A 55-year-old man with episodic flushing, watery diarrhoea and bronchospasm is suspected to have carcinoid syndrome. Which investigation best confirms the diagnosis biochemically?
- A Plasma chromogranin A level alone
- B Serum tryptophan concentration
- C 24-hour urinary 5-hydroxyindoleacetic acid (5-HIAA) excretion ✓
- D Urinary vanillylmandelic acid excretion
Correct answer: C. 24-hour urinary 5-hydroxyindoleacetic acid (5-HIAA) excretion
Explanation
Carcinoid tumours secrete serotonin, which is metabolised by monoamine oxidase to 5-hydroxyindoleacetic acid and excreted in urine. A 24-hour urinary 5-HIAA is the standard diagnostic test, though it can be falsely elevated by serotonin-rich foods such as bananas, tomatoes, walnuts and certain drugs. Vanillylmandelic acid is the catecholamine metabolite measured in phaeochromocytoma, and chromogranin A is supportive but nonspecific.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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