What laboratory tests are needed after a tick bite?

Laboratory Testing After a Tick Bite

Laboratory testing is not routinely recommended for asymptomatic individuals following a tick bite, but specific tests should be ordered if symptoms develop, focusing on the most likely tick-borne diseases based on geographic location and clinical presentation.

Initial Assessment After Tick Bite

• No laboratory testing is recommended for asymptomatic individuals following tick bites, as stated by multiple guidelines including the American College of Rheumatology 1
• Testing ticks for infection is not recommended as results do not necessarily correlate with risk of human infection 1
• Instead of immediate testing, patients should be monitored for 30 days after a tick bite for development of symptoms 1

Key Information to Gather

• Determine if the tick was an Ixodes species vector tick 1
• Assess if the bite occurred in a highly endemic area 1
• Determine if the tick was attached for ≥36 hours 1
• Document any early symptoms such as:
  • Fever
  • Headache
  • Muscle/joint pain
  • Rash (especially erythema migrans)
  • Fatigue

Laboratory Testing When Symptoms Develop

If symptoms develop after a tick bite, the following tests should be considered based on the suspected disease:

For Suspected Lyme Disease

• Two-tier testing approach as recommended by CDC 1:
  1. First-tier: Enzyme-linked immunosorbent assay (ELISA) or immunofluorescence assay (IFA)
  2. Second-tier: Western blot confirmation if first test is positive or equivocal
• Timing is critical: Tests are generally negative during the first week post-bite, with highest sensitivity 2-3 weeks post-bite 1
• Paired acute and convalescent sera (collected 2-4 weeks apart) provide the most reliable serological confirmation 1

For Suspected Rickettsial Diseases (e.g., Rocky Mountain Spotted Fever)

• Blood collection in appropriate tubes 2:
  • Heparin or citrate tubes (not EDTA) for culture
  • EDTA tubes for molecular diagnosis (PCR)
• Complete blood count (CBC) - look for:
  • Thrombocytopenia (low platelets)
  • Leukocyte count abnormalities 2
• Liver function tests - look for elevated hepatic enzymes 2
• Serological testing for specific rickettsial species 2:
  • Acute sample before antimicrobial therapy
  • Convalescent sample 2 weeks later
  • Consider third sample after 4-6 weeks if no four-fold rise in antibody titer is observed

For Suspected Ehrlichiosis/Anaplasmosis

• CBC with differential - look for:
  • Leukopenia
  • Thrombocytopenia
  • Elevated liver enzymes 2
• Examination of peripheral blood smears for morulae in leukocytes (though sensitivity is low, 1-20%) 2
• PCR testing of whole blood specimens 2
• Serological testing (paired acute and convalescent samples) 2

Common Pitfalls to Avoid

1. Testing too early: Antibodies take weeks to develop, and early testing leads to false negatives 1
2. Relying solely on serology: PCR or culture may be needed for early diagnosis, especially for rickettsial diseases 2
3. Missing co-infections: Consider testing for multiple tick-borne pathogens as ticks can transmit more than one pathogen simultaneously
4. Waiting for test results to treat: If clinical suspicion is high for a tick-borne disease, appropriate antibiotic therapy should be started while awaiting test results 2
5. Overlooking the clinical picture: Laboratory findings should be interpreted in the context of clinical presentation and epidemiological factors 2

Prophylaxis Considerations

• Antibiotic prophylaxis (single-dose doxycycline) is only recommended for high-risk tick bites meeting all three criteria:
  • Ixodes species tick
  • Endemic area
  • ≥36 hours attachment 1
• The number needed to treat to prevent one case of Lyme disease is approximately 51 3
• Prophylaxis is not effective for all tick-borne diseases, particularly anaplasmosis 4

Remember that the clinical presentation and geographic location should guide the specific laboratory workup, as different tick-borne diseases are endemic to different regions.