Diagnosis is made by identifying clinical features and ruling out conditions that mimic impetigo. Images of impetigo and its differential diagnoses can be found at www.dermnetnz.org.

• Non-bullous impetigo (also known as impetigo contagiosa or crusted impetigo) is the most common form.
• Lesions begin as vesicles or pustules, but these are rarely visible as they rapidly burst and evolve into gold-crusted plaques, typically 2 cm in diameter (these have been described as resembling glued-on cornflakes).
• The area around the mouth and nose is most commonly affected, although other areas of the face and the extremities may also be involved. Satellite lesions may occur due to autoinoculation.
• Non-bullous impetigo is usually asymptomatic, although there may be some itching. Systemic symptoms (such as fever) are uncommon unless the infection is widespread.
• Bullous impetigo commonly affects neonates, although older children and adults can also be affected.
• Bullous impetigo presents with flaccid, fluid-filled vesicles and blisters (bullae), that are usually at least 1–2 cm in diameter. These easily burst leaving raw skin, and eventually form thin, flat, brown-to-golden crusts.
• The face is less commonly affected than with non-bullous impetigo; instead bullous impetigo tends to involve the axillae, neck folds, and nappy area. Lesions are often multiple and spread rapidly.
• Unlike non-bullous impetigo, lesions tend to be painful. Systemic symptoms (weakness, fever, and diarrhoea) are more common, and there may be regional lymphadenopathy.
• Skin swabs are not necessary to diagnose impetigo. Take a swab (for bacterial identification and sensitivity) if the infection is:
• Very extensive or severe.
• Recurrent (consider nasal swab for staphylococcal carriage).
• Suspected as being a community outbreak.
• Suspected as being caused by meticillin-resistant Staphylococcus aureus (MRSA), for instance if the person has been in contact with a person who has been diagnosed with MRSA.

Recommendations for the diagnosis and assessment of impetigo are based on expert opinion from narrative reviews [Sladden and Johnston, 2004; Watkins, 2005; Cole and Gazewood, 2007].

Skin infections and infestations

• Cellulitis is bacterial infection of the dermis and epidermis. It presents with an acute onset of red, painful, hot, swollen, tender, and well-demarcated skin, with possible blister or bullae formation. It is more common in adults.
• Erysipelas is an acute streptococcal infection of the dermis, usually affecting the face or extremities. It initially forms erythematous skin lesions with a sharply demarcated, raised edge, that rapidly enlarge. These may form a red, swollen, warm, hardened, and painful rash, similar in consistency to orange peel.
• Ecthyma is similar to impetigo and is caused by staphylococcal or streptococcal infection, but affects deeper layers of the skin.
• Candidiasis causes erythematous papules or pustules, or reddened moist plaques, and is usually confined to mucous membranes or areas such as the axilla, groin, between the fingers, and skin folds.
• Dermatophytosis causes scaly, red lesions with a slightly raised 'active border' that may contain pustules. It may be vesicular, especially on the feet.
• Herpes simplex virus usually affects the lips or genitalia, although other areas of skin may be affected. Vesicles form on an erythematous base that rupture to become erosions covered by crusts.
• Varicella (chicken pox and shingles) presents as vesicles on an intensely erythematous base, usually starting on the trunk and spreading to the face and extremities. The vesicles break down to give a crusted appearance similar to impetigo.
• Scabies causes intense itching, especially at night. Lesions consist of burrows and small, discrete vesicles, often in finger webs.

Non-infective skin diseases

• Atopic eczema presents with dry and itchy skin and is often a secondary cause of impetigo.
• Contact dermatitis causes intense itching and reddened, weeping skin. A causative irritant or allergen can sometimes be identified.
• Insect bites can cause papule formation which may be confused with impetigo. Lesions may be painful or itchy, and may become super-infected by bacteria to cause secondary impetigo.
• Other skin disorders that may be mistaken for impetigo include burns and scalds, drug reactions, Stevens–Johnson syndrome, and toxic epidermal necrolysis.

Rare causes

• Discoid lupus erythematosus may present with well-defined plaques on the face, ears, and scalp. Lesions develop as a red, inflamed patch, with a scaling and crusty appearance.
• Pemphigus foliaceus is rare but may mimic impetigo with scaling, crusting, or bullae forming a butterfly distribution on the face or on the scalp, chest, and upper back.
• Bullous pemphigoid is a benign skin disease most common in older people that is characterized by widespread blistering; it can be confused with bullous impetigo.
• Sweet's syndrome is associated with haematological disorder (in particular neutrophilia) and is characterized by the sudden onset of tender or painful plaques or nodules, with occasional pseudo-vesicle or pustule formation, and fever.

This list is derived from narrative reviews [Watkins, 2005; Cole and Gazewood, 2007].