A blood smear is a blood test that gives information about the number and shape of blood cells.
How the test is performed:
A blood sample is needed. For information on how this is done, see: Venipuncture
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.
How to prepare for the test:
No special preparation is necessary.
How the test will feel:
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.
Why the test is performed:
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
The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples.Talk to your doctor about the meaning of your specific test results.
What abnormal results mean:
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.
What the risks are:
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.
References: Bain BJ. The Peripheral blood smear. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 160.