Blood tests are available for specific antigens that are produced against some types of cancer (e.g. breast, pancreas, liver, colon, ovary, uterus and prostate cancer). The most commonly ordered of these tests is the prostate specific antigen (PSA).
Cancer associated antigen (CA or carbohydrate antigen) tests are unreliable as a way of detecting cancer. This is because there is no absolute value in any one person above which a cancer can be said to be present. A series of tests over a period of months or years may show increasing levels of the antigen which may indicate the presence of a particular cancer.
The tests are particularly useful in following the progress of treatment in a patient (if successful, antigen levels should decrease steadily) or in watching a patient who has a bad family history of a particular type of cancer. Raised levels may also occur with infection, inflammation or enlargement of the organ.
Most of the different CA tests are differentiated from each other by number and an above normal result can be interpreted in the following ways:-
- CA 15-3 – Metastatic breast cancer (70%+), localised breast cancer (10%+). False positive with liver failure.
- CA 19-9 – Cancer of the pancreas (80%+), bile duct cancer (66%+), stomach cancer (50%+), liver cancer (hepatoma – 50%+), large bowel (colorectal) cancer (25%+). False positive with liver cirrhosis, cholangitis (inflamed bile duct) and pancreatitis.
- CA 50 – Cancer of the pancreas (75%+), large bowel (colorectal) cancer (45%+). False positive with pancreatitis.
- CA 125 – Some types of ovarian cancer (85%+), cancer of the uterus (endometrial cancer). False positive with endometriosis.
- CA 195 – Cancer of the pancreas (85%+), bowel (gastrointestinal) cancer .
- CA 549 – Breast cancer (50%+), lung cancer, large bowel (colon) cancer, prostate gland cancer. False positive with endometriosis, liver and ovarian disease.