TAAs can be useful tumor markers in the diagnosis and management of various tumors. An ideal tumor marker is released only from tumor tissue, is specific for a given tumor type (to direct diagnostic assessment), is detectable at low levels of tumor cell burden, has a direct relationship to the tumor cell burden and the marker concentration in blood or other body fluid, and is present in all patients with the tumor. Most tumors release antigenic macromolecules into the circulation that can be detected by immunoassay. Although useful in monitoring patients for tumor recurrence after therapy, no tumor marker has undisputed specificity or sensitivity for application in early diagnosis or mass cancer screening programs.

Carcinoembryonic antigen (CEA) is a protein-polysaccharide complex found in colon carcinomas and in normal fetal intestine, pancreas, and liver. A sensitive immunoassay can detect increased levels in the blood of patients with colon carcinoma, but the specificity is relatively low because positive tests also occur in heavy cigarette smokers and in patients with cirrhosis, ulcerative colitis, and other cancers (eg, breast, pancreas, bladder, ovary, cervix). Monitoring CEA levels may be useful for detecting cancer recurrences after excision of a tumor that had been associated with elevated CEA.

α-Fetoprotein, a normal product of fetal liver cells, is also found in the sera of patients with primary hepatoma, yolk sac neoplasms, and, frequently, ovarian or testicular embryonal carcinoma.

β Subunit of human chorionic gonadotropin (β-HCG), measured by immunoassay, is the major clinical marker in women with gestational trophoblastic neoplasia (GTN)-a disease spectrum that includes hydatidiform mole, nonmetastatic GTN, and metastatic GTN - and in about 2/3 of men with testicular embryonal or choriocarcinoma. The β subunit is measured because it is specific for HCG.

Prostate-specific antigen (PSA), a glycoprotein located in ductal epithelial cells of the prostate gland, can be detected in low concentrations in the sera of healthy men. Using an appropriate upper limit of normal, assays with monoclonal antibodies detect elevated serum levels of PSA in about 90% of patients with advanced prostate cancer, even in the absence of defined metastatic disease. It is more sensitive than prostatic acid phosphatase. However, because PSA is elevated in benign prostatic hypertrophy, it is less specific. PSA can be used to monitor recurrence after prostatic carcinoma has been diagnosed and treated.

CA 125 is clinically useful for diagnosing and monitoring therapy for ovarian cancer, although any peritoneal inflammatory process can cause increased circulating levels.

Radiolabeled monoclonal antibody B72.3, which recognizes a pancarcinoma antigen (one that recognizes carcinomas from all tissues) termed TAG-72, is being used in tumor localization studies to find occult tumor deposits. The clinical benefit of finding such occult tumors is under study.