Complete Blood Count

A complete blood count (CBC) is a blood test used to evaluate a patient's state of health by measuring the number of blood cells in the body and the ratio of one type of blood cell to another in the bloodstream. This information is valuable in helping to diagnose or rule out a number of medical problems, including anemia, various infections. and many serious disorders. A CBC is frequently the first test taken during a comprehensive medical examination both because it is simple to administer and because it provides a great deal of data concerning the patient's health.

Reasons for a CBC

Complete blood counts are taken for several reasons. These include the following:

  • To confirm the positive findings of a wellness check-up
  • To diagnose the reason for symptoms, such as fatigue, fever or bruising
  • To monitor an already diagnosed blood disorder
  • To monitor the effectiveness of certain medications or treatments

Diseases or conditions that may be diagnosed or monitored with CBCs include:

  • Anemia
  • Leukemia
  • Allergies
  • Infections
  • Blood-clotting disorders, like hemophilia
  • Other blood diseases, like sickle cell anemia

Sometimes a blood smear test is done at the same time as the CBC. In a blood smear, a drop of blood is spread thinly across a microscopic slide and stained with a special dye. When the doctor looks at this slide under the microscope the number, shapes and sizes of the various blood cells are visible, giving the physician an even clearer understanding of any disease process that may be present.

The CBC Procedure

During a CBC procedure, a small quantity of blood is withdrawn from the patient through a process known as a venipuncture. The doctor, nurse, or phlebotomist wraps a plastic tube around the patient's arm to enlarge the veins and inserts a slender needle into the targeted vein in the patient's arm, usually in the crook of the elbow. The needle is attached to a vial that will hold the blood specimen. There is very little discomfort involved. After the blood sample is obtained, a bandaid may be applied to stop any residual bleeding. The patient can resume normal activities immediately after the procedure.

Once the blood sample is obtained, the blood is sent to a laboratory for analysis, where the following tests are performed:

  • Red blood cell count
  • White blood cell count
  • Platelet count
  • Hemoglobin count
  • Hematocrit

These tests assist in determining how well the red blood cells are nourishing and oxygenating the other cells of the body, how well the white blood cells are fighting disease, how well the platelets are helping the blood to clot, and how well the hemoglobin is assisting in the oxygenation and clotting processes. The hematocrit measures the ratio of red blood cells to plasma (the fluid portion of the blood). If it is too high or too low, it may be indicative of dehydration, nutritional deficiencies, blood loss, lung or heart disease.

Results of a CBC

The CBC provides a wide range of important information. A lower than normal red blood count, for example, may indicate anemia, possibly indicative of iron deficiency or blood loss. A higher than normal red blood cell count, known as erythrocytosis, or a high hematocrit, may point to a medical disorder called polycythemia vera or to heart disease. A high count of white blood cells may indicate an infection or inflammation where a low white count can be caused by an autoimmune disorder or a cancer. A low platelet count (throbocytopenia) or one that is abnormally high (thrombocytosis) may also be indicators of underlying medical conditions.

In some cases, abnormal results of a CBC are the result of a medication the patient is taking. Often, CBCs are repeated to make sure that the readings remain constant. When a CBC provides abnormal results, further diagnostic tests are typically necessary to pinpoint the cause.

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