A blood smear is a blood test that gives information about the number and shape of blood cells.
How the Test is Performed
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.
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:
- Any known or suspected blood disorder
- Hairy cell leukemia
- Monitoring the side effects of chemotherapy
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.
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:
- Decreased osmotic fragility
- Deficiency of an enzyme called lecithin cholesterol acyl transferase
- Hemoglobin abnormalities (hemoglobinopathies)
- Iron deficiency
- Liver disease
- Spleen removal
The presence of sphere-shaped cells (spherocytes) may be due to:
The presence of fragmented cells (schistocytes) may be due to:
- Artificial heart valve
- Disseminated intravascular coagulation
- Hemolytic uremic syndrome (HUS)
- Microangiopathic hemolytic anemia
- Thrombotic thrombocytopenic purpura (TTP)
The presence of a type of immature red blood cell called a normoblast may be due to:
- Cancer that has spread to bone marrow
- Erythroblastosis fetalis
- Leukoerythroblastic anemia (myelophthisis process)
- Miliary tuberculosis
- Removal of spleen
- Severe hemolysis
The presence of burr cells (echinocytes) may indicate:
The presence of spur cells (acanthocytes) may indicate:
- Severe liver disease
The presence of teardrop-shaped cells may indicate:
- Leukoerythroblastic anemia
- Severe iron deficiency
- Thalassemia major
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:
- Lead poisoning
- Myelophthisic process
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.
Newland J. The peripheral blood smear. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 161.
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.