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. 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 normal blood flow. Once the blood has been collected, the needle is removed. 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 small glass tube (pipette), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if bleeding continues.

A technologist, blood specialist (hematologist), or disease specialist (pathologist) looks at the blood under a microscope. Or, the blood may be examined by an automated "calculator." 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 it may be done when the health care provider suspects an abnormality of any type of blood cell.

• Normal differential
• Normal appearance of cells

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

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

The presence of target cells may indicate:

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

• Increased osmotic fragility
• Hereditary spherocytosis
• Autoimmune hemolytic anemia

The presence of elliptocytes may indicate hereditary elliptocytosis.

The presence of schistocytes may indicate:

• Microangiopathic hemolytic anemia
• Disseminated intravascular coagulation
• Thrombotic thrombocytopenic purpura (TTP)
• Artificial heart valve (aortic valve prosthesis)
• 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 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.

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

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.

Dhaliwal G, Cornett PA, Tierney LM. Hemolytic anemia. Am Fam Physician. June 2004;69:2599-2566.

Kaplan RN, Bussel JB. Differential diagnosis and management of thrombocytopenia in childhood. Pediatr Clin North Am. August 2004;51:1109-1140.