Carcinoembryonic Antigen (CEA) — Tumour Marker Overview

Carcinoembryonic antigen is an oncofoetal glycoprotein involved in cell adhesion. While widely expressed in gastrointestinal tissue during embryonic life, its synthesis diminishes dramatically before birth, leaving only very low concentrations detectable in healthy adult blood serum.

In oncological practice, CEA is classified as a circulating tumour marker. Persistently high or rising levels often correlate with cellular proliferation in specific carcinomas, particularly when malignant cells overexpress the protein and shed it into the systemic circulation.

For patients undergoing cancer therapy, tracking CEA provides an objective measure of whether treatment is working. A falling CEA level often suggests that surgery, chemotherapy, or radiotherapy is reducing tumour burden. Conversely, a sustained increase can be an early warning of disease progression or recurrence, prompting timely imaging and potential adjustments to the management plan.

CEA is evaluated via a standard peripheral blood draw using an immunoassay technique. Baseline levels are typically established before initiating treatment. Routine reference values are usually below 3 to 5 nanograms per millilitre (ng/mL), though thresholds are slightly higher in individuals who smoke cigarettes.

CEA is most closely associated with colorectal adenocarcinoma. However, it can also be elevated in carcinomas of the pancreas, stomach, lung, breast, medullary thyroid, and ovary. Non-malignant conditions, including chronic smoking, cirrhosis, inflammatory bowel disease, and peptic ulcer disease, can also cause mild to moderate elevations in circulating levels.

CEA itself is not a therapeutic target, meaning treatments do not attack the protein directly. Instead, trends in CEA levels guide clinical decisions: failing to decline post-operatively may signal residual disease, while serial increases during surveillance can trigger diagnostic scans and subsequent systemic therapy changes or surgical re-evaluations.