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.
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:
Red blood cells normally are the same in size and color and have a lighter-colored area in the center. The blood smear is considered normal if there is:
- Normal appearance of cells
- Normal white blood cell differential
Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
Abnormal results mean there is an abnormality in the size, shape, color, or coating of the red blood cells.
Some abnormalities may be graded on a 4-point scale:
- 1+ means 25% of cells are affected
- 2+ means half of cells are affected
- 3+ means 75% of cells are affected
- 4+ means all of the cells are affected
The presence of target cells may be due to:
The presence of sphere-shaped cells (spherocytes) may be due to:
The presence of elliptocytes may be a sign of hereditary elliptocytosis or hereditary ovalocytosis.
The presence of fragmented cells (schistocytes) may be due to:
The presence of a type of immature red blood cell called a normoblast may be due to:
The presence of burr cells (echinocytes) may indicate:
The presence of spur cells (acanthocytes) may indicate:
The presence of teardrop-shaped cells may indicate:
- Leukoerythroblastic anemia
- Severe iron deficiency
The presence of Howell-Jolly bodies may indicate:
The presence of Heinz bodies may indicate:
- Alpha thalassemia
- Congenital hemolytic anemia
- G6PD deficiency
- Unstable form of hemoglobin
The presence of slightly immature red blood cells (reticulocytes) may indicate:
- Anemia with bone marrow recovery
- Hemolytic anemia
The presence of basophilic stippling may indicate:
The presence of sickle cells may indicate sickle cell anemia.
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.
Other risks associated with having blood drawn are slight but may include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
The accuracy of this test depends, in part, on the experience of the person looking at the sample. Experienced cell examiners can get a lot of information from the blood smear.