Blood tests for the presence of substances called tumor markers can be helpful in diagnosing cancer and assessing how well treatment is working. But such tests alone generally can’t tell for certain whether someone has cancer or not. That’s why they are used in conjunction with other methods, such as imaging scans and biopsies.
Hematological cancers such as leukemia and lymphoma can be diagnosed on the basis of blood tests that reveal abnormal types or quantities of blood cells, but these are not termed tumor markers. Tumor markers are proteins or other compounds shed into the blood, urine, or body tissues that can be detected and measured. The markers are made by both normal and cancer cells, but they may occur in elevated levels when cancer is present. Examples include:
- PSA (prostate-specific antigen) in prostate cancer
- CA-125 in ovarian cancer
- AFP (alpha-fetoprotein) in liver cancer
- CEA in colon cancer
Limitations of tumor markers
A limitation of these tests is that tumor markers can sometimes be elevated because of noncancerous conditions, and, conversely, markers may not be elevated in all patients with a certain type of cancer. For example, an increased level of PSA in a man’s blood can be caused by benign prostate conditions as well as by prostate cancer, and most men with elevated PSA levels don’t have prostate cancer.
Another example is ovarian cancer. The CA-125 marker is sometimes elevated in women with the disease, but it can also be raised because of benign conditions. Studies have shown that CA-125 is not sufficiently sensitive or specific enough to be used to screen for ovarian cancer in women at average risk.
Using tumor markers to decide treatment
While tumor markers may not always be effective in diagnosing cancer, doctors sometimes measure certain markers in a patient’s blood prior to treatment to help determine a prognosis and plan therapy. Markers may also be measured periodically during therapy: A fall in tumor marker levels may indicate the cancer is responding to treatment, while no change or an increase in the marker may indicate the therapy isn’t working.
Tumor markers may also be measured after the end of treatment to check for recurrence.