How Prevalence Influences the Interpretation of Lyme Disease Test Results in a High-Incidence State (Wisconsin) Versus a Low-Incidence State (Arkansas)

Objective: Clinicians’ understanding of prevalence is key in diagnostic test interpretation. In high-prevalence scenarios, the post-test probability of a positive test result can approach 100%, whereas in low-prevalence situations, the post-test probability might only slightly exceed the initial low pre-test probability. Herein, we present an example of this issue in the context of interpreting Lyme disease test results in high- versus low-incidence states. Methods: We present a thought experiment demonstrating the effects of prevalence on the interpretation of early Lyme disease serologic testing. Results: A positive test result was found to be more than 30 times more likely to be a false positive than a true positive in Arkansas. Conclusions: In locations with low Lyme disease incidence, efforts might be better spent on thoroughly working up alternative diagnoses before considering testing for Lyme disease. Furthermore, we suggest that clinicians in low-incidence states in the southern U.S. consider waiting before treating patients empirically with antibiotics for clinical scenarios consistent with early Lyme disease, because southern tick-associated rash illness in the southern U.S. commonly mimics the classic erythema migrans rash associated with Lyme disease, but is mild and self-limiting.