Before a doctor can treat cancer, he or she needs to make a diagnosis of whether there is cancer in the body. This is called cancer screening.
A blood test is a normal procedure and one of the least expensive and intrusive cancer screening used. However this test is not a stand-alone or conclusive test and usually performed with other types of testing such as urine testing, biopsy of the tumor and others depending on where the suspected cancer cells are.
A cancer screening of the blood alone may show the presence of cancer but a negative result is not conclusive enough to show the absence of cancer. Nonetheless, it gives an indication to the doctor of what is happening inside your body.
What is a blood-test for?
Samples of Blood are normally extracted and sent to an independent laboratory for a detail analysis of your blood. The tests may show cancer cells or other substances that are a by-product of cancer cells activity. Usually your urine sample is also enclosed for testing as well.
What do they look for?
Your blood count. The laboratory examination of your blood will measure the count of your red blood cells, white blood cells, platelets, plasma content and any abnormal blood cell condition. If cancer cells are present, further tests may be required such as a biopsy on your bone marrow which is the production center for your blood cells.
Protein levels in the blood will indicate whether your immune system is in optimal condition. The antibodies in your immune system are basically proteins (known as immunoglobulin) that identify and neutralize bacteria and viruses
Tumor markers in the blood. Tumor cells produce chemicals called tumor markers. These chemicals are present in the blood and can be detected with a blood sample if the medical laboratory is checking for them.
A bit more about tumor markers
Testing of Blood for tumor markers are increasing popular and even commercially marketed as part of a regular medical checkup. The widespread indiscriminate screening for tumor markers is unnecessary.
Some oncologists have found that tumor markers have limited value in cancer screening and cancer diagnosis. They prefer to use the testing of blood to confirm what has already been detected.
Examples of tumor markers are prostate-specific antigen (PSA) for prostate cancer, cancer antigen 125 (CA 125) for ovarian cancer,calcitonin for thyroid cancer, alpha-fetoprotein (AFP) for liver cancer andhuman chorionic gonadotropin (HCG) for testicular cancer and ovarian cancer. Further reading on cancer tumor markers (CA 125, CA 15-3, CA 19-9, PSA) can be found Here. Even then, NOT ALL tumors express tumor markers.
A positive test for CA 19.9 may not tell exactly which organ in the body the cancer cells are present. It could be in the pancreas, liver, stomach or intestines. By the time the tumor markers are detected, you would have shown other signs and symptoms of cancer. Imaging tests such as CAT Scans, MRI scans etc. may be more accurate in detecting localized cancer occurrences.
Even though a cancer screening of the blood for tumor markers has some value, it may result in seeking further costly tests in search for the non-existent cancer or tumor, if you do not have cancer. Who would dare tell you that you don't have cancer, especially when a negative test itself may not mean you do not have cancer?
In some cases, cancer may be present even though the medical test results show normal. See story below:
What to do after Blood Testing
Blood testing may assist your doctor to discover some trace of cancer, but other tests are necessary to make a proper definite diagnosis.
Some doctors insist on a biopsy, a procedure to extract some tissue samples for laboratory testing, before making a conclusive statement.
In other cases, blood tests are carried out regularly and monitored over time for a specific individual. By checking historical data, doctors are better equipped to diagnose any bodily changes or presence of tumor substances.
Generally, if you are already diagnosed with cancer, doctors will still use the lab testing of your blood to determine whether your body is responding to their cancer treatment or not.