28/01/2026
CA 125 (Cancer Antigen 125) is a protein that is often measured in the blood and used as a tumor marker, especially in the context of ovarian cancer.
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🔬 What is CA 125?
Full Name: Cancer Antigen 125
Type: Glycoprotein
Produced by: Cells derived from coelomic epithelium (e.g., ovaries, peritoneum, pleura, and pericardium).
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🩺 Clinical Use of CA 125:
Purpose Details
1. Tumor Marker Most commonly associated with epithelial ovarian cancer.
2. Monitoring Used to monitor response to treatment and recurrence in patients already diagnosed with ovarian cancer.
3. Not for Screening Alone Not specific or sensitive enough to be used as a general screening test for ovarian cancer.
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📈 Normal and Abnormal Levels:
Normal range: Usually < 35 U/mL
Elevated CA 125 can occur in:
Ovarian cancer
Endometriosis
Uterine fibroids
Pelvic inflammatory disease (PID)
Liver disease
Pregnancy
Menstruation
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⚠️ Limitations:
Not cancer-specific: Can be elevated in non-cancerous conditions.
Not useful in all ovarian cancers: Some types do not produce CA 125.
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🧪 Test Method:
Blood test (usually immunoassay-based).
✅ Purpose:
To measure the concentration of CA 125 antigen in the blood, which can help monitor ovarian cancer and other related conditions.
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🧷 Sample Type:
Venous blood sample
Collected in a plain (red-top) or serum separator tube (SST / gold-top)
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🧫 Step-by-Step Procedure:
1. Patient Preparation
No fasting required.
Explain the procedure to the patient.
Confirm identity and consent.
2. Sample Collection
Use standard venipuncture technique.
Collect 3–5 mL of blood into the appropriate tube.
Let it clot at room temperature (~30 minutes if using a plain tube).
3. Centrifugation
Centrifuge the sample at 3000 rpm for 10–15 minutes.
Separate the serum carefully without disturbing the clot.
4. Storage (if not tested immediately)
Store serum at 2–8°C for up to 48 hours, or freeze at -20°C for longer storage.
5. Testing Method
Most commonly performed using:
Enzyme-Linked Immunosorbent Assay (ELISA)
Electrochemiluminescence immunoassay (ECLIA) — e.g., on Roche Cobas analyzers
Chemiluminescent Microparticle Immunoassay (CMIA) — e.g., on Abbott Architect analyzers
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🔍 Principle (e.g., ELISA-based Method):
1. Wells are coated with anti-CA 125 antibody.
2. Add serum sample → CA 125 antigen binds to antibody.
3. Add enzyme-linked secondary antibody → forms antigen-antibody-enzyme complex.
4. Add substrate → enzyme acts on substrate to produce color reaction.
5. Measure the optical density (OD) in a microplate reader.
6. Compare with standard curve to calculate CA 125 concentration.
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📊 Reference Range:
Normal: < 35 U/mL
Elevated: > 35 U/mL (interpret with clinical context)
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⚠️ Quality Control
Use low and high controls with each run.
Ensure calibrators are up to date.
Run duplicates if needed.
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📌 Summary:
CA 125 is most useful for:
Monitoring known ovarian cancer
Detecting recurrence
Sometimes used in conjunction with transvaginal ultrasound or other markers like HE4 and ROMA index.
