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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 CBC test also provides information about the following measurements:

  • Average red blood cell size (MCV)
  • Hemoglobin amount per red blood cell (MCH)
  • The amount of hemoglobin relative to the size of the cell (hemoglobin concentration) per red blood cell (MCHC)

The platelet count is also usually included in the CBC.

See also:

Alternative Names

Complete blood count

How the Test is Performed

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

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, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How to Prepare for the Test

There is no special preparation needed.

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 test may be performed under many different conditions and to assess many different symptoms or diseases. The results can reflect problems with fluid volume (such as dehydration) or loss of blood.

The test can reveal problems with RBC production and destruction, or help diagnose infection, allergies, and problems with blood clotting.

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

Normal Results

  • RBC count (varies with altitude):
    • Male: 4.7 to 6.1 million cells/mcL
    • Female: 4.2 to 5.4 million cells/mcL
  • WBC count: 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

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

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What Abnormal Results Mean

High numbers of RBCs or a high hematocrit may be due to:

Low numbers of RBCs or low hematocrit indicates anemia, which can result from:

  • Autoimmune/collagen-vascular diseases such as lupus erythematosus or rheumatoid arthritis
  • Blood loss (hemorrhage)
  • Bone marrow failure (for example, from radiation, infection, or tumor)
  • Erythropoietin deficiency (usually secondary to kidney disease)
  • Hemolysis (red blood cell destruction)
  • Leukemia
  • Malnutrition (nutritional deficiencies of iron, folate, vitamin B12, or vitamin B6)
  • Multiple myeloma

A lower-than-normal white blood cell count is called leukopenia. A decreased WBC count may be due to:

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

High numbers of WBCs is called leukocytosis. It can result from:

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

Low hemoglobin values may indicate:

  • Anemia (various types)
  • Blood loss

Risks

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

  • Excessive bleeding
  • Fainting or feeling light-headed
  • Hematoma (blood accumulating under the skin)
  • Infection (a slight risk any time the skin is broken)

Considerations

RBCs transport hemoglobin which, in turn, carries oxygen. The amount of oxygen received by body tissues depends on the amount and function of RBCs and hemoglobin.

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

References

Newland J. The peripheral blood smear. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 161.

Zuckerman K. Approach to the anemias. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 162.


Review Date: 3/4/2010
Reviewed By: A.D.A.M. Editorial Team: David Zieve, MD, MHA, David R. Eltz. Previously reviewed by David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine (2/9/2010).
The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. 1997- A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.
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