What CA 27-29 Tests For
CA 27-29 is a blood test that detects circulating MUC-1 antigen, a glycoprotein shed by breast cancer cells, and is FDA-approved specifically for monitoring disease activity in patients with known metastatic breast cancer during active treatment. 1, 2
What the Test Measures
- CA 27-29 measures a soluble form of the MUC-1 gene product (glycoprotein) that is overexpressed and shed into the bloodstream by breast cancer cells, particularly those involving glandular epithelial tissue 3, 4
- The test detects this circulating antigen in peripheral blood using monoclonal antibody BR 27.29 1, 5
- Normal reference range is typically <38 U/mL (or <25 U/mL depending on laboratory) 3, 5
Clinical Performance in Breast Cancer
- In metastatic disease: CA 27-29 is elevated in approximately 81-86% of patients with metastatic breast cancer 2, 5
- In early-stage disease: Sensitivity is extremely poor at only 6.4% in non-metastatic breast cancer, making it clinically useless for screening or early detection 5
- CA 27-29 appears slightly more sensitive than CA 15-3 across all breast cancer stages, though both provide comparable clinical utility 2, 6
ASCO-Approved Clinical Uses
The American Society of Clinical Oncology explicitly recommends CA 27-29 for only ONE indication: monitoring patients with metastatic breast cancer during active therapy, and only when used in conjunction with imaging, history, and physical examination—never alone. 1, 2
Specific Monitoring Guidelines:
- A median increase of 32% indicates progressive disease 2
- A median decrease of 19% indicates stable or regressing disease 2
- A rising level ≥20% suggests treatment failure, particularly when measurable disease is absent 2
- Critical timing caveat: Do not interpret CA 27-29 during the first 4-6 weeks of new therapy, as spurious early rises can occur 1, 2
What CA 27-29 Should NOT Be Used For
ASCO explicitly recommends AGAINST using CA 27-29 for: 1, 2
- Screening for breast cancer in asymptomatic individuals (insufficient sensitivity and specificity)
- Diagnosis of breast cancer (cannot establish diagnosis)
- Staging at initial diagnosis (does not help with treatment decisions in early-stage disease)
- Routine surveillance after primary treatment to detect recurrence (despite detecting recurrence 5-6 months earlier, no evidence this improves survival, quality of life, or cost-effectiveness) 1
Important Clinical Pitfalls
False-Positive Elevations:
CA 27-29 can be falsely elevated in several benign conditions: 3, 4
- Pulmonary fibrosis (including idiopathic pulmonary fibrosis and hypersensitivity pneumonitis)
- Benign breast disease
- Ovarian cysts
- Liver disease
Clinical pearl: In one documented case, a patient with breast cancer and severe pulmonary fibrosis had persistently elevated CA 27-29 that normalized completely after bilateral lung transplantation, with no evidence of cancer recurrence 4
Common Ordering Errors:
- Many clinicians incorrectly order CA 27-29 for post-surgical follow-up, which is not evidence-based and may lead to unnecessary anxiety and testing without survival benefit 1, 7
- Elevated CA 27-29 always requires confirmation with imaging before modifying therapy 7
Comparison with CA 15-3
- CA 27-29 and CA 15-3 both measure MUC-1 antigen and provide comparable clinical utility 2, 6
- Either marker can be used for metastatic disease monitoring, but they should not be used interchangeably in the same patient 2
- CA 27-29 shows slightly better sensitivity in cases with low antigen concentrations (below the cut-off point) 6