30th July 2026

Diagnosis

Classification

Table 2: Classification of Erythema Multiforme

Clinical features

Target lesions eruption over 72 hours over the distal, extensor surfaces. Prodromal symptoms are rare.

Mucosal involvement includes oral blistering and genital ulceration. Symptoms usually heal within 2-3 weeks with minimal scarring although hyperpigmentation has been reported.

Target lesion on the dorsum of the hand.

Image Source: Wikimedia Commons (2009). Erythema Multiforme Minor of the Hand, Licensed under a creative commons licence. Available for free at https://commons.wikimedia.org/wiki/File:Erythema_multiforme_minor_of_the_hand.jpg (Accessed 19 May 2025)

Differential Diagnosis

• Rowell’s syndrome (targetoid lesions in Systemic Lupus Erythematosus)
• Pemphigus Vulgaris
• Urticaria
• Polymorphous light eruption
• Steven Johnson’s syndrome/Toxic Epidermal Necrolysis
• Paraneoplastic pemphigus

Investigations

• Full blood count 
• Urea & electrolytes
• Liver function tests 
• HSV serology and other infectious disease serology 
• Chest X-ray: to look for evidence of mycoplasma pneumoniae as a cause of EM 
• Skin biopsy: upper dermal oedema and epidermal keratinocyte necrosis. 
  • Biopsy is not usually required however may be considered to differentiate from other serious skin reactions.