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Understanding Lab tests

Cancer blood tests: Lab tests used in cancer diagnosis


Cancer blood tests and other laboratory tests may help your doctor make a cancer diagnosis. To help reduce your anxiety, learn about cancer blood tests and how they're used.


Blood tests alone can rarely, if ever, show the presence or absence of cancer. If your doctor suspects you may have cancer, he or she may order certain cancer blood tests or other laboratory tests, such as an analysis of your urine, to help guide the diagnosis. While cancer blood tests generally can't 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.


What your doctor is looking for

Your doctor may order cancer blood tests or other types of laboratory tests after conducting a physical exam. The signs and symptoms you report may give your doctor clues about what could be wrong. Tests to analyze your blood, urine or body tissues may give your doctor further information about your condition.


Blood and urine samples are analyzed in a lab. If the doctor finds cancer cells, too many or too few cells of a particular type, or abnormal types of cells, or if any of various other substances are detected, it may indicate cancer. For example, if you have leukemia — a blood cancer — cancerous white blood cells can be seen under a microscope. A common blood test called complete blood count (CBC) measures the amount of various types of blood cells in a sample of your blood. CBC may give your doctor an idea of what's causing your signs and symptoms.

Blood and urine samples may also be tested for various substances, called tumor markers, which may indicate cancer. Tumor markers are typically chemicals made by tumor cells, but tumor makers are also produced by some normal cells in your body.


For instance, prostate-specific antigen (PSA) is a tumor marker sometimes used to screen men for prostate cancer, though this is somewhat controversial. Any man who hasn't had his prostate removed has a detectable level of PSA in his blood. An abnormally elevated PSA level may prompt your doctor to recommend further testing for prostate cancer. Another tumor marker is cancer antigen 125 (CA 125), which may be elevated in women with ovarian cancer, though levels can be elevated in people with other types of cancer and with many benign conditions. A high CA 125 result may prompt your doctor to recommend further testing to determine the cause.


What the results mean


Test results must be interpreted carefully because several factors can influence test outcomes, such as variations in your own body or even what you eat. In addition, it's important to remember 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.

What happens next?


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. Cancer blood markers are most 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 your test results with your doctor. Ask your doctor what your results say about your health and what the next steps should be.

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