Lyme Western Blot Positivity Criteria
For IgM Western blot, at least 2 of 3 specific bands (23-24 kDa, 39 kDa, and 41 kDa) must be reactive; for IgG Western blot, at least 5 of 10 specific bands (18,23-24,28,30,39,41,45,58,66, and 93 kDa) must be reactive to be considered positive 1.
Critical Timing Considerations
The interpretation depends entirely on symptom duration:
Symptoms <30 days: Order both IgM and IgG Western blots as second-tier tests (after positive/equivocal ELISA). IgM appears first and may be the only positive result in early disease 1.
Symptoms ≥30 days: Order IgG Western blot only. Do NOT order IgM at this stage—it increases false-positive risk and is unnecessary since IgG response should be established by this point 1.
Band Requirements Breakdown
IgM Positivity (Early Disease Only)
- Requires >2 bands (meaning at least 2 of the 3 specified bands)
- The 3 bands evaluated: 23-24 kDa, 39 kDa, 41 kDa
- Only interpret in patients with symptoms <30 days 1
IgG Positivity
- Requires >5 bands (meaning at least 5 of the 10 specified bands)
- The 10 bands evaluated: 18,23-24,28,30,39,41,45,58,66, and 93 kDa
- This is the definitive test for established infection 1
Critical Pitfalls to Avoid
Never interpret fewer bands as positive. The 41 kDa band (flagellin protein) cross-reacts with other bacterial flagellar proteins and was found in 43% of healthy controls with minimal Lyme exposure risk 1. Interpreting 1 IgM band or fewer than 5 IgG bands as positive dramatically reduces specificity and leads to misdiagnosis 1.
Do not order IgM testing beyond 30 days of symptoms. This is a common error that generates false-positive results and leads to inappropriate treatment 1.
Band Intensity Requirement
Bands are only considered present if their intensity equals or exceeds the control band intensity on the blot 1. Visual assessment alone without proper intensity comparison is inadequate.
Two-Tiered Testing Context
Western blot is the second-tier confirmatory test—it should only be performed after a positive or equivocal ELISA result. The Western blot alone without initial ELISA screening is not appropriate for diagnosis 1, 2.