What is CA 125?

Updated: Jan 10, 2020
  • Author: Gentry George T King, MD; Chief Editor: Eric B Staros, MD  more...
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CA 125 is an antigenic determinant on a high–molecular-weight glycoprotein recognized by a monoclonal antibody (OC 125). The full-length CA 125 glycoprotein contains more than 11,000 amino acids in its proteinaceous core and has been termed Muc16 to reflect the mucinlike nature of the antigen and is now identified as a new member of the protein family of mucins. [23]

The precise function of the antigenic determinant is unknown but can be found in both benign and malignant tissue.

CA 125 is expressed by more than 80% of nonmucinous epithelial ovarian carcinomas and is found in most carcinomas of Müllerian origin, including fallopian tube and primary serous peritoneal carcinoma. [11] In addition, it can be expressed by several benign and physiologic conditions, as detailed in Interpretation.

The first-generation immunoassay for CA 125 was commercialized in 1983 (CA-125 I) and used the OC 125 antibody for both capture and detection. [24] In 1991, a second-generation assay was developed (CA-125 II), using M11 mouse monoclonal antibody as capture and OC 125 as a tracer. Incorporation of these two distinct and nonoverlapping epitopes improved sensitivity and decreased variability. [25] As such, the type of immunoassay used should be considered, as results are not interchangeable.

The first-generation assay is no longer manufactured in the United States.

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