how do I work up a red eye in a dog and prioritize differentials between uveitis, glaucoma, and conjunctivitis?

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The first priority in a red eye workup is ruling out glaucoma and corneal ulceration before prescribing any topical medication. Tonometry and fluorescein staining must be performed before empiric topical antibiotics or steroids are applied, as steroids are contraindicated over an ulcer and glaucoma requires urgent pressure-lowering intervention rather than anti-inflammatory therapy.1 Perform the diagnostic sequence in this order: fluorescein staining, Schirmer tear test, and tonometry — all before any topical treatment. This sequence rules out the two conditions most likely to cause permanent vision loss if missed or mistreated: corneal ulceration and glaucoma. Conjunctivitis is a diagnosis of exclusion once these are cleared.1 Organize your differentials by the dominant clinical sign to narrow the list before diagnostics. Ocular redness, cloudiness, discomfort, apparent globe enlargement, and vision loss each carry distinct differential lists, and categorizing the presentation on this basis focuses the workup.1 A dog presenting with a red, painful, cloudy eye and apparent globe enlargement should have glaucoma at the top of the list until tonometry proves otherwise. A dog with redness, discharge, and no pain or cloudiness is more consistent with conjunctivitis. Examination technique directly affects diagnostic accuracy. Ophthalmic examination should be performed in dim light with a bright focal light source and competent head restraint — these conditions are required to properly characterize anterior segment changes such as aqueous flare, miosis, or corneal edema that distinguish uveitis from glaucoma.1 Uveitis and glaucoma can coexist and mimic each other clinically, making tonometry non-negotiable when either is suspected. Both can present with a red, painful, cloudy eye, but their treatments are opposing — uveitis is managed with anti-inflammatories and mydriatics, while glaucoma requires pressure reduction. Prescribing empiric steroids without tonometry in a gl