Complete Blood Count

Also known as: CBC, Hemogram, CBC with differential
Related tests: Blood smear, Hemoglobin, Hematocrit, Red blood cell (RBC) count, White blood cell (WBC) count, White blood cell differential count, Platelet count

The Test
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How is it used?
The CBC is used as a broad screening test to check for such disorders as anemia, infection, and many other diseases. It is actually a panel of tests that examines different parts of the blood and includes the following:

    - White blood cell (WBC) count is a count of the actual number of white blood cells per
      volume of blood. Both increases and decreases can be significant.

    - White blood cell differential looks at the types of white blood cells present. There are
      five different types of white blood cells, each with its own function in protecting us
      from infection. The differential classifies a person's white blood cells into each type:
      neutrophils (also known as segs, PMNs, grans), lymphocytes, monocytes, eosinophils,
      and basophils.

    - Red blood cell (RBC) count is a count of the actual number of red blood cells per volume
      of blood. Both increases and decreases can point to abnormal conditions.

     -
Hemoglobin measures the amount of oxygen-carrying protein in the blood.

     -
Hematocrit measures the amount of space red blood cells take up in the blood. It is
      reported as a percentage.


     -
The platelet count is the number of platelets in a given volume of blood. Both increases
      and decreases can point to abnormal conditions of excess bleeding or clotting. Mean
      platelet volume (MPV) is a machine-calculated measurement of the average size of your
      platelets. New platelets are larger, and an increased MPV occurs when increased
      numbers of platelets are being produced. MPV gives your doctor information about
      platelet production in your bone marrow.


     -
Mean corpuscular volume (MCV) is a measurement of the average size of your RBCs. The
      MCV is elevated when your RBCs are larger than normal (macrocytic), for example in   
      anemia caused by vitamin B12 deficiency. When the MCV is decreased, your RBCs are
      smaller than normal (microcytic), such as is seen in iron deficiency anemia or
      thalassemias.


     -
Mean corpuscular hemoglobin (MCH) is a calculation of the amount of oxygen-carrying
      hemoglobin inside your RBCs. Since macrocytic RBCs are larger than either normal or
      microcytic RBCs, they would also tend to have higher MCH values.


     -
Mean corpuscular hemoglobin concentration (MCHC) is a calculation of the concentration
     of hemoglobin inside the RBCs. Decreased MCHC values (hypochromia) are seen in
     conditions where the hemoglobin is abnormally diluted inside the red cells, such as in
     iron deficiency anemia and in thalassemia. Increased MCHC values (hyperchromia) are
     seen in conditions where the hemoglobin is abnormally concentrated inside the red cells,
     such as in hereditary spherocytosis, a relatively rare congenital disorder.


     -
Red cell distribution width (RDW) is a calculation of the variation in the size of your
      RBCs. In some anemias, such as pernicious anemia, the amount of variation
      (anisocytosis) in RBC size (along with variation in shape - poikilocytosis) causes an
      increase in the RDW.


When is it ordered?
The CBC is a very common test that used to be ordered on every person during his or her yearly physical. While it is not run quite as frequently now, it is still used routinely to screen for, help diagnose, and to monitor a variety of conditions. Many patients will have baseline CBC tests to help determine their general health status. If they are healthy and they have cell populations that are within normal limits, then they may not require another CBC until their health status changes or until their doctor feels that it is necessary.

If a patient is having symptoms associated with anemia, such as fatigue or weakness, or has an infection, inflammation, bruising, or bleeding, then the doctor may order a CBC to help diagnose the cause. Significant increases in WBCs may help confirm that an infection is present and suggest the need for further testing to identify its cause. Decreases in the number of RBCs (anemia) can be further evaluated by changes in size or shape of the RBCs to help determine if the cause might be decreased production, increased loss, or increased destruction of RBCs. A platelet count that is low or extremely high may confirm the cause of excessive bleeding or clotting.


Many conditions will result in increases or decreases in the cell populations. Some of these conditions may require treatment, while others will resolve on their own. Some diseases, such as cancer (and chemotherapy treatment), can affect bone marrow production of cells, increasing the production of one cell at the expense of others or decreasing overall cell production. Some medications can decrease WBC counts, and some vitamin and mineral deficiencies can cause anemia. The CBC test may be ordered by the doctor on a regular basis to monitor these conditions and drug treatments.

What does the test result mean?
NOTE: A standard reference range is not available for this test. Because reference values are dependent on many factors, including patient age, gender, sample population, and test method, numeric test results have different meanings in different labs. Your lab report should include the specific reference range for your test. Lab Tests Online strongly recommends that you discuss your test results with your doctor. For more information on reference ranges, please read Reference Ranges and What They Mean.

The following table explains what increases or decreases in each of the components of the CBC may mean. To download an expanded, printable version of this table, click here.

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