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Blood tests alone can rarely, if ever, show the presence or absence of cancer. If it's suspected that you have cancer, your doctor may order certain cancer blood tests or other laboratory tests, such as an analysis of your urine or a biopsy of a suspicious area, to help guide the diagnosis. While cancer blood tests generally can't absolutely tell whether you have cancer or some other noncancerous condition, they can give your doctor clues about what's going on inside your body.

Because your doctor has ordered cancer blood tests or other laboratory tests to look for signs of cancer doesn't mean that a cancer diagnosis has been made and you have cancer. Find out what your doctor might be looking for when cancer blood tests are done.

Samples collected with cancer blood tests or other tests of urine, fluid or tissue are analyzed in a lab for signs of cancer. The samples may show cancer cells, proteins or other substances made by the cancer. Blood and urine 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 and urine 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.
• Urine cytology. Examining a urine sample under a microscope may reveal cancer cells that could come from the bladder, ureters or kidneys.
• Blood protein testing. A test to examine various proteins in your blood (electrophoresis) 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 makers 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.

  The best ways 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.

Test results must be interpreted carefully because several factors can influence test outcomes, such as variations in your body or even what you eat. In addition, keep in mind that noncancerous conditions can sometimes cause abnormal test results. And, in other cases, cancer may be present even though the blood test results are normal.

Your doctor usually uses your test results to determine whether your levels fall within a normal range. Or your doctor may compare your results with those from past tests.

Though blood and urine tests can help give your doctor clues, other tests are usually necessary to make the diagnosis. For most forms of cancer, a biopsy — a procedure to obtain a sample of suspicious cells for testing — is usually necessary to make a definitive diagnosis.

In some cases, tumor marker levels are monitored over time. Your doctor may schedule follow-up testing in a few months. Tumor markers are most often helpful after your cancer diagnosis. Your doctor may use these tests to determine whether your cancer is responding to treatment or whether your cancer is growing.

Discuss test results with your doctor. Ask your doctor what your results say about your health and what the next steps should be.