CBC (Complete Blood
Count) Test

CBC (Complete Blood Count) Test: Overview

The Complete Blood Count (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, such as anemia and infection.

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The CBC is usually carried out as part of a routine medical exam, or as determined by a doctor.  It is performed on a blood sample drawn from a vein in the arm or a fingerstick (children and adults) or heelstick (in the case of newborns).

Blood consists of cells suspended in a liquid called plasma.  These cells – the RBCs, WBCs, and platelets – are produced and mature primarily in the bone marrow.  Under normal circumstances, they are released into the bloodstream as needed.

White Blood Cells (WBC)
There are five different types of WBC that the body uses to fight infections or other causes of injury.  These types – neutrophils, lymphocytes, basophils, eosinophils, and monocytes – are present in relatively stable percentages that may temporarily shift higher or lower depending on what is going on in the body.  For instance, in the case of an infection, there may be a higher concentration of neutrophils (a "shift to the left").  With allergies, there may be an increased number of eosinophils, and with leukemia, there may be a much higher percentage of a single type of cell, such as a lymphocyte.  In this case, the cell may be present in large numbers, in a mature form and in a variety of immature forms.  The CBC determines whether there are sufficient WBCs present to fight infection, notes when there are more than expected, and determines the percentages and numbers of each type.

Red Blood Cells (RBC)
RBCs are reddish in color and shaped like a donut with a thinner section in the middle instead of a hole.  They have hemoglobin inside them, a protein that transports oxygen throughout the body.  The CBC determines whether there are sufficient RBCs present and whether the RBC population appears to be normal.  RBCs are normally all the same size and shape; however, variations can occur with vitamin B12 and folate deficiencies, iron deficiency, and with a variety of other conditions.  If there are insufficient normal RBCs present, the patient is said to have anemia and may have symptoms, such as fatigue and weakness.  Much less frequently, there may be too many RBCs in the blood (erythrocytosis or polycythemia).  In extreme cases, this can interfere with the flow of blood through the veins and arteries.

Platelets are special cell fragments that play an important role in blood clotting.  If a patient does not have enough platelets, he will be at an increased risk of excessive bleeding and bruising.  The CBC measures the number and size of platelets present.  With some conditions and in some people, there may be giant platelets or platelet clumps that are difficult for the hematology instrument to accurately measure.  In this case, a blood smear test may be necessary.

Function; Why it is Recommended

The Complete Blood Count (CBC) test is an automated count of the cells in the blood.  It provides information about the white blood cell (WBC), red blood cell (RBC), and platelet populations that are present.  This information includes the number, type, size, shape, and some of the physical characteristics of the cells.  In only a minute or two, the hematology instrument (the machine that is used to run the test) can measure thousands of RBCs, WBCs, and platelets and compare them against established normal ranges.  Any abnormalities found are noted, and the clinical laboratory scientist (CLS) running the instrument then uses his or her expertise and experience to accept the automated findings and/or to target the sample for further analysis.

In most cases, the automated CBC is very accurate and the test is complete at this point.  If, however, there are significant abnormalities in one or more of the cell populations, a blood smear test may be performed.  In this test, a drop of blood is placed on a slide, smeared into a thin layer, allowed to dry, and then dyed with a special stain.  A CLS then looks at the slide under the microscope and is able evaluate the cells present.  Any additional information is added to that found by the automated count, and all of the findings are reported to the doctor.

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.
  • 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.

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.

Expected Outcome

It is important to note that 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.

Some tests provide a simple yes-or-no answer.  Was the culture positive for strep throat?  Did the test find antibodies to a virus that indicates an infection?

But for many more tests, the meaning of the results depends on their context.  A typical lab report will have your results followed by a normal or reference range.  For example, your results for a thyroid-stimulating hormone (TSH) test might look something like: 2.0 m-IU/L, ref range 0.5-5.0 m-IU/L. The test results indicate that it falls within the "normal" range.

Laboratories will generally report your test results accompanied by a reference range keyed to your age and sex.  Your physician then will still need to interpret the results based on personal knowledge of your particulars, including any medications or herbal remedies you may be taking.  A plethora of additional factors can affect your test results: your intake of caffeine, tobacco, alcohol, and vitamin C; your diet (vegetarian vs. omnivorous); stress or anxiety; or a pregnancy.  Even your posture when the sample is taken can affect some results, as can recent heavy exertion.  For example, albumin and calcium levels may increase when shifting from lying down to an upright position.

What can be done about a bad CBC result?
Patients who have a keen interest in their own healthcare frequently want to know what they can do to change their WBCs, RBCs, and platelets.  Unlike "good" and "bad" cholesterol, cell populations are not generally affected by lifestyle changes unless the patient has an underlying deficiency (such as vitamin B12 or folate deficiency or iron deficiency).  There is no way that a patient can directly raise the number of his WBCs or change the size or shape of his RBCs.  Addressing any underlying diseases or conditions and following a healthy lifestyle will help optimize your body's cell production and your body will take care of the rest.

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