Unit Converter
CA 15-3 (Cancer Antigen 15-3)
(Mucin-1 Glycoprotein Fragment – Tumor Marker for Breast Cancer)
Synonyms
- CA 15-3
- Cancer Antigen 15-3
- MUC1 antigen fragment
- Breast cancer marker
- DF3 antigen
Units of Measurement
- U/mL (Units per milliliter - standard)
- kU/L (kilo-units per liter)
1 U/mL = 1 kU/L
Description
CA 15-3 is a circulating fragment of the transmembrane glycoprotein Mucin-1 (MUC1).
It is shed into the bloodstream from breast epithelial cells, especially in breast carcinoma.
Major clinical use:
- Monitoring breast cancer treatment and recurrence
- Not recommended for average-risk screening or early diagnosis
CA 15-3 is elevated in:
- Metastatic breast cancer (highest)
- Some early-stage breast cancers
- Certain benign conditions
Physiological Source
Produced by:
- Breast ductal epithelial cells
- Normal epithelial cells of lung, ovary, GI tract (low amounts)
Tumor cells overexpress MUC1, releasing larger quantities of CA 15-3.
Clinical Significance
Elevated CA 15-3
1. Breast Cancer (Most Important Use)
- Levels correlate with disease burden
- Most sensitive in metastatic disease
- Used to:
- Track treatment response
- Detect recurrence
- Monitor metastatic spread (bone/liver)
- Track treatment response
2. Benign Breast / Gynecologic Conditions
May cause mild elevations:
- Benign breast disease
- Endometriosis
- Pregnancy
- Ovarian cysts
- Mastitis
3. Non-Breast Malignancies
Moderate CA 15-3 elevation may occur in:
- Ovarian cancer
- Pancreatic cancer
- Lung cancer
- Liver cancer
- GI cancers
4. Liver Disease
Decreased clearance → increased CA 15-3.
Low CA 15-3
Normal; no clinical concern.
Reference Intervals
(Tietz 8E + NCCN + Mayo + ARUP)
Normal Range
- < 30 U/mL
- (Some labs use <25 U/mL)
Interpretation
- 30–60 U/mL → Mild elevation (often benign or early disease)
- > 60–100 U/mL → Concerning; consider metastatic disease
- > 100 U/mL → Strongly suggests advanced/metastatic breast cancer
Monitoring
- Falling CA 15-3 → good therapeutic response
- Rising trend → recurrence or metastasis
(Trend is more important than any single value.)
Unit Meanings
| Unit | Meaning |
| U/mL | Units per milliliter |
| kU/L | kilo-units per liter |
Diagnostic Uses
1. Monitoring Breast Cancer Treatment
- Chemotherapy
- Hormonal therapy
- Targeted therapy (HER2 drugs)
2. Recurrence Surveillance
- CA 15-3 rises months before clinical detection in many patients.
3. Evaluating Metastatic Spread
Especially in:
- Liver metastases
- Bone metastases
4. Adjunct in Diagnostic Workup
Not diagnostic alone, but used alongside:
- Imaging (US/Mammography/MRI/PET)
- Histopathology
- Other markers (CEA, CA 27-29)
Analytical Notes
- Method: immunoassay (chemiluminescence/electrochemiluminescence)
- Avoid interpretation in pregnancy (can elevate).
- Liver dysfunction may increase levels.
- Serial testing should use same assay platform for consistency.
- Hemolysis/lipemia → minimal effect.
Clinical Pearls
- CA 15-3 is not recommended for breast cancer screening.
- Best utility → monitoring metastatic disease, not early detection.
- Rapid increase = recurrence; slow increase = disease progression.
- Combination with CEA improves sensitivity in metastatic breast cancer.
- CA 15-3 can remain normal in 30–50% of early-stage breast cancers.
Interesting Fact
MUC1, the parent glycoprotein of CA 15-3, is overexpressed in most epithelial cancers and is being used in developing personalized cancer vaccines.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Tumor Markers.
- NCCN Guidelines Breast Cancer.
- ASCO Tumor Marker Guidelines.
- IFCC - Tumor Marker Standardization.
- Mayo Clinic Laboratories - CA 15-3.
- ARUP Consult - Breast Cancer Markers.
- MedlinePlus / NIH - Tumor Marker Overview.