CBC (Complete Blood Count) Test

CBC and Cancer

Reviewed By:
Norman Klein, M.D., FAAAAI


The CBC (complete blood count) is a routine blood test that measures the composition of blood cells within the blood. The test provides a count of the type and numbers of specific blood cells, including white blood cells, red blood cells and blood platelets. While used to screen for a variety of conditions, the test can provide specific data regarding the presence of allergies in an individual.

The CBC test takes only a few minutes, and is performed by taking a blood sample from a patient. During this simple procedure, blood is withdrawn from the patient’s arm through a needle inserted into the vein. The blood sample is sent to a laboratory and analyzed.

A CBC test will not conclusively demonstrate the presence of allergies in an individual. However, test results that show increased levels of eosinophils and basophils (types of white blood cells) can suggest an allergic reaction has recently taken place in a patient. The test is often used along with other types of allergy tests (e.g., ELISA, RAST) to more accurately determine the presence of an allergy.

There is generally no special preparation necessary for the CBC test. However, some medications may interfere with a CBC. Patients are urged to follow their physician’s orders carefully about how to take their medications before the test in order to ensure its accuracy.

About CBC

The CBC (complete blood count) blood test is used to diagnose and manage a wide range of conditions. It provides valuable information about blood cells, including the number, sizes and shapes of the cells. The CBC is sometimes performed as part of a routine medical examination.

The CBC test may be used for several conditions including:

  • Allergies. Exaggerated immune system response to a foreign substance.

  • Infection. Invasion of the body by disease-causing microorganisms.

  • Dehydration problems. Excessive water loss from the body.

  • Heart attack. Impaired heart function due to decreased blood flow.

  • Anemia. Low red blood cell count.

  • Leukemia. Cancer of the white blood cells.

  • Crohn’s disease. Illness characterized by irritation in the digestive tract.

  • HIV (human immunodeficiency virus). Virus that causes AIDS (auto immune deficiency syndrome).

The CBC test is used to determine the counts of various types of blood cells. These cells include: 

  • White blood cells (WBC, leukocytes). Produced in the bone marrow and useful for repelling infections in the body. Increased amounts of white blood cells often indicate an infection or inflammation somewhere in the body. A low count may indicate a disease process (e.g., HIV) is interfering with the bone marrow’s ability to create white blood cells. The CBC test counts total WBC, with the normal range for this test usually falling within 4,300 and 10,800 white blood cells per cubic millimeter of blood. The test also measures the number of specific types of some white blood cells, including:
    • Neutrophils. Most numerous type of white blood cell in circulation. Their numbers become elevated when infection is present. A subtype of neutrophils are band neutrophils. These are immature forms of neutrophils that represent only 0 to 3 percent of the white blood cells in peripheral blood. When their number increases, it can signal an infection such as appendicitis. Neutrophils normally make up 47 percent to 77 percent of white blood cells.

    • Lymphocytes. Found to be elevated in some types of leukemia or more commonly with viral infections. Lymphocytes normally make up 16 percent to 43 percent of white blood cells.

    • Monocytes. Found to be elevated in different kinds of leukemia or infections. Monocytes normally make up 0.5 percent to 10 percent of white blood cells.

    • Eosinophils. Found to be elevated in many allergic conditions or parasitic infections. Eosinophils normally make up 0.3 percent to 7 percent of white blood cells.

    • Basophils. Can be elevated in different types of blood disease and poisonings, as well as in allergic conditions. Basophils normally make up 0.3 percent to 2 percent of white blood cells.

  • Red blood cells (RBC, erythrocytes). Used to distribute oxygen from the lungs to the rest of the body. A low count (anemia) often causes fatigue. The CBC test provides a total RBC count. The normal range may vary slightly from lab to lab but it usually falls between 4.2 and 5.9 million cells per cubic millimeter of blood. This part of the test also measures:
    • Hemoglobin. Component of red blood cells that transports oxygen. The hemoglobin test measures the amount of hemoglobin in a patient’s blood and gives a good indication of a blood cell’s ability to transport oxygen. The normal range for hemoglobin varies slightly between the sexes, with a normal value of 13 to 18 grams per deciliter for men and 12 to 16 grams per deciliter for women.

    • Hematocrit. Percentage of the blood that is made up of red blood cells. A hematocrit of 45 would mean that red blood cells compose 45 percent of a blood sample. The normal range for hematocrit varies slightly between the sexes with a range of 45 percent to 52 percent for men and 37 percent to 48 percent for women.

  • Platelets (thrombocytes). Produced in the bone marrow, platelets are cell fragments that play an essential role in the blood clotting process. Reduced numbers of blood platelets are often caused as a side effect of some medications. A CBC test provides a total platelet count. Normal range values vary slightly between laboratories but usually fall between 150,000 and 400,000 per cubic millimeter.

The levels of various cells can suggest the patient has different conditions. For example:

  • High WBC levels may indicate a heart attack, infection, an inflammatory disease, allergies or leukemia.

  • Low WBC levels can be caused by kidney or liver disease, radiation exposure or the presence of a toxic substance in the body.

  • High RBC levels may indicate a lack of fluid in the body (dehydration). RBC levels are also higher than normal in people who smoke.

  • Low RBC levels are usually an indicator of anemia.

  • High platelet levels may be due to severe bleeding, infection, strenuous exercise, pregnancy or other factors.

  • Low platelet levels are usually caused by an infection, cancer or a lack of either folic acid or vitamin B-12.

CBC tests and allergies

The CBC test can be used to screen for a range of conditions, including allergies. To test for the presence of allergies, a physician will usually look at a specific part of the CBC test results that detail the number of eosinophils (type of white blood cell that produces histamine) found in the blood. A high eosinophil count often indicates that a recent allergy attack has taken place. Increased levels of basophils, another type of white blood cell, may also indicate an allergic condition.

The results of the CBC test do not conclusively determine whether an individual has allergies. A high eosinophil count may also indicate a parasitic infection. However, the test can provide strong evidence that an individual does not have an allergy, provided the patient recently experienced what they suspected was an allergy attack. The eosinophil count should remain elevated for several days following an allergic response in the body.

To better determine the presence of allergies in a patient, a physician will often conduct a more specific type of allergy test in addition to the CBC test (e.g., ELISA, RAST).

Before, during and after the CBC test

There is generally no special preparation necessary for the CBC test. However, patients may be advised to reduce or stop the use of certain medications at some point before the test, as a number of medications interfere with the test’s results. Patients can talk to their physicians prior to the test to request specific instructions. The test is usually performed at the physician’s office or at the hospital on an outpatient basis. It should not take more than a few minutes.

During the CBC test blood will be drawn from a vein in the inside of the elbow or in the back of the hand. The procedure for this will be as follows:

  1. The person conducting the test will use an antiseptic to clean the area of the patient’s skin where the needle is to be inserted.

  2. A tourniquet (strong rubber tube) is wrapped around the upper arm, allowing the veins in the lower part of the arm to swell as restricted blood flows into them.

  3. A needle – attached to a vial or syringe – is inserted into a vein of the arm, usually at the inside of the elbow or back of the hand. The rubber tourniquet is removed, allowing blood to flow properly through the vein.

  4. Blood flows from the vein through the needle and into the vial or syringe, filling it.

  5. The needle is removed from the arm and the puncture site on the skin is covered with gauze or a bandage for several minutes to prevent bleeding.

  6. The blood sample is sent to a lab for testing. A machine known as a hematology analyzer is used to evaluate the sample. In some cases, further analysis with a microscope is needed.

If the patient is a newborn or young child, the sample is drawn from the heel instead. After the area is sterilized with antiseptic, a sharp needle or lancet is used to puncture the skin. The blood is then collected in a pipette (a small glass tube), on a slide, on a test strip or into a small container. A bandage is applied, if necessary, and the sample is sent to a lab for analysis.

During the procedure there may be some discomfort associated with the insertion of the needle into the arm. This pain should be minor, however, such as a prick or stinging sensation, and last only briefly. Patients who feel severe pain during the procedure should immediately inform the individual performing the test. Slight throbbing or bruising at the point of insertion is normal following the test. Patients who experience significant pain that lasts for several days should contact their physician.

Potential risks with CBC

The risks associated with puncturing a vein and drawing blood are not significant. However, some individuals experience one or more of the following:

  • Excessive bleeding. Excessively bleeding from such a small wound is rare, and anyone with a blood clotting condition (e.g., hemophilia) should inform their physician before the procedure.

  • Fainting or dizziness. This condition can result from blood loss. However, these symptoms are more likely a psychological reaction to the procedure, especially if the patient has a fear of needles or an aversion to the sight of blood. The amount of blood taken during testing is usually not enough to cause such symptoms.

  • Hematoma. Commonly known as a bruise, this is a blood accumulation under the skin.

  • Infection. There is only a slight risk of infection due to the puncturing of the skin.

  • Multiple punctures to locate veins. Individuals who are more difficult to draw blood from (often due to vein size) may experience some added discomfort with additional punctures.

Treatments that may follow CBC

If a physician concludes from a CBC test and other factors that a patient has an allergy, the next step is to identify the allergen and limit the patient’s contact with it. Any treatment plan for an allergy will include avoidance – completely avoiding the allergen known to trigger an allergic reaction.

However, avoiding an allergen is not always possible. The allergen may be airborne and constantly present (e.g., pollen) or the individual may be so highly sensitive that even trace amounts will trigger a reaction. Therefore, a number of other strategies may be necessary to treat allergic conditions.

The majority of allergy treatments are designed to ease symptoms of an allergic reaction that has already occurred. However some treatments are designed for other reasons, including:

  • Preventing the onset of allergic reactions in people with known allergies

  • Reducing the frequency of reactions

  • Reducing the severity of symptoms if a reaction does occur

Most allergy treatments for symptom relief are in the form of medications (e.g., antihistamines, corticosteroids). Some medications target specific symptoms (e.g., itchiness, difficulty breathing). Others may target the relief of a variety of related symptoms, such as those associated with hay fever (e.g., runny nose, sneezing, watery eyes).

Questions for your doctor about CBC

Preparing questions in advance can help patients to have more meaningful discussions with their physicians. Patients may wish to ask their doctors the following CBC-related questions:

  1. Why are you recommending that I undergo a CBC?

  2. What risks do I face by undergoing a CBC test?

  3. Will I experience any pain when the blood sample is drawn? What about after?

  4. How long will it take for my blood to be drawn?

  5. Are there any special steps I should take on the day of the CBC test?

  6. Will any of my medications interfere with the test results?

  7. How long will I have to wait for the results? Who will explain them to me?

  8. How accurate are CBC tests? Can I trust the results?

  9. Will I need additional tests to confirm that I have allergies?

  10. Will you be prescribing me drugs based on my CBC results?
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