CA 125 Test
The CA 125 test measures serum cancer antigen 125, a mucin glycoprotein expressed by ovarian and mesothelial epithelia. It is used to stratify pre-operative risk in pelvic masses, monitor treatment response and detect recurrence in epithelial ovarian cancer, and assess ascites; benign gynaecological and serosal conditions limit specificity.
1
Biomarkers
Blood (serum)
Sample
2–3 business days
Turnaround
No
Fasting
What is the CA 125 test?
The CA 125 Test measures serum concentration of a mucin glycoprotein predominantly expressed by ovarian epithelium and mesothelial surfaces. It is not a screening test for the general population but a targeted tool in three main settings: evaluation of a pelvic mass when ovarian malignancy is in the differential, monitoring treatment response and detecting recurrence in patients with known epithelial ovarian cancer, and occasionally assessing other malignancies where CA 125 is a minor tumour marker. CA 125 is one component of multimodal risk algorithms (such as the Risk of Malignancy Index and Risk of Ovarian Malignancy Algorithm) that combine biomarker level, menopausal status, and ultrasound findings to triage adnexal masses for specialist review or surgery. In established ovarian cancer the kinetics of CA 125 — how rapidly it falls with chemotherapy and whether it rises again — correlate with prognosis and guide decisions about second-look surgery or change of regimen. Practitioners order CA 125 to stratify risk and inform next steps rather than to diagnose or exclude cancer in isolation.
What does the CA 125 test measure?
CA 125 is a high-molecular-weight glycoprotein encoded by the MUC16 gene and expressed on the cell surface of coelomic epithelia, including mesothelial cells lining the peritoneum, pleura, and pericardium, and epithelial cells of the fallopian tube, endometrium, and endocervix. Physiologically it is shed into body fluids and cleared by the reticuloendothelial system; serum concentrations are low in health but rise when epithelial turnover increases or when peritoneal irritation drives mesothelial shedding.
CA 125 is the most widely used serum biomarker in ovarian cancer, both for pre-operative risk stratification in pelvic masses and for monitoring response to therapy and detecting recurrence in established epithelial ovarian cancer. Its sensitivity and specificity as a diagnostic tool are limited by benign gynaecological and non-gynaecological causes of elevation, so it is interpreted in context — a rising trend in a post-menopausal woman with a complex adnexal mass has different weight than an isolated elevation in a pre-menopausal woman with fibroids.
Why is the CA 125 test ordered?
- Pre-operative evaluation of a pelvic or adnexal mass in a post-menopausal woman, where elevated CA 125 raises concern for epithelial ovarian cancer and prompts referral to gynaecological oncology
- Baseline and serial monitoring in patients with diagnosed epithelial ovarian, fallopian tube, or primary peritoneal carcinoma, to assess response to chemotherapy and detect biochemical recurrence
- Investigation of ascites of uncertain origin, where a very high CA 125 suggests peritoneal carcinomatosis, though benign causes (cirrhosis, heart failure, tuberculosis) can also elevate it
- Risk stratification in conjunction with pelvic ultrasound findings and menopausal status, using algorithms such as RMI or ROMA to decide on specialist referral
- Follow-up surveillance in women with a history of treated ovarian cancer, where a doubling of CA 125 or a level rising above the institutional threshold may precede clinical or radiological recurrence
- Occasional use in monitoring other CA 125-secreting malignancies (endometrial, pancreatic, lung adenocarcinoma), though it is not the primary marker for those sites
- Supporting the diagnosis of ovarian hyperstimulation syndrome in fertility treatment, where CA 125 can be markedly elevated alongside clinical and ultrasound findings
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
2–3 business days
Collection notes
Phlebotomy at the processing laboratory's draw site (LabCorp, Quest, or Access Medical Laboratories, depending on routing); no special preparation. Avoid drawing immediately post-operatively or during menstruation if baseline or surveillance values are being established, as these transiently elevate CA 125.
Specimen requirements
Minimum 0.5 mL serum (gel separator acceptable). Stable refrigerated for up to one week; frozen serum is acceptable. Note the performing laboratory and assay platform for serial comparability, as reference intervals and absolute values differ.
What can affect CA 125 results?
Endometrial tissue expresses CA 125; menstruation, endometriosis, and adenomyosis can raise levels modestly, particularly in pre-menopausal women, reducing specificity.
Any process that irritates the peritoneum or increases turnover of coelomic epithelium can elevate CA 125, so fibroids, functional cysts, and infection are common benign causes.
CA 125 rises physiologically in early pregnancy and falls as gestation progresses; it is not useful for cancer assessment during pregnancy.
Serosal inflammation and mesothelial irritation from any cause — cardiac, hepatic, infectious, inflammatory — can drive CA 125 into the hundreds or occasionally thousands, mimicking malignancy.
Some studies report modestly higher CA 125 in smokers, though the effect is inconsistent and not large enough to alter interpretation materially.
Surgical trauma to peritoneal surfaces transiently raises CA 125; allow several weeks for post-operative levels to return to baseline before interpreting surveillance results.
What do CA 125 results mean?
Elevated
Ovarian, fallopian tube, or primary peritoneal carcinoma (especially high-grade serous); endometrial cancer; pancreatic, gastric, lung, or breast adenocarcinoma (less commonly); peritoneal carcinomatosis from any primary. Benign: endometriosis, fibroids, ovarian cysts, pelvic inflammatory disease, pregnancy, menstruation, liver cirrhosis, heart failure, pleuritis, peritonitis. The magnitude, trend, menopausal status, and imaging context determine whether elevation is concerning.
Normal
A normal CA 125 does not exclude ovarian cancer — mucinous, clear-cell, and early-stage tumours frequently do not secrete CA 125, and roughly 20–30 per cent of epithelial ovarian cancers present with a normal level. Imaging and clinical assessment remain essential.
Reduced
CA 125 does not have a clinically meaningful 'low' threshold; low or undetectable values are normal and reassuring in the context of adnexal mass evaluation.
Note: CA 125 is not a screening test and should not be ordered in asymptomatic women at average risk. It is assay-dependent, so serial measurements should use the same platform and laboratory. Menopausal status, imaging findings, and clinical context are mandatory to interpretation; the marker is a component of risk algorithms, not a standalone diagnostic.
Frequently asked questions
CA 125 is a glycoprotein expressed by ovarian epithelium and mesothelial surfaces. Elevated levels can indicate epithelial ovarian cancer, but also benign conditions such as endometriosis, fibroids, menstruation, liver cirrhosis, and heart failure, so context and imaging are essential to interpretation.
How to access the CA 125 test
The CA 125 reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
Create Free AccountAbout this page
Published: October 7, 2026
Last reviewed: October 7, 2026
- Menon U, et al. Ovarian cancer population screening and mortality after long-term follow-up in the UK Collaborative Trial of Ovarian Cancer Screening (UKCTOCS): a randomised controlled trial. Lancet. (2021)
- American College of Obstetricians and Gynecologists Committee Opinion No. 477: the role of the obstetrician–gynecologist in the early detection of epithelial ovarian cancer in women at average risk. Obstet Gynecol. (2011)
- Duffy MJ, et al. Tumor markers in ovarian cancer: European Group on Tumor Markers (EGTM) guidelines for clinical use. Int J Gynecol Cancer. (2005)
- Moore RG, et al. A novel multiple marker bioassay utilizing HE4 and CA125 for the prediction of ovarian cancer in patients with a pelvic mass. Gynecol Oncol. (2009)
- NICE guideline [NG12]: Suspected cancer — recognition and referral. (2015)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
Related tests
cancer biomarkers
CEA
The CEA test measures serum carcinoembryonic antigen, a glycoprotein re-expressed and shed by many epithelial cancers. It is principally used to monitor colorectal cancer after treatment, detect recurrence, and assess response to therapy, with baseline and serial trends more informative than isolated values.
cancer biomarkers
CA 19-9
CA 19-9 measures a sialylated Lewis A antigen shed by pancreaticobiliary and gastrointestinal epithelia. It is the principal tumour marker for pancreatic ductal adenocarcinoma, used to monitor therapy response, detect recurrence, and inform prognosis. Elevations occur in benign biliary obstruction and pancreatitis; Lewis-negative individuals cannot synthesise the antigen.