eTable 1. Classification of the causes of red eye by disease site.

Characteristics	Diseases of the ocular surface
	Conjunctivitis		Allergy	Dry eye	Subconjunctival hemorrhage	Erosion	Corneal ulcer	Photokeratitis	Episcleritis/scleritis
	Bacterial	Viral
Incidence	+++	+	+++	+++	+	++	–	++	–
(U)nilateral/ (B)ilateral	U/B	U/B	B	B	U	U	U	B	U
Worsening of vision	+	+++	+	+	–	++	++	+++	+
Photophobia	(+)	+++	+	(+)	–	+	+	+++	+
(L)ocalized/ (D)iffuse reddening	D	D	D	D	L	D	D	D	L
(S)uperficial/ (D)eep reddening	S	S	S	S	S	S	S	S	S/D
Secretion: (P)urulent/ (W)atery/ (M)ucinous	P	W	W	M/W	–	W	W/P	W	W
Conjunctival hemorrhage	–	(+)	–	–	+	–	–	–	–
Conjunctival chemosis	+	+++	++	–	–	–	–	+++	(+)
Corneal lesions	(+)	–	(+)	+	–	+	++	+	–
Foreign body event	–	–	–	–	+ / –	+ / –	–	–	–
Elevated ocular pressure	–	–	–	–	–	–	–	–	–
Further features	Gradual onset, often initially unilateral	Extreme symptoms, first on one side and then bilaterally	Ask about causative agent, seasonal accumulation, general history: atopy	Chronic problem, often with a long history	Arterial hypertension, blood pressure crises, trauma, foreign body event	Trauma, foreign body, eyelid malposition, eyelid closure defect	Nerve palsy, eyelid malposition, eyelid closure defect	Welding in the previous 6–8 h	Frequently recurrent, deep injection of the sclera, investigate for systemic diseases
Urgency of definitive diagnosis	Normal referral to ophthalmologist	Immediate referral to ophthalmologist following notification and hygiene measures	Normal referral to ophthalmologist	Normal referral to ophthalmologist	Normal referral to ophthalmologist	Urgent referral to ophthalmologist	Emergency, consult ophthalmologist immediately	Normal referral to ophthalmologist	Urgent referral to ophthalmologist

Incidence: +++ very often, ++ often, + occasionally, – rarely

Remainder of table: (+) may occur, +/– can occur with or without changes