What your doctor is looking for

Samples collected for cancer blood tests are analyzed in a lab for signs of cancer. The samples may show cancer cells, proteins or other substances made by the cancer. Blood tests can also give your doctor an idea of how well your organs are functioning and if they've been affected by cancer.

Examples of blood tests used to diagnose cancer include:

• Complete blood count (CBC). This common blood test measures the amount of various types of blood cells in a sample of your blood. Blood cancers may be detected using this test if too many or too few of a type of blood cell or abnormal cells are found. A bone marrow biopsy may help confirm a diagnosis of a blood cancer.
• Blood protein testing. A test (electrophoresis) to examine various proteins in your blood can aid in detecting certain abnormal immune system proteins (immunoglobulins) that are sometimes elevated in people with multiple myeloma. Other tests, such as a bone marrow biopsy, are used to confirm a suspected diagnosis.

• Tumor marker tests. Tumor markers are chemicals made by tumor cells that can be detected in your blood.

  But tumor markers are also produced by some normal cells in your body, and levels may be significantly elevated in noncancerous conditions. This limits the potential for tumor marker tests to help in diagnosing cancer. Only in extremely rare circumstances would such a test be considered enough to make a firm diagnosis of cancer.

  The best way to use tumor markers in diagnosing cancer hasn't been determined. And the use of some tumor marker tests is controversial.

  Examples of tumor markers include prostate-specific antigen (PSA) for prostate cancer, cancer antigen 125 (CA 125) for ovarian cancer, calcitonin for medullary thyroid cancer, alpha-fetoprotein (AFP) for liver cancer and human chorionic gonadotropin (HCG) for germ cell tumors, such as testicular cancer and ovarian cancer.

• Circulating tumor cell tests. Recently developed blood tests are being used to detect cells that have broken away from an original cancer site and are floating in the bloodstream. One circulating tumor cell test has been approved by the Food and Drug Administration to monitor people with breast, colorectal or prostate cancer. This test isn't commonly used in a clinical setting.