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

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 the 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 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.

Additional conditions under which the test may be performed:

• Clinical hemoglobin C
• Hairy cell leukemia
• Non-Hodgkin's lymphoma
• Any known or suspected blood disorder
• Monitoring the side effects of chemotherapy

Red 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 differential
• Normal appearance of cells

Abnormal results mean there is an abnormalities in the size, shape, coloring or coating of the red blood cells.

Some abnormalities may be graded on a 4-point scale:

• 1+ means 25% of cells affected
• 2+ means half of cellsare 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
• Liver disease (especially obstructive jaundice)
• Hemoglobin abnormalities (hemoglobinopathies)
• Iron deficiency
• Spleen removal
• Thalassemia

• Autoimmune hemolytic anemia
• Hereditary spherocytosis
• Increased osmotic fragility

The presence of elliptocytes may be a sign of hereditary elliptocytosis.

The presence of fragmented cells (schistocytes) may be due to:

• Microangiopathic hemolytic anemia
• Disseminated intravascular coagulation
• Thrombotic thrombocytopenic purpura (TTP)
• Artificial heart valve (aortic valve prosthesis)
• Hemolytic uremic syndrome (HUS)

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
• Myelofibrosis
• Removal of spleen
• Severe hemolysis
• Thalassemia

• Uremia

• Abetalipoproteinemia
• Severe liver disease

• Myelofibrosis
• Leukoerythroblastic anemia
• Thalassemia major
• Severe iron deficiency

• Post-splenectomy
• Sickle cell anemia
• Myelodysplasia

• G6PD deficiency
• Congenital hemolytic anemia
• Unstable form of hemoglobin (unstable hemoglobin variant)
• Alpha thalassemia

The presence of reticulocytes (more than 2% of total red cells; seen with special stain) may indicate hemolytic anemia or hemorrhage.

The presence of basophilic stippling may indicate:

• Myelofibrosis
• Myelophthisic process
• Lead poisoning

The presence of sickle cells may indicate sickle cell anemia.

The risks associated with having blood drawn are minimal:

• Excessive bleeding
• Fainting or feeling light-headed
• Blood collecting under the skin (hematoma)
• Infection
• Many needle sticks to find veins

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.