Blood Tests After a Tick Bite
No laboratory testing is recommended for asymptomatic individuals following tick bites. 1 Instead, risk assessment and monitoring for symptoms for 30 days is the appropriate approach.
Risk Assessment After Tick Bite
The need for testing after a tick bite depends on several key factors:
- Tick species: Ixodes ticks pose the highest risk for Lyme disease 1
- Attachment duration: ≥36 hours significantly increases transmission risk 1
- Geographic location: Different regions have different endemic tick-borne diseases 1
- Degree of tick engorgement: Higher engorgement correlates with longer attachment and higher risk 2
Appropriate Testing When Symptoms Develop
If symptoms develop after a tick bite, the following tests should be considered:
Initial laboratory panel:
- Complete blood count (CBC) with differential
- Liver function tests
- Serological testing for tick-borne diseases 1
For Lyme disease:
For rickettsial diseases and anaplasmosis:
- PCR testing of whole blood specimens
- Examination of peripheral blood smears for morulae in leukocytes (though sensitivity is low at 1-20%) 1
Important Timing Considerations
The timing of testing is crucial for accurate results:
| Time Post-Bite | Antibody Detection |
|---|---|
| 0-7 days | Generally negative and insensitive |
| 7-15 days | Both IgM and IgG antibodies usually detectable |
| 2-3 weeks | Highest sensitivity for antibody detection |
Testing immediately after a tick bite is not recommended as antibodies take weeks to develop, leading to false negatives 1.
Common Pitfalls to Avoid
- Testing too early: Antibodies are typically not detectable before the second week of illness 1
- Delaying treatment for test results: If clinical suspicion is high, appropriate antibiotic therapy should be initiated while awaiting test results 1
- Ignoring geographic risks: Different tick-borne diseases are endemic to different regions 1
- Assuming a tick bite is required: Up to 40% of patients with Rocky Mountain Spotted Fever do not report a history of tick bite 1
- Focusing on a single pathogen: Multiple tick-borne pathogens may co-exist in the same geographic area 1
Prophylaxis vs. Testing
Rather than testing, antibiotic prophylaxis should be considered when:
- The tick is identified as an adult or nymphal Ixodes scapularis
- Estimated attachment duration is ≥36 hours
- Local infection rate of ticks with B. burgdorferi is ≥20%
- Prophylaxis can start within 72 hours of tick removal 1
A single dose of doxycycline (200 mg for adults, 4 mg/kg up to 200 mg for children ≥8 years) has shown a 67% relative risk reduction in developing Lyme disease 3.
Human granulocytic anaplasmosis and other tick-borne diseases may still develop despite prophylaxis, highlighting the importance of monitoring for symptoms even after prophylactic treatment 4.