Interpretation of an Isolated IgG p58 Band on Lyme Disease Western Blot
An isolated IgG p58 band does not meet CDC criteria for a positive Lyme disease Western blot and should be reported as negative. 1
CDC Diagnostic Criteria
The CDC requires ≥5 of 10 specific bands for IgG Western blot positivity, which include the 58 kDa band plus at least 4 additional bands (18,21,28,30,39,41,45,66, and 93 kDa). 1, 2 A single band of any type—including p58—does not constitute a positive result and represents a common diagnostic error when misinterpreted as evidence of infection. 1
Clinical Context and Testing Algorithm
Two-tiered testing is mandatory for proper interpretation:
Western blot should only be performed after a positive or equivocal first-tier EIA/ELISA. 1, 3 Performing Western blot without a preceding positive screening test violates CDC recommendations and increases false-positive rates. 3
For symptoms >30 days duration: Only IgG Western blot should be performed (not IgM), as IgM testing beyond 4-8 weeks is not clinically interpretable. 1
For symptoms <30 days duration: Both IgM and IgG Western blots should be performed if the first-tier test is positive/equivocal. 1 However, the IgM criteria require ≥2 of 3 specific bands (23,39,41 kDa) and do not include the 58 kDa band. 1
Specificity Concerns with Single Bands
The p58 band, while included in CDC IgG criteria, requires additional bands for specificity. 2, 4 European studies have identified p58 as one of several immunodominant proteins, but interpretation criteria consistently require multiple bands to distinguish true infection from cross-reactivity. 4, 5
Management Recommendations
Report the result as negative per CDC criteria. 1
If clinical suspicion for early Lyme disease remains high (symptom duration <30 days), repeat serologic testing in 2-4 weeks to allow antibody development rather than treating based on an insufficient Western blot. 1
If the patient has characteristic erythema migrans rash with appropriate epidemiologic exposure, diagnose and treat clinically without requiring serologic confirmation, as early Lyme serology has decreased sensitivity in the first weeks of infection. 1
Avoid using alternative laboratories that apply non-standard interpretation criteria, as these have demonstrated false-positive rates as high as 58% in healthy controls. 3
Critical Pitfall to Avoid
Interpreting fewer than 5 IgG bands as positive reduces test specificity and leads to misdiagnosis. 1 This is particularly problematic in low-incidence regions where the positive predictive value of Lyme serology can be as low as 10%. 3 If there is any question regarding interpretation, consult an infectious disease specialist. 3