January 21, 2011
Dear Mayo Clinic:
At my annual physical, my doctor recommended I have a complete blood count. What is it, and why is it necessary? I'm healthy and, as far as I know, I'm not at high-risk for any medical problems.
A complete blood count is a blood test that can be used to evaluate your overall health. The test measures a variety of components and features of your blood. Having a complete blood count each year as part of an annual physical is a good idea, even for healthy people. The test is an excellent way to screen for blood abnormalities that could signal an underlying disorder such as anemia or infection, which may not show any other signs or symptoms.
A complete blood count examines several parts of your blood, including hemoglobin, white blood cells and platelets. An easy way to think of hemoglobin is as a transport vehicle. As a protein within your red blood cells, hemoglobin carries the oxygen that you inhale into your lungs to the rest of the tissues in your body. It also takes carbon dioxide from those tissues and brings it back to your lungs, so you can get rid of it when you exhale.
White blood cells are part of your immune system. There are several types of white blood cells, but they're all important for the body's defense against infections. For example, one type of white cell called neutrophils fights off bacterial infections. So if you have a bacterial ear infection or pneumonia, those white cells actually eat up the bacteria. Once they ingest the bacteria, they release enzymes that kill it.
Platelets work as a type of scanning system. They examine the inner surfaces of blood vessels looking for holes. If the platelets find a hole, they seal it. So their job is to prevent or stop bleeding.
A complete blood count shows the amount of each of these substances in your blood. If the results of a complete blood count reveal levels of blood components that fall outside normal ranges, there could be a problem that needs further evaluation. But the ranges aren't the same for everyone. Your gender, ethnic group and age all have an impact on what's considered normal. Talk to your doctor about levels for each measurement that are appropriate for you. If you have abnormal levels, additional testing to find more information is almost always needed.
A complete blood count can point to a possible problem, but it generally cannot be used to specifically diagnose a disorder. For example, an elevated white blood count may signal an infection. Low hemoglobin levels signal anemia. A decrease in your platelet count might be a sign of an autoimmune disorder. But there are many other possible causes for blood count numbers to be high or low, ranging from relatively benign disorders such as vitamin or iron deficiencies to potentially more serious blood diseases. Additional follow-up tests can identify more clearly what the underlying cause might be.
The complete blood count is an important step in assessing your health. Many diseases and conditions can affect your blood. But these disorders may show only minimal symptoms or, in some cases, no symptoms at all. The first sign of an abnormality may appear on the complete blood count. For that reason, it's important that everyone have this test once a year. Your blood can reveal vital clues about what's going on throughout your body.
ā Rajiv Pruthi, M.B.B.S., Hematology, Mayo Clinic, Rochester, Minn.