Erythema Migrans Rash: Why Overreliance Can Harm Lyme Disease Diagnosis

Two studies published in 1996 and 1998 required participants to meet the
Centers for Disease Control and Prevention (CDC) surveillance case definition
for Lyme disease. Under this definition, an erythema migrans (EM) rash must be
at least 2 inches in diameter to qualify.

How Common Is the Erythema Migrans Rash?

The incidence of erythema migrans is lower in studies that are more inclusive
and do not exclude patients who lack a rash. For example, only 25% of patients
had an EM rash in the original study that first described Lyme disease in
Lyme, Connecticut.

Similarly, in a National Institutes of Health (NIH)–sponsored study of Lyme
encephalopathy, only 56% of patients had an erythema migrans rash. Importantly,
all participants had confirmed Lyme disease based on a positive IgG Western
blot test.

Why Accurate EM Rash Incidence Matters

Why is it important to have a realistic estimate of the incidence of an
erythema migrans rash?

Lyme disease patients may be denied timely diagnosis or treatment if clinicians
expect an erythema migrans rash to be present before considering Lyme disease.

In addition, the true incidence of Lyme disease in the United States may be
significantly underestimated if surveillance and diagnosis rely too heavily
on the presence of an EM rash.