Cancer Antigen 125


Cancer antigen 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 the diagnosis of epithelial ovarian cancers was first suggested in 1983.[3] It was detected by using a monoclonal antibody OC 125, which was developed by immunizing mice through exposure to epithelial ovarian carcinoma cell lines.[4] It was later in 2001 that the genomic sequence of the antigen was elicited and was referred to as the MUC16.[5] 

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

Etiology and Epidemiology

CA 125, an antigenic membrane protein, is a high molecular weight glycoprotein. It is expressed by the cells originating from the coelomic epithelium, such as the Mullerian duct, cell lining of the pleura, peritoneum, and pericardium.[1][2] Researchers initially identified CA 125 using monoclonal antibody OC 125, followed by the development of several other antibodies in the upcoming years.[4] Currently, three groups of 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 involving M 11 like antibodies, and the third involving Ov197 like antibodies.[2] 

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% 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] Amongst the gynecologic malignancies, elevated CA 125 levels also correlate with adenocarcinoma of the endometrium and the endocervix.[7] Apart from these, CA125 becomes elevated in certain physiologic conditions such as during menstruation, the first trimester of pregnancy, and the postpartum period.[8][9][10] It is also associated with fibroids and pelvic endometriosis.[11]


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.[12] CA 125 may have a role in inhibiting cytotoxic responses of the Natural Killer cells (NK cells).[13][14] 

Under physiologic conditions, CA-125 expresses on the membrane of the cell, but it is unable to cross over to the bloodstream, as it is unable to cross across the junctional complexes of the cells. Pathological states which are 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][5] Upon their malignant transformation, the tumor cells invade and disrupt the architecture to enter the bloodstream.[7] In a benign ovarian cyst, while the antigen may be shed into the cystic fluid, it is not present in the bloodstream.[7]

Specimen Requirements and Procedure

CA 125 is detectable either through serological based tests or tissue-based studies of malignant ovarian or endometrial tissue specimen.[15] It may appear in body fluids such as the pleural fluid, peritoneal fluid in benign or malignant conditions, and ovarian cystic fluid.[7][15]

Serological assays, on serum or plasma samples, are the most commonly used quantitative method of analysis 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).[16] An immunoradiometric assay was initially the technique to detect CA 125 levels in the serum of patients.[3] However, due to elevated levels noted while menstruating, it is best performed outside of the menstrual period.[8] 

The serum samples should not be collected within two weeks of surgery as the levels may become falsely elevated secondary to tissue damage.[17] 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.[17]

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- like and M 11.[18] 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.[19] 

These tests are made available by different commercial manufacturers in different versions, such as IRMA, 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.[7]

Testing Procedures

The original CA 125 test is a homologous double determinant assay wherein the capture antibody and the tracer antibody 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.[7] 

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.[11]

Interfering Factors

The original CA 125 assay involves the use of 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 human anti-murine antibodies (HAMA).[11][20] These may interfere with the test by binding to the capture and tracer antibody, leading to a falsely elevated level of CA 125.[20] 

The presence of the HAMA is common in patients who have had exposure to murine monoclonal antibodies for therapeutic or diagnostic purposes. This interference is less observable in the newer assays.[11]

Results, Reporting, Critical Findings

In the original CA 125 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 as compared to the original CA 125 test.[18] 

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.[21] 

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.[7] 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 amongst 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 found to be between 50 to 74% and the specificity between 69 to 78%.[22]

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.[21] 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.[23] 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.[24] 

Current guidelines suggest the utilization of 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.[7] There is a significant degree of correlation between the disease progression and serum CA 125 levels, with doubling or halving of serum values considered to be clinically significant.[7] 

CA 125 levels also serve as an indicator of 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 having minimal residual disease among half of the patients.[7] 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 occurs before the tumor recurrence in 75% of the patients.[2] However, it is not feasible to use CA 125 as a screening test for patients for ovarian cancer. In subsequent studies, evaluating the benefit of using multimodal screening using a combination with transvaginal ultrasound for screening ovarian cancer, it was found to have no benefit in improving mortality.[25]

Quality control and Lab Safety

Pre-analytical errors while testing for tumor markers such as CA 125 are related to simple specimens 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. Blood collection must not be during menstruation, which can lead to falsely elevated levels (up to threefold).[16] One must exercise care during the interpretation of elevated CA 125 results in women with suspected endometriosis, pregnancy, and ascites.[16] 

Human anti-murine antibodies are known to interfere with the test, and the addition of non-immune murine serum can neutralize this interference.[11][19] For patients who will have monitoring with serial CA 125 levels, care is necessary to use the same manufacturer and laboratory, preferably. In a case where 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.[26]

Enhancing Healthcare Team Outcomes

CA 125 has found great importance as a tumor marker to evaluate patients with suspected ovarian cancer and in monitoring the disease. It is an important criterion used as per the guidelines, for referral of 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 are an indication for 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.[27] [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 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 shown to improve the accuracy of the diagnosis, to achieve accurate staging and grading of the disease, and thus improving the management of the patient. In addition to improving patient care, it provides a learning opportunity for the team members.[28] [Level 5]

Nurses involved in the care of a patient with ovarian cancer have a unique position from which to provide information and education to the patient and her family members about the disease. They may be able to identify patients in need for psychological support and counseling, thus can help improve the quality of life for their patients by coordinating referrals with nutritionists, physiotherapists, and psychologists[29]. Finally, experience shows that patients managed under interprofessional team care, involving clinicians and physicians from different specialties, had a significant survival advantage.[30] [Level 4]

Article Details

Article Author

Tanmay Gandhi

Article Editor:

Harshil Bhatt


8/8/2022 8:54:34 PM

PubMed Link:

Cancer Antigen 125



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