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Subject: Borrelia Burgdorferi (Lyme Disease)—Western Blot
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The western blot assay for antibodies to Borrelia burgdorferi, the etiologic agent of Lyme disease, is a qualitative method of categorizing specific immunoreactivities in serum or plasma to B. burgdorferi proteins that have been formatted according to molecular weight into discrete bands on nitrocellulose strips.
Normal range: Negative.
The western blot assay for B. burgdorferi is used as a second-tier test to characterize the specificity of an individual's immune response to the component proteins of B. burgdorferi by identifying the presence, relative level, and pattern of reactivities to the complete set of the bacterial proteins. The assay is used routinely to provide supportive serologic evidence of infection following a more sensitive but less specific screening test (such as EIA) for general reactivity to B. burgdorferi. Both IgM and IgG reactivities to the bacterial proteins are assayed to provide information on the evolution of the immune response relative to the stage of infection (i.e., early localized, early disseminated, or late disseminated). A caveat to this use is that IgM testing is not recommended in patients with symptoms lasting greater than 1–2 months. For such patients, IgG testing alone should be performed.
Reactivity scores: Specimen reactions with protein bands are first scored in terms of relative reaction intensity versus a cutoff control or minimally positive (“+”) band reaction intensity by a positive control specimen.
Test interpretation (IgM class reactivities)
Positive: Reactivity scores of “+” or greater for at least two of three clinically significant proteins at the early stage of the disease (2–3 weeks after infection): 41, 39, 23 kDa
Negative: Absence of any band reactivity on the test strip or reactivity for only one of the three clinically significant proteins
Test interpretation (IgG class reactivities)
Positive: Reactivity scores of “+” or greater for at least 5 of 10 clinically significant proteins at the later stages of the disease (weeks to months after infection): 93, 66, 58, 45, 41, 39, 30, 28, 23, 18 kDa
Negative: Absence of any band reactivity on the test strip or reactivity for <5 of the 10 clinically significant proteins
Minimum specimen volume is 40 μL (20 μL each for the IgM and IgG tests).
Like any second-tier test, the positive predictive value for a western blot assay is a function of the a priori likelihood of the disease by clinical and epidemiologic criteria, whereas the negative predictive value is not as well defined because of the variability of the immune response among infected individuals. Cross-reactive diseases are most frequently evidenced by reactivity to the 41-kDa flagellar protein and at much lower frequency to the 66-kDa heat shock protein. Specimens from patients diagnosed with Ehrlichia or Babesia infections can show other Borrelia-specific bands.