Laboratory Testing for Bullseye Rash (Erythema Migrans)
Do not order laboratory tests for a patient presenting with a typical bullseye rash (erythema migrans) in an endemic area—the diagnosis is clinical and testing is not indicated. 1
Clinical Diagnosis is Preferred
The 2020 IDSA/AAN/ACR guidelines provide a strong recommendation that patients with potential tick exposure in a Lyme disease endemic area who have one or more skin lesions compatible with erythema migrans should receive clinical diagnosis rather than laboratory testing 1. This is based on moderate-quality evidence and represents the current standard of care.
Why No Testing for Typical EM?
- Serology is often negative early: Up to 50% of patients with erythema migrans are seronegative at initial presentation, particularly when the rash has been present for a short duration 2
- Testing delays treatment: Clinical diagnosis allows immediate antibiotic initiation, which is highly effective at preventing complications
- False positives are problematic: In low-risk populations, serologic testing carries a high risk of false-positive results that lead to unnecessary treatment 3, 4
When Laboratory Testing May Be Considered
For atypical presentations only, consider antibody testing with the following approach 1, 5:
Two-Tiered Testing Algorithm
- First tier: ELISA screening test
- Second tier: If ELISA is positive or equivocal, confirm with specific immunoblot (Western blot)
- Timing matters:
Atypical Features Warranting Testing
Testing should be reserved for cases where the rash is suggestive but atypical for erythema migrans 1. However, the guidelines emphasize this is a weak recommendation with low-quality evidence, and direct detection methods like PCR or culture are not recommended over serology.
Common Pitfalls to Avoid
- Do not order "test of cure": Positive serology can persist for years after adequate treatment and does not indicate active infection 5
- Do not test asymptomatic patients: This leads to inappropriate testing and false positives 3
- Do not delay treatment for test results: If clinical diagnosis is clear, start antibiotics immediately (doxycycline 200 mg single dose for adults, or 10-day course for treatment) 1
- Patient-requested testing: Be aware that 26% of Lyme tests are patient-requested and are more likely to be inappropriate 3
Geographic Considerations
In regions where both Lyme disease and STARI (Southern tick-associated rash illness from lone star tick) are endemic, clinical distinction may be impossible without tick identification. In these cases, treat as Lyme disease 1.
The bottom line: For typical erythema migrans in an endemic area with appropriate exposure history, skip the lab work and start treatment.