Blood tests constitute one element in the detection of cancer and other diseases. Blood test results that differ substantially from normal ranges give your doctor valuable information that will help her understand your health situation better. Normal blood test results have a similar informative value. But no single blood test result clearly indicates that you do or do not have cancer. You can have cancer with normal blood counts.
Why your doctor orders blood tests
Your doctor may order cancer blood tests for several reasons, one of them being to determine how well your organs function. Note also that your doctor cannot detect certain cancers with blood tests. Prostate cancer, for example, if confined to the prostate, will probably not show up in one common blood test, the Complete Blood Count test, and may not show up even in a PSA (Prostate-Specific Antigen) test. Your doctor will recommend at least one further test, usually a needle biopsy, before a determination of prostate cancer. A needle biopsy is a relatively painless test that your doctor will normally perform in his office.
Complete Blood Count (CBC)
One commonly used blood test, the CBC test, will reveal the comparative amounts of different blood cells in a blood sample. If you have too many or too few cells of certain types of blood cells, this suggests to your doctor that you should have further diagnostic tests, such as a bone marrow biopsy. By itself, a CBC test does not confirm that you do not have cancer, even for those cancers that commonly show up in a blood sample.
Blood protein test
Another common blood test, blood protein testing, can help your doctor by detecting abnormal proteins, or immunoglobulins. Persons with elevated immunoglobulin levels may have multiple myeloma. But a person may have an elevated immunoglobulin level for other reasons. In the presence of other factors associated with cancer, even if you have normal blood protein you will need additional tests
After blood tests that confirm abnormal cells, abnormal blood counts or abnormal immunoglobulin levels, your doctor will order additional tests before concluding that you have a particular cancer. Urine cytology (basically a urine sample examination) may reveal cancers specific to the bladder, urinary tract or kidneys. Tumour marker tests will also help confirm or reject cancer hypotheses. But tumour marker tests do not give definitive results either. Like a detective, your doctor assembles evidence that cumulatively leads her toward a diagnosis. No single diagnostic test can determine the presence or absence of cancer.
Even if your doctor concludes that you do or do not have a particular cancer, you may want to seek a second opinion from another group of doctors or cancer centre. Often a different medical staff will want to run the same tests, including blood tests again. Different laboratories can come to different conclusions. Also, a second specialist in reading stains (visual replications of biopsies), even if he concludes that you do have the cancer previously diagnosed, may interpret the results differently, leading a second staff to recommend a different course of treatment.