Lyme Evidence Sheet
Reviewed 2026-07-24When does a possible Lyme rash matter more than a blood test?
An expanding rash compatible with erythema migrans can be clinically important, and early antibody tests can be negative. A clinician needs to evaluate the rash, symptoms, exposure, and timing.
What the evidence says
- CDC describes erythema migrans as an expanding rash that may appear in different forms.
- CDC notes that antibody tests can be falsely negative in the first few weeks after infection.
What this can tell you
- Why an expanding rash deserves timely clinical attention.
- Why a very early blood test may not settle the question.
What this cannot tell you
- What caused a rash from a photo or written description.
- Whether a person should wait for this website to decide about care.
A useful question for your clinician
Could this be erythema migrans, and should the rash and exposure guide care before relying on an early blood test?
Sources (2)
- Centers for Disease Control and Prevention. ‘Lyme Disease Rashes.’ https://www.cdc.gov/lyme/signs-symptoms/lyme-disease-rashes.html. Accessed July 24, 2026. Open source ↗
- Centers for Disease Control and Prevention. ‘Testing and Diagnosis for Lyme Disease.’ https://www.cdc.gov/lyme/diagnosis-testing/index.html. Accessed July 24, 2026. Open source ↗