This blood test provides information about the number and shape of blood cells by visual inspection.

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to swell with blood.

A needle is inserted into the vein, and the blood is collected in an airtight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

The blood is examined under a microscope. It may be examined by an automated "calculator," a technologist, a hematologist, or a pathologist. The smear shows the relative number and kinds of white blood cells (differential), abnormalities in the shape of all types of blood cells, and 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, while 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 examination, to help in the diagnosis of many illnesses, or when an abnormality of any type of blood cell is suspected.

• Normal differential
• Normal appearance of cells

Red cell abnormalities such as size, shape, hypochromia (lack of color), polychromatophilia (multiple colors), rouleaux (nonspecific coating by serum globulins) are determined. Some abnormalities may be graded on a 4 point scale:

• 1+: 25%
• 2+: 50%
• 3+: 75%
• 4+: 100% of cells affected

The presence of target cells may indicate:

• Decreased osmotic fragility
• Liver disease (especially obstructive jaundice)
• Hemoglobinopathies (hemoglobin abnormalities)
• Thalassemia
• Post-splenectomy
• Deficiency in an enzyme called lecithin cholesterol acyl transferase
• Deficiency in iron

• Increased osmotic fragility
• Hereditary spherocytosis
• Immunohemolytic anemia

The presence of elliptocytes may indicate hereditary elliptocytosis.

The presence of schistocytes may indicate:

• Microangiopathic hemolytic anemia
• Disseminated intravascular coagulation (may be due to gram-negative sepsis or malignancy)
• Thrombotic thrombocytopenic purpura (TTP)
• Aortic valve prosthesis (artificial heart valve)
• Hemolytic uremic syndrome (HUS)

• Severe hemolysis
• Leukoerythroblastic anemia (myelophthisis process)
• Myelofibrosis
• Cancer that has spread to bone marrow
• Thalassemia
• Post-splenectomy
• Miliary tuberculosis
• Erythroblastosis fetalis

• Artifact (induced result) from specimen preparation
• Uremia

• Severe liver disease
• Abetalipoproteinemia

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

• Post-splenectomy
• Sickle cell anemia
• Myelodysplasia

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

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

The risks associated with having blood drawn are minimal:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

The accuracy of this test depends, in part, on the experience of the person examining the sample. Experienced cell examiners can determine significant amounts of information from the blood smear.