Unit Converter
CA 125 (Cancer Antigen 125)
(Mucin-16 Glycoprotein – Tumor Marker for Ovarian Cancer)
Synonyms
- CA-125
- Cancer Antigen 125
- Mucin-16
- Ovarian cancer marker
- OC125 antigen
Units of Measurement
- U/mL (Units per milliliter) - standard unit
- kU/L (kilo-units per liter) - equivalent
1 U/mL = 1 kU/L
Description
CA-125 is a high-molecular-weight glycoprotein (MUC16) expressed on mesothelial surfaces of the:
- Ovary
- Peritoneum
- Pleura
- Endometrium
It is the most widely used tumor marker for:
- Epithelial ovarian cancer (especially serous type)
- Monitoring recurrence
- Assessing treatment response
Not recommended as a screening test for the general population because of low specificity.
Physiological Role
CA-125 is normally present in:
- Endometrial tissue
- Fallopian tube epithelium
- Peritoneum/pleura
- Cervical mucus
Levels vary across the menstrual cycle and pregnancy.
Clinical Significance
Elevated CA-125
1. Ovarian Cancer (Most Major Use)
- Highest levels seen in epithelial ovarian carcinoma
- Used for:
- Initial evaluation
- Treatment response
- Detection of recurrence
- Initial evaluation
- CA-125 > 200 U/mL with adnexal mass in postmenopausal woman → high suspicion
2. Benign Gynecologic Conditions
- Endometriosis
- Menstruation
- Pregnancy
- Benign ovarian cysts
- Uterine fibroids
3. Non-Gynecologic Causes
- Liver cirrhosis
- Pancreatitis
- Peritonitis
- Heart failure (congestion)
Low CA-125
Normal and clinically insignificant.
Reference Intervals
(Tietz 8E + NCCN + Mayo + ARUP)
Reference Range
- < 35 U/mL (normal)
Mild elevation (35–200 U/mL)
- Often benign conditions
- Consider imaging + clinical context
Moderate to high elevation (>200 U/mL)
- High suspicion for malignancy, particularly in postmenopausal women with an adnexal mass
Ovarian cancer monitoring
- Falling during treatment → good response
- Rising after remission → recurrence
Unit Meanings
| Unit | Meaning |
| U/mL | Units per milliliter |
| kU/L | kilo-units per lite |
Diagnostic Uses
1. Diagnosis (Adjunct)
Not diagnostic alone, but helpful with:
- Pelvic ultrasound
- Risk of Malignancy Index (RMI)
- ROCA algorithm (limited use)
2. Ovarian Cancer Monitoring
- Track treatment response
- Assess residual/recurrent disease
- Rising trend = recurrence
3. Evaluation of Pelvic Mass
Especially in postmenopausal women:
- CA-125 >35 U/mL increases suspicion
- CA-125 >200–500 U/mL → strongly concerning
4. Non-Ovarian Malignancies
May be raised in:
- Pancreatic cancer
- Lung cancer
- Breast cancer
- GI malignancies
Analytical Notes
- Immunoassay-based → method-specific variability
- Hemolysis minimal effect
- Avoid interpretation in menstruating women (false elevation)
- Pregnancy may elevate CA-125 early in the first trimester
- Serial measurements are more meaningful than single results.
Clinical Pearls
- CA-125 is not a screening test for ovarian cancer in healthy women.
- High CA-125 with normal imaging may still be benign - always correlate clinically.
- In endometriosis, CA-125 can reach >100–200 U/mL without malignancy.
- In ovarian cancer follow-up, doubling from nadir is more significant than absolute value.
- CA-125 may be normal in up to 50% of early-stage ovarian cancers.
Interesting Fact
CA-125 was discovered in 1981 using the OC125 monoclonal antibody and remains one of the most clinically used tumor markers today.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Tumor Markers.
- NCCN Guidelines - Ovarian Cancer.
- ACOG - Management of Adnexal Masses.
- IFCC - Tumor Marker Standardization.
- Mayo Clinic Laboratories - CA-125.
- ARUP Consult - CA-125 & Ovarian Cancer.
- MedlinePlus / NIH - Tumor Markers.