Cancer Antigen 15-3

What is the Cancer Antigen 15-3 Test?

Cancer Antigen 15-3 (CA 15-3) is a protein fragment derived from the MUC-1 glycoprotein, which is over-expressed by many breast cancer cells. Elevated levels may reflect tumour burden or disease activity in certain types of breast cancer.

The CA 15-3 blood test measures the concentration of this marker in the bloodstream. It is not used for routine breast cancer screening, but plays an important role in monitoring disease progression, evaluating response to therapy and detecting recurrence in patients with established disease.

CA 15-3 results are always interpreted alongside imaging studies, clinical findings and other tumour markers.

Why do I need a test for Cancer Antigen 15-3?

You may need this test if you have a known diagnosis of breast cancer and require ongoing monitoring. Changes in CA 15-3 levels over time can help assess how well treatment is working or whether disease activity is increasing.

The test is also commonly used during follow-up after treatment to detect early signs of recurrence. Rising levels may prompt further investigation, while stable or falling levels often indicate effective disease control.

Symptoms and scenarios where testing is recommended:

• Monitoring known breast cancer
• Assessing response to chemotherapy or hormonal therapy
• Follow-up after breast cancer treatment
• Investigating possible disease recurrence
• Supporting imaging or clinical findings
• Monitoring metastatic breast cancer
• Tracking disease progression over time

Benefits of the CA 15-3 Test

• Key marker for breast cancer monitoring
  Widely used to track disease activity in breast cancer.
• Assesses treatment effectiveness
  Falling levels often indicate a positive response.
• Detects recurrence or progression
  Rising levels may signal disease activity before symptoms appear.
• Supports long-term follow-up plans
  Useful in structured oncology surveillance.
• Complements imaging and clinical review
  Adds biochemical insight alongside scans.
• Non-invasive monitoring tool
  Requires only a blood sample.
• Allows trend analysis over time
  Serial testing provides meaningful clinical patterns.
• Supports timely clinical decision-making
  Helps guide further investigations or treatment changes.

Step-by-Step: How Testing Works

Choose your preferred testing method

You can attend one of our UK partner clinics or arrange a home or hotel visit with a mobile phlebotomist.

Option 1 – In-Clinic Blood Draw

1. Select your nearest UK clinic and book an appointment.
2. A clinician or phlebotomist collects a venous blood sample.
3. The sample is analysed for CA 15-3 levels.
4. Results are delivered securely online.

Option 2 – Home or Hotel Phlebotomy (+£60)

1. Book a UK-wide home or hotel visit at a convenient time.
2. A trained phlebotomist collects your blood sample on-site.
3. The sample is packaged and transported to the laboratory.
4. Results are uploaded digitally without the need to travel.

Access your report

You can download and share your results with your GP or oncologist.

Your Results Explained

Low Levels

Low CA 15-3 levels are considered normal.
This suggests no biochemical evidence of active CA 15-3-producing tumour activity.
Typical UK reference range: usually <30 U/mL (ranges may vary by laboratory).

Normal Levels

Normal results indicate expected antigen levels in the bloodstream.
This reduces the likelihood of active disease but does not completely exclude cancer.

High Levels

Elevated CA 15-3 levels may indicate active or progressing breast cancer.
Levels may also rise in advanced disease or metastatic spread.
Results must always be interpreted alongside imaging, symptoms and clinical context.