How can you diagnose Lyme disease after a tick bite? - briefly
Diagnosis depends on recognizing the characteristic erythema migrans rash and, when needed, performing a two‑tier serologic test (ELISA screening followed by confirmatory Western blot). If the rash is absent and the bite occurred within the past month, order the antibody assay and interpret results in the clinical context.
How can you diagnose Lyme disease after a tick bite? - in detail
After a tick attachment, the first step is to assess the exposure. Record the date of the bite, the geographic region, and the duration of attachment, because Borrelia burgdorferi transmission typically requires ≥ 24 hours of feeding.
Physical examination should focus on the presence of an erythema migrans lesion. The rash appears as an expanding, erythematous area, often with central clearing, and may be up to several centimeters in diameter. Its appearance, even without laboratory confirmation, is sufficient for a clinical diagnosis.
If the rash is absent or atypical, laboratory testing becomes essential. The recommended two‑tier serologic approach includes:
- Initial screening – enzyme‑linked immunosorbent assay (ELISA) or chemiluminescent immunoassay (CLIA) detecting IgM and IgG antibodies against B. burgdorferi.
- Confirmatory test – Western blot performed on samples that test positive or equivocal in the screening assay. Interpretation follows CDC criteria: IgM positivity requires ≥ 2 of 3 specific bands (24 kDa, 39 kDa, 41 kDa) within 30 days of symptom onset; IgG positivity requires ≥ 5 of 10 bands after 30 days.
Serology may be negative during the first weeks after exposure because antibodies have not yet reached detectable levels. In such cases, repeat testing after 2–3 weeks is advised if clinical suspicion persists.
Molecular methods are reserved for specific situations:
- Polymerase chain reaction (PCR) on synovial fluid, cerebrospinal fluid, or skin biopsy can identify bacterial DNA, useful for neurologic or articular manifestations.
- Culture is rarely performed because of low sensitivity and lengthy incubation.
Additional diagnostic considerations include:
- Neurologic involvement – lumbar puncture to assess cerebrospinal fluid for pleocytosis, elevated protein, and intrathecal antibody production.
- Cardiac involvement – electrocardiography to detect atrioventricular block; serology supports the diagnosis.
- Joint involvement – arthrocentesis of inflamed joints, with PCR or culture of synovial fluid when indicated.
Interpretation must integrate clinical findings, exposure risk, and laboratory results. A positive two‑tier serology in the absence of a rash confirms infection, whereas a negative result early in the disease does not exclude it. Follow‑up evaluation should be scheduled to monitor symptom progression and treatment response.