The complete blood count (CBC) measures the following:

• The number of red blood cells (RBCs)
• The number of white blood cells (WBCs)
• The total amount of hemoglobin in the blood
• The fraction of the blood composed of red blood cells (hematocrit)
• The mean corpuscular volume (MCV) -- the size of the red blood cells

CBC also includes information about the red blood cells that is calculated from the other measurements:

• MCH (mean corpuscular hemoglobin)
• MCHC (mean corpuscular hemoglobin concentration)

Adult or child:

Blood is 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 air-tight 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.

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 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.

Adult:

There is no special preparation needed.

Infants and 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/procedure preparation (birth to 1 year)
• Toddler test/procedure preparation (1 to 3 years)
• Preschooler test/procedure preparation (3 to 6 years)
• Schoolage test/procedure preparation (6 to 12 years)
• Adolescent test/procedure preparation (12 to 18 years)

The CBC is a screening test, used to diagnose and manage numerous diseases. The results can reflect problems with fluid volume (such as dehydration) or loss of blood. It can show abnormalities in the production, life span, and rate of destruction of blood cells. It can reflect acute or chronic infection, allergies, and problems with clotting.

MCV, MCH, and MCHC values reflect the size and hemoglobin concentration of individual cells and are useful in the diagnosis of various types of anemia.

• RBC (varies with altitude):
  • male: 4.7 to 6.1 million cells/mcL
  • female: 4.2 to 5.4 million cells/mcL
• WBC: 4,500 to 10,000 cells/mcL
• Hematocrit (varies with altitude):
  • male: 40.7 to 50.3 %
  • female: 36.1 to 44.3 %
• Hemoglobin (varies with altitude):
  • male: 13.8 to 17.2 gm/dL
  • female: 12.1 to 15.1 gm/dL
• MCV: 80 to 95 femtoliter
• MCH: 27 to 31 pg/cell
• MCHC: 32 to 36 gm/dL

• Low oxygen tension in the blood
  • congenital heart disease
  • cor pulmonale
  • pulmonary fibrosis
• Polycythemia vera
• Dehydration (such as from severe diarrhea)
• Renal (kidney) disease with high erythropoietin production

• Blood loss
  • Anemia (various types)
  • Hemorrhage
• Bone marrow failure (for example, from radiation, toxin, fibrosis, tumor)
• Erythropoietin deficiency (secondary to renal disease)
• Hemolysis (RBC destruction)
• Leukemia
• Multiple myeloma
• Malnutrition (nutritional deficiencies of iron, folate, vitamin B12, or vitamin B6)

• Bone marrow failure (for example, due to infection, tumor or fibrosis)
• Presence of cytotoxic substance
• Autoimmune/collagen-vascular diseases (such as lupus erythematosus)
• Disease of the liver or spleen
• Radiation exposure

• Infectious diseases
• Inflammatory disease (such as rheumatoid arthritis or allergy)
• Leukemia
• Severe emotional or physical stress
• Tissue damage (SUCH AS burns)

• Anemia (various types)
• Blood loss (hemorrhage)
• Bone marrow failure (for example, due to radiation, toxin, fibrosis, tumor)
• Hemolysis (RBC destruction) related to transfusion reaction
• Leukemia
• Malnutrition or specific nutritional deficiency
• Multiple myeloma
• Rheumatoid arthritis

• Dehydration
  • Burns
  • Diarrhea
• Polycythemia vera
• Low oxygen tension (smoking, congenital heart disease, living at high altitudes)

• Anemia (various types)
• Blood loss

• 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

Red blood cells transport hemoglobin which, in turn, transports oxygen. The amount of oxygen received by tissue depends on the amount and function of RBCs and hemoglobin. The MCV, MCH, and MCHC reflect the size and hemoglobin content of individual red blood cells.

The hematocrit is an expression of the proportion of whole blood that is composed of red blood cells (since the contribution by the WBCs is almost negligible). The hematocrit is a compound measure of RBC number and size.

WBCs are mediators of inflammation and the immune response. There are various types of WBCs that normally appear in the blood:

• Neutrophils (polymorphonuclear leukocytes)
• Band cells (slightly immature neutrophils)
• T-type lymphocytes (T cells)
• B-type lymphocytes (B cells)
• Monocytes
• Eosinophils
• Basophils
  

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.