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.
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.
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:
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.
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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