When taking the presenting history of red eye, it is important to consider the serious vision-threatening diagnoses along with more common causes. By including key questions and noting down pertinent negative features, the differential diagnosis can be narrowed and a decision can be made on whether referral for further ophthalmological treatment is required or treatment can be given in the primary care setting.
Key questions to consider include: 
When did the condition start?
Is the condition unilateral or bilateral?
A foreign body or trauma is usually unilateral, whereas conjunctivitis may start as unilateral and then become bilateral.
Was the onset of the symptoms acute or gradual?
Acute onset may indicate a corneal foreign body or abrasion or foreign body trauma.
The most important associated symptoms to note in the history are the presence of reduced visual acuity or a deep aching pain within the eye, indicating the presence of a more serious underlying diagnosis, such as angle-closure glaucoma, anterior uveitis, or scleritis.
If the patient reports a foreign body sensation, the possible diagnoses are conjunctivitis, conjunctival/subtarsal foreign body, corneal foreign body, keratitis, and corneal ulcer. If a foreign body is suspected, ask whether the patient has undertaken any recent activity that could have resulted in this and, if so, whether he or she was wearing eye protection. The nature of the activity will also point to potential penetrating injuries: for example, the use of mechanical saws and hammering can produce high-velocity foreign bodies, which have the ability to penetrate the surface of the globe and become intra-ocular.
If the patient wears contact lenses, contact lens-related red eye should be referred for further ophthalmological review, as corneal ulceration must be excluded.
If there is any discharge present, factors that can help to identify the presence of conjunctivitis and the possible underlying aetiology are: 
Watery, purulent, or mucopurulent discharge; for example:
A watery discharge is seen in viral conjunctivitis
A profuse mucous discharge is seen in chlamydial conjunctivitis
A purulent discharge is seen in gonococcal conjunctivitis
Discharge that is worse in the mornings:
May be due to allergy
Presence of itch:
Usually due to allergy
Minimal itch may be present in chlamydial conjunctivitis
History of atopy.
If the patient is photophobic, this can indicate possible underlying anterior uveitis or corneal epithelial disturbance. The systemic associations of photophobia, such as meningitis, should always be considered in an unwell patient. 
The physician should consider whether the patient has had previous similar episodes or whether there are any underlying systemic associations of conditions known to cause red eye, such as:
HLA-B27 histocompatibility complex-positive patients
Pauciarticular juvenile chronic arthritis
Connective tissue disorders (including rheumatoid arthritis, Sjogren's syndrome, and systemic lupus erythematosus)
Granulomatosis with polyangiitis (Wegener's)
The current use of any ophthalmological medications as well as any systemic medications known to precipitate causes of red eye should be noted. These include mydriatics and systemic anticholinergic medications. Patients on anticoagulants may be predisposed to subconjunctival haemorrhage. Persistence of conjunctivitis despite topical antibiotics should prompt evaluation for a different aetiology.
Examination of the eye in a primary care setting requires the use of a Snellen chart, a light source, fluorescein, and a cotton wool bud to evert the upper lid.  A step-wise approach can be used, with consideration of the differential diagnosis from the history.
Visual acuity should be checked in all patients, as a reduction may indicate a more serious underlying cause for the red eye.
Inspection of the lids and brow should be performed to exclude peri-orbital injury. The position of the lid margins should be checked for the presence of trichiasis, an entropion, or an ectropion. If any discharge can be seen, conjunctivitis should be considered. If the condition is bilateral with purulent discharge, it should be treated as conjunctivitis.
On inspection of the ocular surface and subtarsal surface, the pattern of redness, an important feature, should be assessed. Segmental injection may indicate episcleritis or the presence of a foreign body. Ciliary or limbal (junction of the cornea and sclera) injection occurs in anterior uveitis and corneal conditions. Redness that is localised and well demarcated with quiet surrounding conjunctiva is seen in subconjunctival haemorrhage, prompting the patient's blood pressure to be checked. Generalised injection, with engorgement of the deeper scleral vessels and pain on palpation of the globe, indicates the presence of scleritis.  The tarsal conjunctiva should be inspected for papillae, seen in allergic conjunctivitis, or follicles, seen in chlamydial conjunctivitis. If there is a history of a foreign body, the upper lid should be everted with a cotton wool bud to exclude a subtarsal position. If the foreign body cannot be found and the activity during the incident may have produced a high-speed foreign body, then further ophthalmological opinion should be sought to exclude an intra-ocular position. Instilling fluorescein during inspection of the ocular surface can allow the visualisation of foreign bodies, corneal abrasions, and corneal ulcers. If there is fluorescein staining present on the cornea or the cornea appears cloudy (seen in angle-closure glaucoma), referral for further ophthalmological examination is advised. Rose bengal stain can be used in patients in whom dry eye is suspected as the underlying cause.
Pupillary reactions. The physician should observe for anisocoria (unequal pupil size), and if this is present should refer for further ophthalmological assessment.  Using a pen torch (or equivalent light source), the direct and consensual pupillary responses should be checked. If the pupillary response is abnormal in the presence of red eye, anterior uveitis and angle-closure glaucoma need to be excluded. If the patient is photophobic on examination, further referral is also advised. 
Swabs for bacterial, viral, and chlamydial culture can be taken in patients with suspected conjunctivitis. Investigation into the underlying systemic causes of red eye should be performed in a specialist clinic after a definite ophthalmological diagnosis has been given. Certain local causes of red eye including ectropion, entropion, corneal ulcer, contact lens-related red eye, corneal abrasion, corneal foreign body, penetrating and chemical trauma, scleritis, and angle-closure glaucoma should be evaluated further by an ophthalmologist.
Computed tomography imaging of the orbits should be performed if a high-velocity penetrating injury is suspected.
If acute glaucoma is suspected, intra-ocular pressure should be measured in the emergency department.
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