In accordance with current practice, NICE advise: “Diagnose Lyme disease in people with erythema migrans”. This advice ensures that some patients will receive timely treatment for their infection. NICE do not indicate how many patients this might apply to.

The outbreak of arthritis and other symptoms which later became known as ‘Lyme disease’, occurred in Old Lyme, Connecticut in 1975. This outbreak represented a singular opportunity to objectively document the then, unknown infection in a discrete population. The Connecticut State Department of Health with Yale University investigated. Their report notes that: “One quarter of the patients had an unusual skin lesion before the onset of joint symptoms.”(1)

This unique investigation suggests that only 25% of infected patients will report an EM rash. Higher estimates for the incidence of EM rash can be presumed to be inflated. When it occurs, EM rash is usually the only specific visible sign of Lyme disease and when present with symptoms, is much more likely to lead to diagnosis, biasing the incidence. The NICE guideline does not indicate the frequency of EM rash, but leaves doctors to guess to what extent they should expect this sign to be present.