Cancer Antigen (CA) 125

This immunoassay is intended for the in vitro quantitative determination of OC 125 reactive determinants in human serum and plasma.¹ The Elecsys CA 125 II assay is indicated for use as an aid in the detection of residual or recurrent ovarian carcinoma in patients who have undergone first-line therapy and would be considered for second-look procedures.¹ The Elecsys CA 125 II assay is further indicated for serial measurement of CA 125 to aid in the management of cancer patients.¹ CA 125 belongs to the family of hybridoma-defined tumor markers. The values measured are defined by the use of the monoclonal antibody (MAb) OC 125. The antigenic determinant CA 125 is located on a high-molecular weight glycoprotein (200-1000 kd) isolated from cell culture or serum. The antigenic determination CA 125 has a protein structure with associated carbohydrate side-chains.^2,3 These determinants are associated with a high-molecular weight glycoprotein in serum and plasma of women with primary epithelial invasive ovarian cancer (excluding those with cancer of low malignant potential).

CA 125 is found in a high percentage of nonmucinous ovarian tumors of epithelial origin⁴ and can be detected in serum.^5,6 It does not occur on the surface epithelium of normal ovaries (adult and fetal). Ovarian carcinoma accounts for about 20% of gynecologic tumors; the incidence is 15/100,000.⁷ CA 125 has been found in the amniotic fluid and in the coelomic epithelium; both of these tissues are of fetal origin. In tissues of adult origin, the presence of CA 125 has been demonstrated in the epithelium of the oviduct, in the endometrium, and in the endocervix.⁸

Elevated values are sometimes found in various benign gynecologic diseases, such as ovarian cysts, ovarian metaplasia, endometriosis, uterus myomatous, or cervicitis. Slight elevations of this marker may also occur in early pregnancy and in various benign diseases (eg, acute and chronic pancreatitis, benign gastrointestinal diseases, renal insufficiency, autoimmune diseases, and others). Markedly elevated levels have been found in benign liver diseases, such as cirrhosis and hepatitis. Extreme elevations can occur in any kind of ascites due to malignant and benign diseases. Although the highest CA 125 values occur in patients suffering from ovarian carcinoma, clearly elevated values are also observed in malignancies of the endometrium, breast, gastrointestinal tract, and various other malignancies.

Although CA 125 is a relatively unspecific marker,^9-13 it is today the most important tumor marker for monitoring therapy and progress of patients with serous ovarian carcinoma. At primary diagnosis, the sensitivity of CA 125 depends on the FIGO stage (FIGO = Federation of Gynecology and Obstetrics); higher tumor stages are associated with higher CA 125 levels.¹⁴

The diagnostic sensitivity and specificity of the Elecsys CA 125 II test was calculated by comparing ovarian carcinoma patients at primary diagnosis (FIGO stage I to IV) with patients suffering from benign gynecologic diseases.¹

At a cutoff value of 65 U/mL, the sensitivity is 79% (at a low specificity of 82%). The cutoff level has to be raised if higher specificity is desired. The optimal clinical value is reached at 150 U/mL (sensitivity 69%, specificity 93%).¹