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Blood Tests

Reviewed by: Emily F. Conant, MD

Updated October 1, 2010

If you have been diagnosed with breast cancer or if your doctor thinks you might have it, you may need a blood test. Blood tests check blood cell counts before chemotherapy or other treatments, or they may be used to detect the presence of disease.

The most common types of blood tests for breast cancer are a complete blood count test, which checks the amount of red and white blood cells and platelets in the blood, and blood chemistry tests and blood marker tests, which may help detect whether cancer is present outside of the breast.

Complete Blood Count Test (CBC)

This test may be given before, during and after treatment. A CBC looks at how many red and white blood cells you have, and it may also suggest the presence of cancer in your bone marrow, where blood cells are formed. A low number of white blood cells, which help build up your body’s immunity, may indicate that you have an infection or are at risk for developing one.

Red blood cells, which contain an iron-rich protein called hemoglobin, are responsible for carrying oxygen throughout your body. They also help provide energy. If you have a low red blood cell count, you may be anemic. Anemia often is a side effect of treatment, especially chemotherapy.

A CBC also will test for platelets, the substances in the blood that help you form blood clots so that any bleeding can be stopped.

Your blood cell count will probably be tested before each chemotherapy treatment. If you do not have enough red or white blood cells, you can take medicines to increase the levels of these cells.

Blood Chemistry Tests

Blood chemistry tests measure the levels of certain substances in your blood that give information about the health of important organs such as the liver, kidneys, bones and blood sugar. Testing these substances can help your doctor determine whether cancer may have traveled from the breast to other areas of the body.

Blood Marker Test

If you have been diagnosed with breast cancer, your doctor also may perform a blood marker test to check for the presence of cancer in the rest of your body. Some tumors create proteins or circulating tumor cells that break away from the tumor and move throughout your body. During a blood test, doctors will look for these proteins or cells in your blood to check for cancer in your breast, ovaries, liver, lung or colon.

Blood marker test results can sometimes be unreliable, and they are expensive. But they are sometimes a way for your doctor to know whether you need more tests to monitor your overall health.