What labs should be ordered for a patient presenting with a tick bite?

Laboratory Testing for Tick Bite

Order a complete blood count (CBC) with differential immediately for any patient presenting with a tick bite, as this is the single most critical initial test that can reveal characteristic findings of thrombocytopenia and leukopenia that distinguish tick-borne illness from other causes and guide urgent treatment decisions. 1

Essential Initial Laboratory Tests

Complete Blood Count with Differential

• CBC with differential is the highest priority test and should be ordered for all patients with suspected tick-borne illness, regardless of symptoms 1, 2
• Leukopenia and thrombocytopenia are characteristic findings in ehrlichiosis, anaplasmosis, and Rocky Mountain spotted fever 3, 1
• These findings help distinguish tick-borne illness from viral gastroenteritis or other nonspecific febrile illnesses 1
• In one study, platelet and white blood cell counts were significantly lower in PCR-positive Crimean-Congo hemorrhagic fever patients 4

Comprehensive Metabolic Panel

• Order a comprehensive metabolic panel to assess liver function, as hepatic abnormalities occur in 50-75% of ehrlichiosis and anaplasmosis cases 1
• Transaminase elevations (AST and ALT) are common supportive findings 1, 4
• AST levels are significantly higher in patients with confirmed tick-borne disease 4

Pathogen-Specific Testing

PCR Testing (Molecular Diagnostics)

• Order PCR testing on EDTA-anticoagulated whole blood for Anaplasma phagocytophilum, Ehrlichia chaffeensis, and spotted fever group rickettsiae 3, 1, 2
• Critical timing: Collect specimens BEFORE initiating doxycycline therapy whenever possible, as antibiotic treatment rapidly decreases PCR sensitivity within 24-48 hours 1
• Blood should be kept at room temperature for no more than 48 hours or frozen at -20°C 3

Serologic Testing

• Order acute-phase serology for Borrelia burgdorferi (Lyme disease), E. chaffeensis, and A. phagocytophilum 1, 2
• For Lyme disease specifically: Use two-tier testing with EIA IgG and IgM antibody screening first, followed by Western blot confirmation ONLY if EIA is positive or equivocal 3, 1, 2
• Never order Western blot as a standalone test 1
• Collect convalescent serum at least 21 days after symptom onset for comparison 2

Blood Smear Examination

• Order Giemsa, Wright's, or Diff-Quik stains of peripheral thin and thick blood smears for suspected babesiosis or relapsing fever 3, 2
• While morulae may be visible in ehrlichiosis, microscopy is generally insensitive (only 1-20% sensitivity) 3

Additional Supportive Tests

Cultures

• Order blood, urine, and stool cultures to exclude other causes of sepsis and fever of unknown origin 3, 1
• This helps rule out bacterial pneumonia, urinary tract infections, and other infectious causes 3

Tissue-Based Testing (When Applicable)

• If eschar is present: Order eschar biopsy or swab for PCR and immunohistochemistry for spotted fever group rickettsioses 1
• If rash is present: Consider skin biopsy for PCR testing 1

Critical Pitfalls to Avoid

Do Not Rely on Negative Acute Serology

• Negative acute-phase serology does NOT exclude tick-borne disease, as antibodies are typically absent during the first 7 days of illness when patients present for care 1, 2
• IgM and IgG antibodies are typically not detectable before the second week of illness 3
• This is why treatment should not be delayed while awaiting laboratory results when clinical suspicion is high 1

Do Not Delay Treatment

• Treatment should not be delayed while awaiting laboratory results, particularly for Rocky Mountain spotted fever which has high mortality 1
• 50% of RMSF deaths occur within 9 days of illness onset 3

Geographic and Clinical Context Matters

• Testing should be guided by geographic location, tick species, and clinical presentation rather than routine testing of all patients with tick bites 2
• Patient travel history, recent outdoor activities in wooded areas, and duration of tick attachment are essential for determining which specific pathogens to test for 1, 2

Specimen Handling

• Transport clinical material to the laboratory as quickly as possible at room temperature, or at -20°C if transport times exceed 48 hours 3
• For isolation attempts, blood should be collected on EDTA and kept at room temperature for not more than 48 hours 3