Test Catalog

Test ID: CA25    
Cancer Antigen 125 (CA 125), Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Evaluating patients' response to ovarian cancer therapy


Predicting recurrent ovarian cancer


This test is not useful for cancer detection screening in the normal population.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Cancer antigen 125 (CA 125) is a glycoprotein antigen normally expressed in tissues derived from coelomic epithelia (ovary, fallopian tube, peritoneum, pleura, pericardium, colon, kidney, stomach).


Serum CA 125 is elevated in approximately 80% of women with advanced epithelial ovarian cancer, but assay sensitivity is suboptimal in early disease stages. The average reported sensitivities are 50% for stage I and 90% for stage II or greater.


Elevated serum CA 125 levels have been reported in individuals with a variety of nonovarian malignancies including cervical, liver, pancreatic, lung, colon, stomach, biliary tract, uterine, fallopian tube, breast, and endometrial carcinomas.


Elevated serum CA 125 levels have been reported in individuals with a variety of benign conditions including: cirrhosis, hepatitis, endometriosis, first trimester pregnancy, ovarian cysts, and pelvic inflammatory disease. Elevated levels during the menstrual cycle also have been reported.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Males: Not applicable

Females: <46 U/mL

Interpretation Provides information to assist in interpretation of the test results

In monitoring studies, elevations of cancer antigen 125 (CA 125) above the reference interval after debulking surgery and chemotherapy indicate that residual disease is likely (>95% accuracy). However, normal levels do not rule out recurrence.


A persistently rising CA 125 value suggests progressive malignant disease and poor therapeutic response.


Physiologic half-life of CA 125 is approximately 5 days.


In patients with advanced disease who have undergone cytoreductive surgery and are on chemotherapy, a prolonged half-life (>20 days) may be associated with a shortened disease-free survival.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Results cannot be interpreted as absolute evidence of the presence or absence of disease.


Serum markers are not specific for malignancy and values may vary by method. Values obtained with different assay methods cannot be used interchangeably.


Some individuals have antibodies to mouse protein (HAMA), which can cause interference in immunoassays that employ mouse antibodies. In particular, it has been reported that serum specimens from patients who have undergone therapeutic or diagnostic procedures that include infusion of mouse monoclonal antibodies may produce erroneous results in such assays. Rerunning the specimen in question after additional blocking treatment may resolve the issue.


No interference was observed from rheumatoid factors up to a concentration of 1200 IU/mL.


There is no high-dose hook effect at cancer antigen 125 (CA 125) concentrations of up to 50,000 U/mL.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Sturgeon CM, Duffy MJ, Stenman UH, et al: National Academy of Clinical Biochemistry laboratory medicine practice guidelines for use of tumor markers in testicular, prostate colorectal, breast, and ovarian cancers. Clin Chem. 2008 Dec;54(12):11-79

2. Salani R, Backles FJ, Fung MFK, et al: Posttreatment surveillance and diagnosis of recurrence in women with gynecologic malignancies: Society of Gynecologic Oncologists recommendations. Am J Obstet Gynecol.2011 Jun;204(6):466-478

3. The Role of the Obstetrician-Gynecologist in the Early Detection of Epithelial Ovarian Cancer. American College of Obstetricians and Gynecologists. 2011. Committee Opinion Number 477