Blood will be drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill 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 tourniquet 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 infants or young children:
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.

After the specimen is collected from an adult or child, a drop of blood is placed on a glass slide. A smear is made and stained to differentiate the various types of WBCs for a manual differential (the cells are counted by a technician, hematologist, or pathologist rather than a computer).

No special preparation is necessary for adults.

For infants and young children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:

• 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)

The differential count can be used to help detect infection, anemia, and leukemia or to follow the progress of treatment.

There are various types of WBCs (also called leukocytes) that normally appear in the blood. The differential determines the relative percentages of the different types of cells in the blood, notes any abnormal appearance of the cells, and the presence of any abnormal immature cells.

• Neutrophils are mainly phagocytic cells (that is, they engulf and destroy invading organisms). They also release some enzymes and substances that affect the function of other cells called cytokines.
• B lymphocytes (B cells) synthesize and secrete antibodies.
• T lymphocytes (T cells) -- one type of T cell is called a helper cell; it secretes proteins that increase B cell function and the functions of cytotoxic T lymphocytes. Cytotoxic T lymphocytes recognize and destroy virus-infected and other abnormal cells.
• Monocytes recognize a variety of microorganisms, especially gram-negative bacteria. Activated monocytes turn into macrophages that can engulf and destroy microorganisms and secrete a variety of cytokines that modulate the activity of other leukocytes.
• Eosinophils and basophils release substances that cause vasoconstriction, smooth muscle contraction, and an increased permeability of small blood vessels. Eosinophils are stimulated by parasites and some bacteria. Basophils are stimulated by allergens.

• Neutrophils: 40% to 60%
• Lymphocytes: 20% to 40%
• Monocytes: 2% to 8%
• Eosinophils: 1% to 4%
• Basophils: 0.5% to 1%
• Band: 0% to 3%

Any infection or acute stress results in increased production of WBCs. This usually entails an increased numbers of cells and an increase in the percent of immature cells (mainly band cells) in the blood. High WBC counts may indicate the presence of an inflammatory and immune response, or it may result from other conditions such as leukemia.

It is important to realize that an abnormal increase in one type of leukocyte can produce an apparent decrease in the percentage of other types.

An increased percentage of neutrophils may indicate:

• Acute infection
• Eclampsia
• Gout
• Myelocytic leukemia
• Rheumatoid arthritis
• Rheumatic fever
• Acute stress
• Thyroiditis
• Trauma

• Aplastic anemia
• Chemotherapy
• Influenza
• Overwhelming bacterial infection
• Radiation therapy

• Chronic bacterial infection
• Infectious hepatitis
• Infectious mononucleosis
• Lymphocytic leukemia
• Multiple myeloma
• Viral infection (such as infectious mononucleosis, mumps, measles)
• Recovery from a bacterial infection

• Chemotherapy
• HIV infection
• Leukemia
• Radiation therapy
• Sepsis

• Chronic inflammatory disease
• Parasitic infection
• Tuberculosis
• Viral infection (for example, infectious mononucleosis, mumps, measles)

• Allergic reaction
• Parasitic infection
• Hodgkin's disease

A decreased percentage of basophils may indicate an acute allergic reaction.

This test may be performed under many other conditions as well.

• 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