QUICK ANSWER
The 58 KD (IgG) Band is one of ten IgG bands evaluated in the Western blot test for Lyme disease (Borrelia burgdorferi).
- Reactive = antibodies to the 58 kDa protein were detected in your blood
- Non-reactive = antibodies to this specific protein were not detected
A single reactive band does not confirm Lyme disease. The CDC standard requires 5 out of 10 IgG bands to be reactive for a positive IgG Western blot result. The specific bands that satisfy this criterion are: 18, 23, 28, 30, 39, 41, 45, 58, 66, and 93 kDa.
The 58 kDa protein is a heat shock protein (HSP58/GroEL) present in many bacteria — it is not exclusive to Borrelia — which means a reactive 58 KD band carries lower Lyme-specificity than bands such as 23 kDa (OspC) or 39 kDa (BmpA).
Key takeaway: A reactive 58 KD (IgG) band is one piece of a multi-band interpretation. Its clinical significance depends on which other bands are reactive alongside it, whether at least 5 of 10 IgG bands are reactive total, and whether Lyme disease symptoms are present. A positive Western blot without symptoms does not confirm active Lyme disease.
WHAT DOES "58 KD (IgG) BAND REACTIVE" MEAN?
"Reactive" on a Western blot or immunoblot report means the test detected IgG antibodies in your blood that bind to the 58 kilodalton (kDa) protein of Borrelia burgdorferi — the bacterium that causes Lyme disease.
"Non-reactive" means no antibodies to this specific protein were detected at a level above the test threshold.
"Abnormal" on some reports is synonymous with "reactive" — it simply means the result was above the negative threshold.
What is the 58 kDa protein? The 58 kDa protein of Borrelia burgdorferi is a heat shock protein, also known as HSP58 or GroEL. Heat shock proteins are produced by bacteria under stress conditions. Because HSP58 proteins are structurally similar across many different bacteria and even human cells, antibodies to the 58 kDa band may appear in response to non-Lyme infections — making this band less Lyme-specific than bands like 23 kDa (OspC) or 39 kDa (BmpA).
THE 5/10 RULE: WHEN IS AN IgG WESTERN BLOT POSITIVE?
The CDC two-tier testing standard for Lyme disease IgG Western blot requires at least 5 of the following 10 bands to be reactive for the test to be considered positive:
| Band | Protein | Lyme-specificity |
|---|---|---|
| 18 kDa | Unknown | Moderate |
| 23 kDa (OspC) | Outer surface protein C | High |
| 28 kDa | Unknown | Moderate |
| 30 kDa | OspA/OspB related | Moderate |
| 39 kDa (BmpA) | Membrane protein | High |
| 41 kDa (FlaB) | Flagellin B | Low (cross-reactive) |
| 45 kDa | Unknown | Moderate |
| 58 kDa (HSP58) | Heat shock protein | Low (cross-reactive) |
| 66 kDa (HSP66) | Heat shock protein | Low (cross-reactive) |
| 93 kDa | Unknown | Moderate |
The 58 kDa band alone — or even 2–4 reactive bands — does not satisfy the CDC positive criteria. A minimum of 5 bands must be reactive.
Why isn't one reactive band enough? Antibodies can cross-react with proteins from unrelated bacteria, viruses, or even the body's own tissues. The 58 kDa band is a good example: because HSP58 is structurally similar across many bacterial species, a person with an unrelated infection may produce antibodies that bind to the 58 kDa Borrelia protein without ever having been infected with Borrelia. Requiring multiple bands to be reactive simultaneously dramatically reduces the chance that a positive result is caused by cross-reaction rather than true Borrelia infection — this is why the CDC threshold is 5 of 10, not 1 of 10.
Combined bands matter: If your report shows both the 41 kDa and 58 kDa bands reactive, that is 2 of 10 — still well below the 5/10 threshold. If your report shows 5 or more bands including 58 kDa, the IgG Western blot meets CDC positive criteria.
WHAT IS "IgG P58 Ab" AND HOW IS IT DIFFERENT?
"IgG P58 Ab" or "IgG P58 Ab. Present" is the LabCorp label for the 58 kDa protein antibody result on the Lyme IgG/IgM Line Blot (LabCorp test code 164972) — a different but related test.
The line blot (immunoblot) is a newer confirmatory test that uses purified recombinant Borrelia proteins rather than the whole-cell lysate used in the classic Western blot. It tests for antibodies to specific proteins including P18, P23, P28, P30, P39, P41, P45, P58, P66, and P93.
"IgG P58 Ab Present" = the line blot detected IgG antibodies to the P58 (58 kDa) protein. The clinical interpretation is similar to "58 KD (IgG) Band Reactive" on the Western blot — a single reactive protein does not confirm Lyme disease; the overall pattern of reactive bands matters.
"IgG P58 Ab. Present Abnormal" = same as "Present" — the "Abnormal" flag on some LabCorp reports is equivalent to "reactive/positive."
If your report says "IgG P58 Ab", you received a Lyme line blot result, not a classic Western blot. The clinical interpretation framework (multiple bands, CDC criteria equivalent, symptom requirement) is the same.
WHAT DOES A POSITIVE IgG WESTERN BLOT MEAN (5/10 BANDS)?
If 5 or more of the 10 IgG bands are reactive — meeting the CDC positive criteria — the result is considered a positive IgG Western blot. This means:
1. Previous exposure or past infection: IgG antibodies can persist for months to years after a successfully treated Lyme infection. A positive IgG Western blot does not necessarily indicate active, ongoing infection.
2. Possible active late infection: If symptoms consistent with Lyme disease are present (especially arthritis, neurological symptoms, or cardiac involvement), a positive IgG Western blot combined with clinical findings supports a diagnosis of late Lyme disease.
3. False positive: Less likely with a full 5-band positive result than with ELISA alone, but possible — particularly in people with other autoimmune conditions (lupus, rheumatoid arthritis), other infections (EBV, other spirochetes), or certain inflammatory states.
4. Post-treatment antibody persistence: People successfully treated for Lyme disease may remain IgG Western blot positive for years. A positive IgG Western blot after treatment does not mean treatment failed.
The critical point: A positive IgG Western blot without current symptoms of Lyme disease does not establish active infection. The CDC and Infectious Diseases Society of America (IDSA) guidelines state that serological tests alone are not sufficient to diagnose active Lyme disease in the absence of compatible clinical signs and symptoms.
IgG VS IgM IN LYME TESTING
IgM antibodies (detected on the IgM Western blot, 3 bands evaluated — 23, 39, 41 kDa):
- Appear first after infection, typically within 1–4 weeks
- Usually disappear after 4–8 weeks in successfully treated early Lyme
- IgM positivity after 4–8 weeks of symptoms may indicate persistent infection, but late IgM positivity without IgG is clinically unusual and can be a false positive
IgG antibodies (detected on the IgG Western blot, 10 bands evaluated):
- Appear later, typically 4–6 weeks after infection
- Can persist for months to years after successful treatment
- The 58 KD (IgG) Band is part of the IgG panel
- A positive IgG Western blot in isolation, without supporting symptoms, should be interpreted cautiously
WHEN TO FOLLOW UP
Discuss your result with a clinician if:
- 5 or more IgG bands are reactive and you have symptoms consistent with Lyme disease (arthritis, neurological symptoms, cardiac symptoms, fatigue, or a history of tick bite in an endemic area)
- You are unsure how many total bands are reactive on your report
- Your result shows IgG P58 Ab Present alongside other reactive proteins on a line blot — ask your clinician how many total proteins are reactive
- You received a positive first-tier ELISA or EIA screening test followed by this Western blot result
- You have been previously treated for Lyme disease and are wondering if a persistent positive IgG represents treatment failure (it usually does not)
INTERNAL LINKS
Related Lyme Western blot bands on HealthMatters: 41 KD (IgG) Band · 18 KD (IgG) Band · 93 KD (IgG) Band · 23 KD (IgM) Band · IgG P41 Ab · IgG P18 Ab
58 KD (IGG) Band
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Frequently Asked Questions
What does "58 KD (IgG) Band Reactive" mean?
Does a reactive 58 KD (IgG) band mean I have Lyme disease?
What does IgG P58 Ab Present mean?
How many IgG bands do I need for a positive Lyme test?
What does it mean if both 41 and 58 kDa IgG bands are reactive?
Can I have Lyme disease with a negative 58 KD band?
Why does my IgG Western blot stay positive after Lyme treatment?
What is the difference between the 58 KD band on the Western blot and IgG P58 Ab on the line blot?
All our content is backed by peer-reviewed studies, academic research, and trusted medical sources. We’re committed to accuracy and transparency — see our editorial policy for details.
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Recommendations for test performance and interpretation from the Second National Conference on Serologic Diagnosis of Lyme Disease
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Prospective study of serologic tests for lyme disease
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Laboratory Diagnosis of Lyme Borreliosis
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Updated CDC Recommendation for Serologic Diagnosis of Lyme Disease