Is a single 58kDa band on an Immunoglobulin M (IgM) Western blot test sufficient for a diagnosis of Lyme disease?

Lyme Disease Diagnosis: Single 58 kDa IgM Band Interpretation

A single 58 kDa band on an IgM Western blot is NOT sufficient for diagnosing Lyme disease and does not meet established diagnostic criteria. This result should be interpreted as negative.

Established Diagnostic Criteria

The CDC-recommended two-tiered testing approach requires specific band thresholds that must be met 1:

IgM Western Blot Positivity Criteria

• Requires ≥2 of 3 specific bands: 21-24 kDa, 39 kDa, and 41 kDa 1
• The 58 kDa band is not included in the IgM positivity criteria 1
• A single band of any type does not constitute a positive result 1

IgG Western Blot Positivity Criteria

• Requires ≥5 of 10 specific bands: 18,21-24,28,30,39,41,45,58,66, and 93 kDa 1
• The 58 kDa band is included in IgG criteria but requires at least 4 additional bands for positivity 1

Why Single Bands Are Insufficient

Cross-reactivity is the fundamental problem with interpreting isolated bands 1:

• Antibodies to several Borrelia burgdorferi antigens cross-react with non-Borrelial antigens 1
• The 41 kDa flagellin band, for example, was found reactive in 43% of healthy controls with minimal Lyme disease exposure risk 1
• Overinterpreting a small number of antibody bands leads to reduced specificity and potential misdiagnosis 1

Clinical Application Algorithm

For Symptoms <30 Days Duration:

1. Perform first-tier EIA/ELISA 1
2. If positive or equivocal, reflex to both IgM and IgG Western blot 1
3. Apply strict band criteria: IgM requires ≥2 of 3 specified bands 1
4. Single band = negative result 1

For Symptoms >30 Days Duration:

1. Perform first-tier EIA/ELISA 1
2. If positive or equivocal, reflex to IgG Western blot only 1
3. Do not order IgM Western blot - increases false-positive risk 1
4. IgG requires ≥5 of 10 specified bands 1

Critical Pitfalls to Avoid

The most common diagnostic error is misinterpreting Western blots with insufficient bands as positive 1:

• Presence of 1 IgM band does not indicate a positive result 1
• Presence of <5 IgG bands does not indicate a positive result 1
• IgM testing beyond 4-8 weeks of symptoms is not clinically interpretable and should not be performed 1
• Antibodies persist for months to years after treatment, so positive serology alone does not indicate active disease 2

When Clinical Diagnosis Supersedes Serology

Patients with erythema migrans rash and appropriate epidemiologic exposure can be diagnosed clinically without serologic confirmation 1:

• Early Lyme disease serology has decreased sensitivity in the first weeks of infection 1
• Treatment should not be delayed awaiting serologic confirmation in patients with characteristic erythema migrans 1

In this specific case, the single 58 kDa IgM band represents a negative Western blot result and does not support a diagnosis of Lyme disease 1. Alternative diagnoses should be pursued based on the clinical presentation.