What are the key clinical features that differentiate bacterial conjunctivitis from viral or allergic conjunctivitis in primary care?

Differentiating between bacterial, viral, and allergic conjunctivitis in primary care involves assessing key clinical features, including the nature of discharge, presence of itching, and associated systemic symptoms NICE CKS,NICE CKS.

• Bacterial Conjunctivitis: This type of conjunctivitis often presents with a purulent discharge, which can be significant enough to cause encrusted eyelids NICE CKS,NICE NG195. Most cases are self-limiting and resolve within 5–7 days, though topical antibiotics can be considered for severe cases or if rapid resolution is required NICE CKS.
• Viral Conjunctivitis: Viral conjunctivitis is typically self-limiting and usually resolves within one to two weeks without antimicrobial treatment NICE CKS. Discharge may be present, which can be managed by bathing and cleaning the eyelids NICE CKS. A key differentiating factor is that topical antibiotics are not appropriate for viral conjunctivitis and may cause adverse reactions NICE CKS. Viral conjunctivitis is also highly contagious, and patients should be advised on measures to prevent spread NICE CKS.
• Allergic Conjunctivitis: A prominent feature of allergic conjunctivitis is prolonged severe itching NICE CKS. It may also be associated with other systemic allergic symptoms, such as nasal and respiratory symptoms NICE CKS. Management focuses on allergen identification and avoidance, and specialist referral may be considered for multisystem disease or persistent symptoms NICE CKS.

While bacterial and viral conjunctivitis are infective, viral conjunctivitis is specifically highlighted as contagious, requiring measures to prevent spread NICE CKS. The presence of significant itching strongly points towards an allergic cause NICE CKS. The type of discharge (purulent for bacterial versus less specific for viral) can also help differentiate NICE CKS,NICE NG195.