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PA23.8 | Colorectal Carcinoma — SDL Guide (Part 4)

Tumour Markers and Screening

A four-panel educational diagram explains CEA re-expression and monitoring in colorectal cancer, false positives, screening test comparison, and colonoscopic polyp removal. — **Panel A:** CEA expression pathway: fetal gut, normal adult colon, colorectal carcinoma, serum CEA, pre-operative baseline, curative resection, falling CEA, rising CEA indicating recurrence before clinical or radiological signs.. **Panel B:** CEA diagnostic limitations: poor sensitivity/specificity, not for diagnosis or screening, false-positive causes including smoking, cirrhosis, and pancreatitis.. **Panel C:** Screening methods: FOBT, FIT, and colonoscopy with test principle, frequency or indication, and target population.. **Panel D:** Colonoscopy view showing colon mucosa, adenomatous polyp, biopsy/snare removal, and prevention of adenoma-to-carcinoma progression..

Carcinoembryonic antigen (CEA)
- CEA is an oncofetal glycoprotein normally expressed in fetal gut; re-expressed in CRC.
- Sensitivity/specificity for diagnosis: poor — elevated in smokers, cirrhosis, pancreatitis; NOT used for diagnosis or screening.
- Primary role: Post-operative monitoring for recurrence. Pre-operative CEA establishes baseline; rising CEA after curative resection is the earliest signal of recurrence (may precede clinical/radiological signs by months).
- Elevated pre-operative CEA correlates with advanced stage and poorer prognosis.

Screening (NMC-relevant):

Method	Description	Population
FOBT (Faecal Occult Blood Test)	Detects haem-peroxidase activity; annual; inexpensive	Average-risk adults ≥50 years
Faecal immunochemical test (FIT)	Human-specific Hb antibody; fewer false positives than guaiac FOBT	Preferred over guaiac
Colonoscopy	Gold standard — visualise + biopsy polyps; removes adenomas	Every 10 years (average risk) or following positive FOBT
CT colonography	For incomplete/contraindicated optical colonoscopy	Selective use

Key teaching point: Screening finds and removes adenomas before they become invasive — interrupting the adenoma-carcinoma sequence. This is why CRC is largely preventable.

CLINICAL PEARL

The two CRC presentations to instantly recognise in clinical exam:

Elderly woman + iron-deficiency anaemia + positive FOBT + no visible bleeding → Right-sided CRC (caecum/ascending) — anaemia from occult blood loss through the exophytic fungating tumour into a wide, liquid-filled lumen.

Middle-aged person + LIF pain + obstruction + pencil-thin stools + rectal bleeding → Left-sided CRC (sigmoid/rectum) — the napkin-ring lesion in a narrow, stool-packed lumen.

Remember: the same carcinoma at the molecular level but completely different presentations due solely to anatomy and luminal content. The classic USMLE/MBBS trap is attributing both presentations to a single image — left-sided lesions obstruct; right-sided lesions bleed silently.

SELF-CHECK

Which of the following is the CORRECT primary clinical application of CEA (carcinoembryonic antigen) in colorectal carcinoma?

A. Population screening in average-risk adults

B. Primary diagnosis when colonoscopy is unavailable

C. Post-operative monitoring for disease recurrence

D. Distinguishing adenocarcinoma from lymphoma histologically

Reveal Answer

Answer: C. Post-operative monitoring for disease recurrence

CEA lacks adequate sensitivity and specificity for diagnosis or screening — it is elevated in many benign conditions (smoking, cirrhosis) and is normal in ~40% of CRC at diagnosis. Its validated role is post-operative monitoring: a rising CEA after curative resection is the earliest indicator of recurrence and prompts imaging/re-staging. Pre-operative CEA level is also used as a prognostic baseline.