A blood smear is a blood test that gives information about the number and shape of blood cells.

This test may be performed as part of a general health exam to help diagnose many illnesses. Or, your doctor may order this test if you have signs of a blood disorder.

Other conditions under which the test may be performed:

• Any known or suspected blood disorder
• Cancer
• Hairy cell leukemia
• Hemoglobinopathies
• Monitoring the side effects of chemotherapy

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

The blood sample is sent to a lab, where the health care professional looks at it under a microscope. Or, the blood may be examined by an automated machine. The smear shows the number and kinds of white blood cells (differential), abnormally shaped blood cells, and gives a rough estimate of white blood cell and platelet counts.

No special preparation is necessary.

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