Complete Blood Count

The Complete Blood Count (CBC) test is an automated count of the cells in the blood. A standard CBC includes the following:

• number of white blood cells (WBC)
• number of red blood cells (RBC)
• hemoglobin content (Hgb)
• hematocrit (Hct)
• mean corpuscular volume (MCV)
• mean corpuscular hemoglobin (MCH)
• mean corpuscular hemoglobin concentration (MCHC)
• platelet count and volume

The results of a CBC can provide information about not only the number of cell types but also can give an indication of the size, shape, and some of the physical characteristics of the cells. In addition, a WBC differential (identifies different types of WBCs) may be ordered and can be done on the same instrument or performed manually.

Significant abnormalities in one or more of the cell populations may require visual confirmation by observing a blood smear under a microscope. In this test, a drop of blood is placed on a glass slide, smeared into a thin layer, allowed to dry, and then dyed with a special stain. A Clinical Laboratory Scientist can then evaluate the physical characteristics of the red and white blood cells present. Any additional information is noted and reported to the doctor.

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 (WBCs)
There are five different types of WBCs that the body uses to maintain a healthy state and to fight infections or other causes of injury. They are neutrophils, lymphocytes, basophils, eosinophils, and monocytes. They are present in the blood at relatively stable percentages. These numbers may temporarily shift higher or lower depending on what is going on in the body. For instance, an infection can stimulate a higher concentration of neutrophils (a “shift to the left”) to fight off bacterial infection. With allergies, there may be an increased number of eosinophils that release certain chemicals (anti-histamines) that minimize the allergic effect. Lymphocytes may be stimulated to produce immunoglobulins (antibodies). And in certain disease states, such as leukemia, abnormal and immature white cells (blasts) rapidly multiply, increasing the WBC count.

Red Blood Cells (RBCs)
RBCs are pale red 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 population of RBCs 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
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.

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, granulocytes, 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 percentage of red blood cells in a given volume of whole blood.
• 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) as is seen in iron deficiency anemia or thalassemias.
• Mean corpuscular hemoglobin (MCH) is a calculation of the average amount of oxygen-carrying hemoglobin inside a red blood cell. Macrocytic RBCs are large so tend to have a higher MCH, while microcytic red cells would have a lower value.
• Mean corpuscular hemoglobin concentration (MCHC) is a calculation of the average concentration of hemoglobin inside a red cell. 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 burn patients and 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.

The CBC is a very common test. 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 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 and can also be associated with diseases of the bone marrow such as leukemia.

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 while 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.

The following table explains what increases or decreases in each of the components of the CBC may mean. 

Components of the CBC

Normal CBC values for babies and children may be different from adults and need to be considered when interpreting data.

Clinical Hematology and Fundamentals of Hemostasis. Third edition. Denise M. Harmening.F. A. Davis Co., 1915 Arch Street Philadelphia, PA 19103. 1997.

Thomas, Clayton L., Editor (1997). Taber’s Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby’s Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

Brose, M., Updated (2004 August 03, Updated). CBC. MEDLINEplus Health Information, Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003642.htm.

Brose, M, Updated (2003 May 08, Updated). Blood differential. MEDLINEplus Health Information, Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003657.htm.

Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby’s Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 290.