This blood test provides information about the number and shape of blood cells by visual inspection.
How the test is performed
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.
How to prepare for the test
No special preparation is necessary.
For infants and children, the preparation you can provide for this test depends on your child’s age and previous experience. For general information regarding how you can prepare your child, see the following topics:
- Infant test or procedure preparation (birth to 1 year)
- Toddler test or procedure preparation (1 to 3 years)
- Preschooler test or procedure preparation (3 to 6 years)
- Schoolage test or procedure preparation (6 to 12 years)
- Adolescent test or procedure preparation (12 to 18 years)
How the test will feel
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.
Why the test is performed
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
What abnormal results mean
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)
- Deficiency in an enzyme called lecithin cholesterol acyl transferase
- Deficiency in iron
The presence of spherocytes may indicate:
- 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)
The presence of normoblasts may indicate:
- Severe hemolysis
- Leukoerythroblastic anemia (myelophthisis process)
- Cancer that has spread to bone marrow
- Miliary tuberculosis
- Erythroblastosis fetalis
The presence of burr cells (echinocytes) may indicate:
- Artifact (induced result) from specimen preparation
The presence of spur cells (acanthocytes) may indicate:
- Severe liver disease
The presence of teardrop cells may indicate:
- Leukoerythroblastic anemia
- Thalassemia major
- Severe iron deficiency
The presence of Howell-Jolly bodies may indicate:
- Sickle cell anemia
The presence of Heinz bodies (with crystal violet stain) may indicate:
- 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:
- 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
What the risks are
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.
by Janet G. Derge, M.D.