Complete blood count

Alternative names
CBC

Definition

A complete blood count (CBC) test 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)

The platelet count is also usually included in the CBC.

How the test is performed

Blood is drawn from a vein, usually from 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 cause the vein to swell 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 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.

In 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. A bandage may be applied to the puncture site if there is any bleeding.

How to prepare for the test

There is no special preparation needed.

The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. 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, you may feel moderate pain, though most people feel only a prick or a stinging sensation. Afterward there may be some throbbing or bruising.

Why the test is performed

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.

Normal Values

     
  • RBC (varies with altitude):       o Male: 4.7 to 6.1 million cells/mcL       o Female: 4.2 to 5.4 million cells/mcL  
  • WBC: 4,500 to 10,000 cells/mcL  
  • Hematocrit (varies with altitude):       o Male: 40.7 to 50.3 %       o Female: 36.1 to 44.3 %  
  • Hemoglobin (varies with altitude):       o Male: 13.8 to 17.2 gm/dL       o 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

(cells/mcL = cells per microliter; gm/dL = grams per deciliter; pg/cell = picograms per cell)

What abnormal results mean
High numbers of RBCs may indicate:

     
  • Low oxygen tension in the blood       o Congenital heart disease       o Cor pulmonale       o Pulmonary fibrosis  
  • Polycythemia vera  
  • Dehydration (such as from severe diarrhea)  
  • Renal (kidney) disease with high erythropoietin production

Low numbers of RBCs may indicate:

     
  • Blood loss       o Anemia (various types)       o 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)

Low numbers of WBCs (leukopenia) may indicate:

     
  • 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

High numbers of WBCs (leukocytosis) may indicate:

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

Low hematocrit may indicate:

     
  • 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

High hematocrit may indicate:

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

Low hemoglobin values may indicate:

     
  • Anemia (various types)  
  • Blood loss

The test may be performed under many different conditions and in the assessment of many different diseases.

What the risks are

     
  • 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

Special considerations

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.

Johns Hopkins patient information

Last revised: December 8, 2012
by Armen E. Martirosyan, M.D.

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