How do you interpret a positive Western blot (WB) result?

Interpreting a Positive Western Blot

A positive Western blot must be interpreted based on the specific disease being tested and requires meeting strict band criteria established by CDC guidelines—for HIV-1, this confirms infection and warrants immediate counseling and management; for Lyme disease, it requires ≥5 of 10 specific IgG bands (or ≥2 of 3 IgM bands if <30 days of symptoms); for other conditions like MOG encephalomyelitis, Western blot is obsolete and should not be used. 1, 2, 1

Context-Specific Interpretation Algorithm

For HIV Testing

Positive HIV-1 Western Blot:

• The person should be considered HIV-infected and counseled/managed as if infected with HIV-1. 1
• No further testing is required for routine clinical purposes, though the result does not always distinguish between HIV-1 and HIV-2 antibodies. 1
• Retesting with a second specimen should be considered for persons who have positive results by HIV-1 Western blot at first testing. 1
• In infants, detection of antibodies soon after birth may indicate either infection or the presence of maternal HIV antibodies, requiring additional follow-up to determine HIV status. 1

Critical Management Points:

• Persons with confirmed HIV-1 Western blot positivity should receive the same counseling regarding transmission prevention and be managed similarly regardless of whether HIV-1 or HIV-2 is ultimately identified. 1
• If epidemiologic risk factors for HIV-2 are present (e.g., West African origin, sexual contact with persons from endemic areas), additional HIV-2 testing is indicated even with a positive HIV-1 Western blot. 1

For Lyme Disease Testing

Positive IgG Western Blot (symptoms >30 days):

• Requires ≥5 of 10 specific bands for CDC positivity criteria, including the 58 kDa band plus at least 4 additional bands. 2
• IgG Western blot alone should be performed for patients with symptoms lasting more than 30 days, as IgM testing beyond 4-8 weeks is not clinically interpretable. 2

Positive IgM Western Blot (symptoms <30 days):

• Requires ≥2 of 3 specific bands for CDC positivity criteria. 2
• The CDC-recommended IgM Western blot positivity criteria do not include the 58 kDa band. 2
• Both IgM and IgG Western blot should be performed if the first-tier EIA/ELISA is positive or equivocal in patients with symptoms lasting less than 30 days. 2

Common Pitfalls to Avoid:

• A single IgM band or fewer than 5 IgG bands does not constitute a positive result—misinterpreting Western blots with insufficient bands as positive is a common diagnostic error. 2
• IgM antibodies persist for months to years after treatment, making IgM testing beyond 4-8 weeks of symptoms uninterpretable and prone to false-positive results. 2
• Patients with characteristic erythema migrans rash and appropriate epidemiologic exposure can be diagnosed clinically without serologic confirmation, as early Lyme disease serology has decreased sensitivity in the first weeks of infection. 2

For MOG Encephalomyelitis

Western Blot is Obsolete:

• Peptide-based ELISA and Western blot are insufficiently specific and obsolete for MOG antibody testing. 1
• Cell-based assays are the recommended method for MOG-IgG detection, not Western blot. 1

Key Interpretive Principles Across All Conditions

Always Interpret in Clinical Context:

• Positive test results should always be interpreted in the context of the patient's overall clinical presentation. 1
• The presence of "red flags" (discordant clinical features) warrants re-testing of the positive sample using a methodologically different assay. 1

Understand Test Limitations:

• Western blot tests are not standardized across manufacturers and show variability in sensitivity and specificity. 3
• False-positive results can occur due to cross-reactivity with non-specific antibodies, particularly in patients with other inflammatory or infectious diseases. 4, 5
• False-negative results can occur in early infection (before antibody development), during immunosuppression, or following certain treatments like antiretroviral therapy with steroids. 6, 5

Timing Considerations:

• For HIV, antibody tests may be negative in the window period of early infection (first 3-4 weeks). 7
• For Lyme disease, early infection serology has decreased sensitivity in the first weeks, and treatment should not be delayed awaiting serologic confirmation in patients with characteristic erythema migrans. 2
• For MOG encephalomyelitis, antibody concentrations depend on disease activity and treatment status, with higher concentrations during acute attacks than remission. 1