Why your doctor might order a cancer blood test (2025)

If your health care provider is worried you might have cancer, you might need more tests to be sure. Cancer blood tests are often part of this process.

Samples taken for cancer blood tests are tested in a lab for signs of cancer. When viewed under a microscope, the samples may show the actual cancer cells. Other blood tests might find proteins or other substances made by the cancer. Blood tests can also tell your provider how well your organs are working.

Most blood tests aren't used on their own to diagnose cancer. But they can provide clues that may lead your health care team to make the diagnosis. For most types of cancer, a procedure to remove a sample of cells for testing is often needed to be sure.

Some blood tests used to diagnose cancer include:

• A test to count your blood cells. A complete blood count (CBC) measures the amount of each type of blood cell in a sample of your blood. Blood cancers may be found using this test.
• A test that looks at the blood proteins. An electrophoresis blood test looks at the various proteins in your blood to find the ones made by your body's germ-fighting immune system. This test is helpful in diagnosing multiple myeloma.

• Tests to find chemicals made by cancer cells. Tumor marker tests use a sample of blood to look for chemicals made by cancer cells.

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  These tests don't always help with diagnosing cancer because many healthy cells also make these chemicals. And some conditions that aren't cancer can cause high levels of tumor markers. Instead, tumor marker tests are mostly used after your cancer diagnosis to see if treatment is working.

  Examples of tumor markers include prostate-specific antigen (PSA) for prostate cancer and cancer antigen 125 (CA 125) for ovarian cancer. Other examples include carcinoembryonic antigen (CEA) for colon cancer and alpha-fetoprotein for testicular cancer.

• Tests to look for cancer cells. Circulating tumor cell tests detect cancer cells in your blood. The cells might be in the blood if they've broken away from where they started and are spreading to other parts of the body. Circulating tumor cell tests are mostly used after a cancer diagnosis.

  Not every person with cancer needs a circulating tumor cell test. These tests are sometimes used for a few types of cancer, including breast cancer, colon cancer and prostate cancer. Researchers are looking at how these tests might help people with other types of cancer.

• Tests to look for cancer cells' genetic material. These tests use a blood sample to look for small pieces of cancer cells' genetic material, called DNA. Healthy cells and cancer cells discard pieces or break apart as part of the natural process of growing and dying. These pieces of cells make their way into the blood so that the body can dispose of them. Special tests look for these cell pieces in a sample of blood.

  In people with cancer, these tests are sometimes used to understand the DNA changes present in the cancer cells. A health care provider uses the results to select the best treatment.

  One day providers might use these tests to detect signs of cancer in healthy people with no symptoms. This is an active area of research.

If your blood test shows a result that's not expected, you might need other tests and procedures to find the cause.

Dec. 05, 2024

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