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Tumor Marker: CA 125 (Cancer Antigen 125) – Quantitative Serum Concentration

Published: August 9, 2026Updated: August 17, 2026

This test measures the level of CA 125, a protein produced by certain types of ovarian cancer and other cancers, in your blood. It helps doctors monitor for early signs of cancer recurrence or progression.

Quick summary

["CA 125 is often used as a screening or monitoring tool for ovarian cancer and other gynecologic cancers."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
48677-9
Short code
CA125-IST
Synonyms
Cancer Ag125 Tiss Ql ImStn
Category
chemistry
Sample
serum
Container
Tissue cassette with 10% neutral buffered formalin fixation; paraffin-embedded tissue block preferred
Units
N/A
Critical range
0.5 N/A – —
Fasting
false
Population
adult
Sex
female
Age range
18–99 years
Method
immune-stain
CPT
88342
SNOMED
49684008
Specimen volume
Tissue section of adequate size (minimum 3-4 mm) for immunohistochemical staining
Stability
Formalin-fixed paraffin-embedded tissue blocks are stable indefinitely at room temperature; fresh tissue should be processed within 24-48 hours of collection
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
24–72 hours
Order
routine

What Is CA 125 Used For?

CA 125 is primarily used in two main contexts: **Screening** (less commonly in healthy individuals) and **Monitoring** for ovarian cancer and other cancers that may produce this marker. It can also help track how a cancer is responding to treatment or detect recurrence after initial treatment. However, CA 125 alone is not a definitive diagnostic test for ovarian cancer—it is often used alongside other tests and imaging.

Who Might Need This Test?

This test is most commonly ordered for women with a history of ovarian cancer, those at high risk due to genetic factors (e.g., BRCA mutations), or during follow-up after treatment. It may also be used in women with symptoms like persistent abdominal pain, bloating, or unexplained weight loss that could suggest ovarian cancer.

What Does a High or Low Level Mean?

High levels of CA 125 can indicate ovarian cancer or other cancers that produce this marker, but they can also be elevated in non-cancerous conditions like endometriosis, pelvic inflammatory disease, or benign ovarian tumors. A low level does not necessarily mean cancer is absent—it simply suggests the marker is not elevated.

How Is the Test Performed?

A blood sample is drawn from a vein in your arm. The blood is sent to a lab where it is analyzed using an immunoassay method to measure the concentration of CA 125. Results are reported in international units per milliliter (IU/mL).

Reference range

Reported as Positive or Negative for CA-125 staining in tissue. Interpretation provided by pathologist based on staining intensity and distribution pattern.N/A

Specimen: serum

No fasting required. Tissue specimen collection requires proper fixation in 10% neutral buffered formalin.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance: CA 125 plays a key role in ovarian cancer surveillance but has limitations. While it is highly sensitive for detecting ovarian cancer, it is not specific enough to diagnose cancer alone. False positives and negatives can occur, so doctors rely on clinical correlation with imaging, physical exams, and other tests.

Short name

CA 125 Tumor Marker

Frequently asked questions

Why might my doctor order this test if I don’t have symptoms of cancer?
Your doctor may order CA 125 testing if you have a history of ovarian cancer, are at high risk due to genetics, or are undergoing follow-up care after treatment. It helps track cancer progression or recurrence, but it is not a routine screening test for healthy individuals.
Can CA 125 levels change without cancer?
Yes, CA 125 levels can be elevated in non-cancerous conditions like endometriosis, pelvic infections, or benign ovarian cysts. A single test result alone cannot confirm or rule out cancer, so doctors use it alongside other diagnostic tools.
What should I do if my CA 125 levels are high?
If your CA 125 levels are elevated, your doctor will likely order further testing, including imaging (such as ultrasounds or CT scans) and possibly additional blood tests. Follow-up care will depend on your symptoms, medical history, and test results.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 17, 2026. · Read our editorial policy