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Osmotic vs Secretory Diarrhoea — OPD Differentiation Guide

A bedside-first guide that turns watery diarrhoea into a practical clinical decision: identify the mechanism, ask the right fasting question, recognise dehydration risk, and choose the next step with confidence.

The decisive OPD question

Does the diarrhoea continue during fasting?

Stops or clearly improves Favors osmotic diarrhoea — remove the poorly absorbed solute and the osmotic pull falls.

Persists despite fasting Favors secretory diarrhoea — epithelial secretion or failed absorption continues without food.

This single distinction rapidly organises the history, likely causes, stool pattern, dehydration risk, and investigation strategy.

Inside This Guide

Mechanism made simple — solute-driven water retention versus active water-and-electrolyte secretion.
Cause-based reasoning — lactose, sorbitol, magnesium and malabsorption versus cholera, ETEC, C. difficile, bile salts and endocrine causes.
Stool-pattern clues — moderate-volume bloating or frothiness versus continuous large-volume watery or rice-water stools.
The fasting test — the highest-yield bedside discriminator and why it works physiologically.
Targeted OPD history — food triggers, medicines, travel, antibiotics, contaminated water, fever, blood, mucus and shared-food illness.
Risk assessment — dehydration, urine output, postural giddiness, thirst, vomiting and when the case needs escalation.
Stool osmotic gap — the practical concept, formula, and high-gap versus low-gap interpretation.

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