Non semper ea sunt quae videntur (Things are not always what they seem)

-Phaedrus

Rubor (redness), tumour (swelling), dolor (pain) and calor (heat) represent the four classical signs of inflammation, as described by the Roman physician Celsus in the first century AD.1 When observed in the skin in the acute setting, these signs are often considered synonymous with bacterial cellulitis, a reasonable assumption given the incidence and importance of early treatment of skin and soft tissue infections.2 Unfortunately, an incorrect initial diagnosis of infection can continue to follow the patient, resulting in unnecessary treatments, prolonged discomfort, and delayed diagnosis. In one study, some 13.6% of patients referred to a specialty centre for cellulitis were found to have alternative diagnoses, suggesting that this is not a rare occurrence.3

Many entities may mimic cellulitis and, compounding the confusion, many of these processes may be complicated by secondary or concomitant bacterial infection.3^–5 For this review, however, I have selected three relatively common cellulitis mimics to discuss in detail: allergic contact dermatitis, erythema nodosum and insect bite-induced hypersensitivity (papular urticaria). By closely examining these cases, I hope to identify some of the pitfalls in the diagnosis of cellulitis and highlight some helpful pearls that may be applied more broadly.

INTRODUCTIONS

Cellulitis is an acute bacterial infection of the skin and subcutis with secondary inflammation, usually characterised by warmth, tenderness, erythema and swelling.6 It is an extremely common clinical problem, representing 2.2% of all office visits for general practitioners in one study.7 Most cases are caused by Streptococci and Staphylococcus aureus, although many other pathogens may be implicated.6 The commonness of cellulitis appears to exploit the bias of the availability heuristic — the decision-making shortcut that places weight on things that are easily called to mind …