What lab tests should be ordered after a tick bite to rule out Lyme disease?

Lab Tests for Lyme Disease After Tick Bite

Serologic testing for Lyme disease is NOT recommended immediately following a tick bite, as antibodies are typically not detectable at this early stage and testing would likely yield misleading results. 1, 2

Appropriate Testing Approach

When NOT to Test

• Immediately after a tick bite with no symptoms
• When erythema migrans (EM) rash is present (clinical diagnosis is sufficient)

When to Consider Testing

• If symptoms develop within 30 days after a tick bite:
  • Expanding red rash (≥5 cm in diameter)
  • Fever, headache, fatigue
  • Muscle/joint pain
  • Neurological symptoms

Testing Protocol When Symptoms Develop

Two-Tier Testing Algorithm

1. First tier: Enzyme-linked immunosorbent assay (ELISA) or immunofluorescence assay (IFA) 1, 3
2. Second tier: If first tier is positive or equivocal, confirm with Western blot 1, 3
   • For symptoms <4 weeks: Test both IgM and IgG Western blot
   • For symptoms >4 weeks: Test only IgG Western blot (IgM not interpretable after 4 weeks)

Western Blot Interpretation

• Positive IgM: Must have 2 of 3 specific bands (24 kDa, 39 kDa, 41 kDa) 1
• Positive IgG: Must have 5 of 10 specific bands (18 kDa, 21 kDa, 28 kDa, 30 kDa, 39 kDa, 41 kDa, 45 kDa, 58 kDa, 66 kDa, 93 kDa) 1

Timing Considerations

• Early localized infection (Stage I): IgM antibodies develop around week 3, IgG around week 6 1
• Early disseminated infection (Stage II): IgG antibodies predominate 1
• Late/persistent infection (Stage III): Usually high IgG titers; IgM typically undetectable 1

Special Testing Situations

• Neuroborreliosis: Test for intrathecally produced antibodies in cerebrospinal fluid (CSF) 1, 4
• Late manifestations: Focus on IgG antibodies, as IgM is usually absent 1

Important Caveats

• A negative test in early disease does not rule out Lyme disease due to delayed antibody response 1, 3
• Antibiotics given early can blunt the antibody response 1
• Erythema migrans is diagnostic of early Lyme disease without laboratory confirmation 1, 2
• False positives can occur with other spirochetal diseases and autoimmune conditions 1, 3

Alternative Management Approach

Instead of testing immediately after a tick bite, consider:

• Single-dose doxycycline prophylaxis (200 mg for adults) if:
  • Tick is identified as Ixodes scapularis
  • Tick was attached ≥36 hours
  • Treatment can be given within 72 hours of tick removal
  • Bite occurred in highly endemic area 2
• "Wait and watch" approach if prophylaxis criteria not met, monitoring for symptoms for 30 days 2

Remember that routine serologic testing at the time of a tick bite is not recommended as it provides no useful information and may lead to unnecessary treatments and costs 1, 5.