

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.