Cancer Antigen 125


Cancer antigen 125 (CA 125) is an antigenic tumor marker that is commonly expressed by the epithelial ovarian neoplasms and other tissues, such as cells lining the endometrium, fallopian tubes, pleura, peritoneum, and pericardium.[1][2] The use of CA 125 as a tumor marker for diagnosing epithelial ovarian cancers was first suggested in 1983.[3] It was detected using a monoclonal antibody OC 125, developed by immunizing mice through exposure to epithelial ovarian carcinoma cell lines.[4] Later, in 2001, the genomic sequence of the antigen was elicited and referred to as the MUC16.[5]

CA 125 is one of the serological tests carried out when suspecting ovarian neoplasm in a woman and is used in monitoring patients diagnosed with epithelial ovarian cancers.[2] However, due to its low sensitivity, the test has limited utility in diagnosing early ovarian cancer. The specificity is particularly low in premenopausal women; thus, it is most useful in postmenopausal women.[6]

Etiology and Epidemiology

The human cancer antigen 125 (CA 125), also known as mucin 16 (MUC16), is a cell-surface, glycoprotein antigen normally expressed in tissues derived from coelomic epithelia, such as ovary, fallopian tube, peritoneum, pleura, pericardium, colon, kidney, and stomach.[2] Researchers initially identified CA 125 using the monoclonal antibody OC 125, followed by developing several other antibodies in the upcoming years.[4] Currently, three antibodies can be useful for identifying the CA 125 antigen, and all three groups recognize non-overlapping epitopes.[2] The first group involves OC 125-like antibodies, the second involves M 11-like antibodies, and the third involves OV197-like antibodies.[7]

In a study carried out by Bast et al. in 1983, elevated levels of serum CA 125 (> 35 U/mL) were present in 82% of patients with epithelial ovarian cancer, 28.5% of patients with non-gynecological cancers such as pancreatic, lung, breast, colorectal, and 6 % of the patients with benign diseases such as an ovarian cyst.[3] Among the gynecologic malignancies, elevated CA 125 levels correlate with adenocarcinoma of the endometrium and the endocervix.[8] Apart from these, CA125 becomes elevated in certain physiologic conditions, such as during menstruation, the first trimester of pregnancy, and the postpartum period.[9][10][11] It is also associated with fibroids and pelvic endometriosis.[12]


The inherent function of CA 125 membrane protein is still not completely clear. The latest studies suspect that the oligosaccharides associated with CA 125 might play a role in cell-mediated immunity.[13] CA 125 may have a role in inhibiting cytotoxic responses of the natural killer cells (NK cells).[14][15] Under physiologic conditions, CA-125 expresses on the cell membrane, but it is unable to cross over to the bloodstream because of junctional complexes of the cells. Pathological states associated with the disruption of this membrane barrier lead to the antigen being shed into the blood and a consequent serological rise in the levels of CA 125.[1] 

In the case of ovaries, it appears that CA 125 expresses when the ovarian epithelium undergoes metaplasia into a Mullerian-type endothelium or a neoplastic transformation.[1][2] Upon their malignant transformation, the tumor cells invade and disrupt the architecture to enter the bloodstream.[16] In a benign ovarian cyst, while the antigen may be shed into the cystic fluid, it is not present in the bloodstream.[8]

Specimen Requirements and Procedure

Cancer antigen 125 (CA 125) is detectable either through serological-based tests or tissue-based studies of malignant ovarian or endometrial tissue specimens.[17] It may appear in body fluids such as the pleural fluid, peritoneal fluid in benign or malignant conditions, and ovarian cystic fluid.[8] Serological assays on serum or plasma samples are the most commonly used quantitative analysis method for CA 125 levels in the blood. To maintain the stability of the specimen, the serum must be separated from the clot and stored at -30 Celsius (long term) or 4 Celsius (short term).[18] 

When the CA 125 assay is used for cancer diagnosis, sampling should not be done immediately before or during menstruation because the physiological elevation of the CA 125 levels may give false positive results.[9] The serum samples for CA 125 should not be collected within two weeks of surgery as the levels may become falsely elevated secondary to tissue damage.CA 125 levels have a half-life of 6 days and may require a few weeks to return to normal levels after surgery. Hence a pre-treatment sample should be used as a reference for evaluating CA 125 levels postoperatively.[19]

Diagnostic Tests

Two tests exist for detecting the levels of CA 125. The original test is a radioimmunoassay using OC 125 monoclonal antibody, which recognizes the antigenic determinants on the CA 125 glycoprotein. The second-generation CA 125 test uses two antibodies; OC 125 and M 11.[20] This test showed a higher level of precision and improved sensitivity with a lower number of false-positive results. However, later research comparing the two tests did not show one test to be superior to the other.[21]

These tests are made available by different commercial manufacturers in different versions, such as immunoradiometric assay (IRMA)and enzyme immunoassay (EIA), etc. When using these tests to monitor patients longitudinally, the tests conducted should preferably be of the same manufacturer, and the same version as different kits and versions can result in different absolute values and test sensitivity.[8]

Testing Procedures

The original CA 125 test is a homologous double-determinant assay wherein the capture and tracer antibodies are the same. The OC 125 antibody would be adsorbed onto the solid phase (capture antibody), followed by the addition of the serum. The OC 125 moieties on the CA 125 antigen would then bind to the antibody. Finally, a second radiolabelled (IRMA) or an enzyme-labeled (EIA) antibody would be added (tracer antibody), which would bind to the antigen-antibody complexes.[8] The second-generation CA 125 test is a heterologous double determinant assay, wherein the capture antibody is a monoclonal M11 antibody, and the tracer antibody is the OC 125 antibody. Since the two antibodies do not have to bind to the same epitope, there is no competition for the same binding site, thus allowing for potentially higher sensitivity.[12]

Interfering Factors

The original CA 125 assay involves using murine monoclonal OC 125 antibody as both the capture and the tracer antibody. While it is supposed to bind to the corresponding epitope on the CA 125 antigen, certain individuals demonstrate the presence of the human anti-mouse antibody or human anti-murine antibody (HAMA).[12] These may interfere with the test by binding to the capture and tracer antibody, leading to a falsely elevated CA 125. The presence of human anti-murine antibodies is known in patients exposed to murine monoclonal antibodies for therapeutic or diagnostic purposes. This interference is less observable in the newer assays.[22]

Pre-analytical errors while testing for tumor markers such as CA 125 are related to simple specimen-related errors such as incorrect sample processing, an insufficient sample, and inappropriate handling. In the case of CA 125, care must be taken to time the specimen appropriately.[18] Blood collection must not be during menstruation, which can lead to falsely elevated levels (up to threefold). One must exercise care when interpreting elevated CA 125 results in women with suspected endometriosis, pregnancy, and ascites.[23]

Human anti-murine antibodies are known to interfere with the test, and adding non-immune murine serum can neutralize this interference.[12][21] For patients who will have monitoring with serial CA 125 levels, care is necessary to use the same manufacturer and laboratory, preferably. If a different manufacturer or testing method needs to be employed, the recommendation is to carry out parallel testing by both methods to establish a new baseline for the patient.[24]

Results, Reporting, and Critical Findings

In the original CA 125  immunoradiometric assay (IRMA) test, the cut-off point for the upper limit of normal was arbitrarily set at 35 U/mL, wherein only 1 % of the healthy population had levels greater than 35 U/mL, and this reduced to 0.2 % of the healthy population when raising the cut-off point for the upper limit to 65 U/mL.[3] The second-generation CA 125 has shown to be more clinically reliable with greater precision in values < 35 U/mL, causing the test to show higher values than the original CA 125 test.[20] A meta-analysis comparing various studies using a CA 125 threshold of >35 U/mL for pre-operative identification of an adnexal mass suspicious for ovarian cancer revealed an overall sensitivity and specificity of 78.7% and 77.9%, respectively.[25]

Patients who have completed treatment for ovarian cancer and appear to be clinically free of the tumor with elevations in CA 125 (>35 U/mL) almost always have a tumor in second-look surgery.[8] CA 125 has low sensitivity (due in part to being elevated only in half of the patients with early-stage epithelial ovarian cancer and seen rarely in patients with mucinous carcinomas of the ovary) and low specificity (especially among premenopausal women, who may have other benign conditions such as fibroids, endometriosis, pregnancy, etc.) when used alone.[6] 

Other studies among postmenopausal women have shown the test to have a sensitivity between 69 to 87% and a specificity between 81 to 93%. In contrast, in premenopausal women, the sensitivity was between 50 to 74%, and the specificity was between 69 to 78%.[26]

Clinical Significance

CA 125 plays a significant role as a tumor marker. It is an essential component in the pre-operative assessment of patients with an adnexal mass suspected to have an ovarian malignancy.[25] Eighty percent of the patients diagnosed with ovarian epithelial carcinoma show an elevated CA 125 level, and levels are monitored post-treatment to assess the progression of the disease.[27] The use of CA 125 for pre-operative assessment is more valuable among postmenopausal women as compared to its use amongst premenopausal women.[6] While an elevated CA 125 level above 35 U/mL is utilized in assessing postmenopausal women, earlier guidelines by the American College of Obstetricians and Gynecologists had recommended using an arbitrary cut-off of 200 U/mL; this was, however, not supported by any research evidence.[28]

Current guidelines suggest using formal risk assessment tests such as the risk of malignancy index or the risk of ovarian malignancy algorithm (ROMA) while assessing the need for gynecological referral in premenopausal women.[6] CA 125 is successfully used for disease monitoring and evaluating the progression of the disease. There is a significant degree of correlation between the disease progression and serum CA 125 levels, with doubling or halving serum values considered to be clinically significant.[8] 

CA 125 levels also indicate the tumor status, wherein most patients with levels > 35 U/mL demonstrate disease recurrence on second-look surgery, and those with levels < 35 U/mL have minimal residual disease among half of the patients.[8] It also acts as an early predictor of outcome wherein a deviation from the ideal CA 125 regression curve is suggestive of a poor outcome. Among patients who had undergone complete remission, elevation in CA 125 occurred before the tumor recurrence in 75% of the patients.[2] However, using CA 125 as a screening test for patients with ovarian cancer is not feasible. In subsequent studies evaluating the benefit of using multimodal screening combined with transvaginal ultrasound for screening ovarian cancer, it was found to have no benefit in improving mortality.[29]

CA 125 may also have a role in monitoring chemotherapy. Serial measurements of CA 125 correlated with clinical disease outcomes in 89% of 531 patients.[19] There is a consensus among current guidelines to use CA 125 to monitor response to chemotherapy. The Gynecologic Cancer Intergroup (GCIC) defines response as a 50% reduction in CA 125 concentration compared to a pretreatment sample.[30][31] The response should be confirmed and sustained for at least 20 days. Patients can only be evaluated if they have a CA 125 concentration in a pretreatment sample that is twice the upper reference limit and taken two weeks prior to initiation of therapy. Additional samples are recommended at 2 to 4 weeks during treatment and at intervals of 2 to 3 weeks during follow-up. The same method should be used to monitor the patient throughout, and patients who receive immunotherapy (mouse antibodies) cannot be evaluated.[32] The FDA approved the serial measurement of CA 125 to aid in monitoring therapeutic response.[33]

Pre- and post-operative CA 125 concentrations may be of prognostic significance.[34] After primary surgery and chemotherapy, persistent elevations of CA 125 concentrations are associated with poor prognosis. Patients with pre-operative CA 125 concentrations > 65 U/mL are reported to have a lower 5-year survival rate and a 6.37-fold risk of death compared to patients with CA 125 levels < 65 U/mL.[19] The half-life of the CA 125 antigen is reported to have an additional prognostic value. A half-life of < 20 days is associated with improved survival compared to a half-life of >20 days.[12] Normalization of CA 125 levels after three cycles of combination therapy also correlates with improved survival. It should be noted that CA 125 concentration is not elevated in 10% to 20% of patients with advanced ovarian cancer. For these patients, using radiological imaging techniques and/or monitoring other tumor markers is necessary.[35]

Quality Control and Lab Safety

For non-waived tests, laboratory regulations require, at the minimum, analysis of at least two levels of control materials once every 24 hours. If necessary, laboratories can assay QC samples more frequently to ensure accurate results. Quality control samples should be assayed after calibration or maintenance of an analyzer to verify the correct method performance.[36] To minimize QC when performing tests for which manufacturers’ recommendations are less than those required by the regulatory agency (such as once per month), the labs can develop an individualized quality control plan (IQCP) that involves performing a risk assessment of potential sources of error in all phases of testing and putting in place a QC plan to reduce the likelihood of mistakes.[37] Westgard multi-rules are used to evaluate the quality control runs. In case of a rule violation, proper corrective and preventive action should be taken before patient testing is performed.[38]

The laboratory must participate in the external quality control or proficiency testing (PT) program because it is a regulatory requirement published by the Centers for Medicare and Medicaid Services (CMS) in the Clinical Laboratory Improvement Amendments (CLIA) regulations. It is helpful to ensure the accuracy and reliability of the laboratory concerning other laboratories performing the same or comparable assays.[39] Required participation and scored results are monitored by CMS and voluntary accreditation organizations. The PT plan should be included as an aspect of the quality assessment (QA) plan and the overall quality program of the laboratory.[40]

Enhancing Healthcare Team Outcomes

CA 125 has found great importance as a tumor marker to evaluate patients with suspected ovarian cancer and monitor the disease. It is an important criterion per the guidelines for referring patients with an adnexal mass suspicious of malignancy to a gynecological oncologist. While it has a higher specificity when used amongst postmenopausal women, very high levels amongst premenopausal women indicate referral. It has also been seen that patient outcomes have been better upon being treated by a specialized gynecological oncologist and when done in a hospital with necessary consult services with multidisciplinary collaboration.[41] [Level 4]

Interprofessional team care involves different disciplines, such as primary care physicians, gynecological oncologists, nurses, and pathologists. Each of them has an essential role in providing care for individuals with suspected ovarian cancer or monitoring the disease process in these patients and thus influencing their management. Clinicopathologic meetings involving a meeting between pathologists and clinicians are a suggested approach as a form of interprofessional care. It has been shown to improve the accuracy of the diagnosis, achieve accurate staging and grading of the disease, and thus improve the management of the patient. In addition to improving patient care, it provides a learning opportunity for the team members.[42] [Level 5]

Nurses involved in the care of a patient with ovarian cancer have a unique position to provide information and education to the patient and her family about the disease. They may be able to identify patients needing psychological support and counseling and thus can help improve the quality of life for their patients by coordinating referrals with nutritionists, physiotherapists, and psychologists.[43] Finally, experience shows that patients managed under interprofessional team care, involving clinicians and physicians from different specialties, had a significant survival advantage.[44] [Level 4]

Article Details

Article Author

Tanmay Gandhi

Article Author

Muhammad Zubair

Article Editor:

Harshil Bhatt


1/18/2023 10:46:15 AM



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