What labs and antibiotics should be ordered for a patient with a rash around a tick bite for 3 weeks?

Management of Tick Bite with 3-Week Rash

For a patient with a rash around a tick bite for 3 weeks, treat with doxycycline 100 mg twice daily for 10 days as first-line therapy, or amoxicillin 500 mg three times daily for 14 days if doxycycline is contraindicated. 1

Diagnostic Approach

1. Clinical diagnosis is recommended for patients with skin lesions compatible with erythema migrans (EM) in a Lyme disease endemic area 1

   • Laboratory testing is not necessary when typical EM is present
   • The rash of EM typically appears 3-30 days after a tick bite and expands gradually over days

2. Laboratory testing should be considered only if:

   • The rash is atypical for EM
   • There are no skin lesions but other symptoms of Lyme disease are present
   • Testing should include serum antibody testing (not PCR or culture) 1

3. Consider co-infections:

   • Evaluate for possible co-infection with Anaplasma phagocytophilum or Babesia microti in endemic areas
   • These require different treatment approaches 1

Treatment Recommendations

First-line therapy:

• Doxycycline 100 mg twice daily for 10 days 1
  • Preferred agent due to coverage of both Lyme disease and potential co-infection with Anaplasma
  • Effective against early localized and early disseminated disease

Alternative therapies (if doxycycline contraindicated):

• Amoxicillin 500 mg three times daily for 14 days 1, 2
• Cefuroxime axetil 500 mg twice daily for 14 days 1
• Azithromycin 500 mg daily for 7 days (less effective, use only when first-line agents cannot be used) 1, 2

Special populations:

• Children ≥8 years: Doxycycline 4.4 mg/kg/day in 2 divided doses (maximum 100 mg per dose) for 10 days 1, 3
• Children <8 years: Amoxicillin 50 mg/kg/day in 3 divided doses (maximum 500 mg per dose) for 14 days 2
• Pregnant women: Amoxicillin 500 mg three times daily for 14 days 1, 2

Important Clinical Considerations

1. The 3-week duration of rash indicates established infection:

   • Prophylaxis is no longer appropriate as it's only effective within 72 hours of tick removal 1, 4
   • Full treatment course is necessary 1

2. Distinguish from STARI (Southern Tick-Associated Rash Illness):

   • In regions where both STARI and Lyme disease are endemic, treat as Lyme disease if distinction cannot be made 1

3. Monitor for treatment response:

   • Improvement should begin within 24-48 hours of antibiotic initiation
   • If no improvement occurs, re-evaluate for alternative diagnoses or co-infections 1

4. Avoid common pitfalls:

   • Do not delay treatment while waiting for test results if EM is present
   • Do not use shorter courses than recommended
   • Do not use macrolides (like azithromycin) as first-line therapy due to lower efficacy 2
   • Do not extend treatment beyond recommended duration without clear evidence of treatment failure 1

Prevention of Future Tick Bites

• Use EPA-registered repellents (DEET, picaridin, IR3535, oil of lemon eucalyptus) 2
• Perform thorough tick checks after outdoor activities 1
• Remove ticks promptly using fine-tipped tweezers 1
• Consider prophylaxis only for future high-risk tick bites if identified within 72 hours 1, 2

This evidence-based approach prioritizes effective treatment of established Lyme disease while considering patient factors that may influence antibiotic selection.