Erythema Migrans

### Make the Diagnosis: Erythema Migrans

#### Prior Probability

Erythema migrans is a macular or popular erythematous skin lesion that evolves over days or weeks and expands to form a large, red lesion. The US Centers for Disease Control and Prevention (CDC) has defined additional size criteria that are to be used for surveillance monitoring (when solitary, the lesion must be at least 5 cm in diameter).1 The European case definition is similar, but it does not specify a particular size. The lesion is described as having a bluish-red hue with a distinct advancing edge that is more intensely colored. Because the clinical evaluation itself defines the presence of erythema migrans, the prior probability is not known. Complicating these considerations is that erythema migrans differs geographically.

#### Population for Whom Erythema Migrans Should Be Considered

While a rash consistent with erythema migrans is a clinical observation, there are many mimics of the lesion (see Table 61-1). The factors that should be considered are the location (tends to be in skin creases, under clothing straps, or on the trunk), evolution (evolves over days or weeks and increases 2-3 cm per day), seasonal tendency (summer months), and associated symptoms. When a patient reports a tick bite, inquire about the type of tick (see Figure 61-1).

Table 61-1Some Diagnostic Mimics of Erythema Migransa

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