🧭 When to suspect

Suspect conjunctivitis in a patient with a red eye that feels gritty, itchy or burning, usually with discharge and normal vision. Conjunctivitis is inflammation of the conjunctiva – it is uncomfortable, not painful – so the diagnostic task in primary care is twofold: identify the likely cause (bacterial, viral or allergic) to guide whether any treatment is needed, and recognise the red eye that is not conjunctivitis at all (keratitis, anterior uveitis, acute angle-closure glaucoma or scleritis), which threatens sight.

The single most useful manoeuvre is to weigh the type of discharge against the dominant symptom. Itch points to allergy; a watery eye with a recent cold points to a virus; a sticky, purulent eye points to bacteria. None of these is perfectly discriminating, but together they steer management.

Feature	Bacterial	Viral	Allergic
Discharge	Purulent, sticky	Watery	Watery, stringy
Hallmark	Lids stuck on waking	Recent/concurrent URTI	Intense itch
Laterality	One then both eyes	One then both eyes	Bilateral
Key clue	Mucopurulent crusting	Pre-auricular node, follicles	Atopy; lid swelling
Contagious?	Yes	Yes (highly)	No

Source: College of Optometrists

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