What does the CA (Cancer Antigen) 19-9 test measure?

CA 19-9 Test: Purpose and Clinical Significance

CA 19-9 is a tumor-associated antigen that primarily tests for pancreatic cancer and other gastrointestinal malignancies, though it is not tumor-specific and has significant limitations in both sensitivity and specificity. 1

What CA 19-9 Measures

CA 19-9 (Cancer Antigen 19-9) is:

• A sialylated Lewis-a blood group antigen 1
• Originally defined by a monoclonal antibody produced from murine spleen cells immunized with a human colorectal cancer cell line 1
• Measured through radioimmunometric assay techniques 1

Clinical Applications

Primary Uses

• Monitoring disease progression in known pancreatic cancer patients
• Evaluating treatment response in pancreatic cancer
• Detecting recurrence after pancreatic cancer treatment
• Supporting diagnosis in symptomatic patients (not as a standalone test)

Not Recommended For

• Screening asymptomatic populations for pancreatic cancer 1
• Determining operability of pancreatic cancer alone 1
• Providing definitive evidence of disease recurrence without confirmation by imaging or biopsy 1

Limitations and Interpretation

Important Limitations

• Lewis antigen-negative individuals (5-10% of population) cannot produce CA 19-9, making the test ineffective in these patients 1, 2, 3
• False positives occur in:
  • Benign biliary obstruction 1
  • Inflammatory conditions of the hepatobiliary system 1
  • Other benign conditions (e.g., thyroid disease) 1
  • Other malignancies (colorectal, hepatocellular, ovarian) 1
• False negatives occur in:
  • Small pancreatic tumors 1
  • Lewis-negative phenotype 3

Interpretation Guidelines

• Normal range is typically <37 U/mL 3
• Levels >100 U/mL may suggest unresectable or metastatic pancreatic disease 3
• Serial measurements are more valuable than single readings 1
• Postoperative normalization correlates with better survival 1, 4, 5

Monitoring Protocol

For patients with pancreatic cancer:

• Measure at the start of treatment for locally advanced/metastatic disease 1
• Monitor every 1-3 months during active treatment 1
• Rising levels may indicate disease progression, requiring confirmation with imaging 1
• Postoperative measurement is recommended before adjuvant therapy 1

Clinical Pearls

• CA 19-9 should be measured after biliary decompression is complete in jaundiced patients 1
• Different testing methodologies exist; results from different methods cannot be directly compared 1
• A decrease in CA 19-9 levels by ≥20-50% from baseline after treatment correlates with better survival 3
• In patients with known IPMNs, elevated CA 19-9 is concerning for malignant transformation 2

Remember that while CA 19-9 is the most widely used biomarker for pancreatic cancer, its clinical utility is significantly limited by its lack of specificity and sensitivity, making it unsuitable as a standalone diagnostic or screening tool 6, 3.