• Assess all people with a unilateral red eye for features of one of the three serious causes of a red eye.
• If any of the following features are present, acute glaucoma, keratitis and iritis should be strongly suspected and the person referred for same-day specialist assessment.
• Moderate to severe eye pain or photophobia.
• Marked redness of the eye in one eye: the greater the redness, the more likely that the cause is serious. Ciliary injection, which is not always obvious, occurs with inflammation of deeper structures. It is indicated by redness and dilated blood vessels seen between the white of the eye and the coloured part (iris) of the eye.
• Reduced visual acuity: any loss of visual acuity, as measured with a Snellen chart.
• If there are no features to suggest a serious cause of a red eye exclude:
• Superficial corneal injury is likely when eye pain follows trauma or possible trauma and a foreign body is present. It is confirmed when an abrasion is seen following staining with fluorescein.
• Subconjunctival haemorrhage is asymptomatic and other than redness there are no other abnormal findings. The redness is well demarcated, does not cover the cornea and obliterates conjunctival blood vessels.
• Irritant conjunctivitis is likely when conjunctivitis is associated with an identifiable mechanical or irritant cause such as a displaced contact lens, a foreign body or eye lashes rubbing against the surface of the eye or a chemical splashing into the eye.
• Allergic conjunctivitis is likely when conjunctivitis is associated with itching and is recurrent following exposure to a known allergen.
• Identify the characteristic features of infective conjunctivitis. An infective cause of conjunctivitis is more likely when:
• There is a history of close contact with another affected person.
• Symptoms of upper respiratory tract infection are present.
• The eyes are glued together by discharge after sleep, or mucopurulent discharge is seen on examination.
• Conjunctivitis starts in one eye then spreads to the other.
• An enlarged lymph node in front of the ear is identified.
• Swab the eye to identify the infective cause when infective conjunctivitis is hyper–acute or persistent. This is not usually considered useful for people with acute infective conjunctivitis.

[American Academy of Ophthalmology, 2003; Yanoff and Duker, 2004]