Lyme Disease Western Blot (IgG and IgM Bands)
A blood test that checks which Lyme bacteria proteins your antibodies react to. It is the second step after a positive or equivocal antibody screen.
AI-verified against cited sources. Awaiting expert reviewAlso called Lyme WB, Lyme IgG WB, Lyme IgM WBLOINC 6320-6
What it is
The Western blot is the second step in the standard two-step Lyme test. Antibodies in your blood are sorted so that each reacts with one protein from the Lyme bacteria. Each reaction appears as a line called a band, named by its size in kilodaltons (kDa). The lab counts the bands and reports one result for IgM, the antibody type that appears first, and one for IgG, the type that appears later.
Why it matters
A positive first-step screen can be a false alarm, so the blot checks whether your antibodies react with enough specific Lyme proteins. Your clinician reads it together with your symptoms, tick exposure and how long you have been ill. IgM results count only in the first 30 days of illness, so timing matters when reading this report.
If it’s low
A negative blot means too few bands were present to meet the criteria. Under the standard two-step approach, a positive first step followed by a negative blot is read as an overall negative result. If your blood was drawn in the first few weeks of illness, antibodies may not have formed yet.
Common causes
- You do not have Lyme disease, and the first-step screen was a false alarm
- The blood was drawn early, before your body made enough antibodies (CDC: the first 4 to 6 weeks)
What can help
- If you were tested within a few weeks of symptoms starting, ask whether to repeat testing later
- Tell your clinician if new symptoms appear
If it’s high
A positive blot means enough bands were present to meet the criteria. CDC criteria call an IgM blot positive with 2 of 3 specific bands, and an IgG blot positive with 5 of 10 bands. A positive IgM with no positive IgG in someone who has been ill for more than 30 days is likely to be a false positive. IgG bands can stay positive for years, so a positive result does not always mean the infection is new.
Common causes
- A past Lyme infection, treated or not (IgG bands can persist for years)
- Cross-reacting antibodies from other conditions, including relapsing fever, syphilis, rheumatoid arthritis and Epstein-Barr virus infection
- A current Lyme disease infection
What can help
- Tell your clinician when your symptoms started, because IgM bands are only meaningful in the first 30 days
- Share any tick bites, rashes or travel to areas where Lyme disease is common
- Ask how the blot result fits with your first-step screen and your symptoms
What can affect your result
- How long you have been ill: IgM bands count only in the first 30 days, and IgG bands take longer to appear
- A past Lyme infection: IgG bands can persist for years
- Cross-reacting antibodies from other infections or autoimmune conditions
- Different labs and kits may label the same band with slightly different sizes (the OspC band appears as 21, 23 or 24 kDa on different reports)
- IgM bands are only meaningful in the first 30 days of illness. CDC says positive IgM results should be disregarded if you have been ill for more than 30 days.
- False positives can occur with other conditions, including relapsing fever, syphilis, rheumatoid arthritis and Epstein-Barr virus infection (CDC).
- Antibody tests can be falsely negative during the first 4 to 6 weeks after infection (CDC).
IgM bands can appear within the first weeks of infection. IgG bands take longer to appear and can persist for months to years, even after treatment.
When to talk to a clinician
Contact a clinician promptly if, after a possible tick exposure, you have a rash that keeps spreading, new drooping of one side of the face, severe headache with a stiff neck, a swollen and painful joint, or a change in your heart rate or rhythm. Do not wait for test results if symptoms are severe.
Questions you could ask
- How does this blot result fit with my first-step screen and my symptoms?
- How do my IgM and IgG results fit with when my symptoms started?
- Could this result be from an old infection or another condition?
- Should I repeat the test later, since I was tested soon after my symptoms began?
Questions people ask
What are the numbers (kDa) on my report?
Each band is a protein from the Lyme bacteria, named by its size in kilodaltons (kDa). The lab marks which proteins your antibodies reacted with and counts them. The CDC's rule is 2 of 3 bands for IgM and 5 of 10 for IgG.
Why is my IgM positive but my IgG negative?
IgM is the antibody type that appears first, and IgG appears later. A positive IgM with a negative IgG can fit a very early infection. But the CDC says positive IgM results should be disregarded if you have been ill for more than 30 days, because of the higher chance of a false positive.
Why did my lab use a second antibody test instead of a Western blot?
Since 2019 the CDC has accepted a modified approach that uses a second antibody test (EIA) instead of a blot. Some labs, including Mayo Clinic Laboratories, use it. Either path is a way of confirming a positive or equivocal first step.
Show the science and sources
For clinicians and researchers
Second tier of the standard CDC two-tier algorithm, after a positive or indeterminate EIA. Interpretive criteria from the Second National Conference on Serologic Diagnosis of Lyme Disease (CDC, MMWR 1995): an IgM immunoblot is positive when 2 of 3 bands are present (24 kDa OspC, 39 kDa BmpA, 41 kDa Fla); an IgG immunoblot is positive when 5 of 10 bands are present (18, 21 kDa OspC, 28, 30, 39 kDa BmpA, 41 kDa Fla, 45, 58 kDa not GroEL, 66 and 93 kDa). The IgM blot is meant for early disease; that report states a positive IgM alone is not recommended for determining active disease in persons with illness greater than 1 month's duration because a false positive is likely, and current CDC guidance says positive IgM results should be disregarded after 30 days of illness. An overall positive requires a positive first tier and a positive second tier. FDA cleared modified two-tier assays (EIA then EIA, July 2019) that replace the blot, and Mayo Clinic Laboratories' modified two-tier algorithm uses a whole-cell sonicate EIA rather than a blot. Serologic assays may be falsely negative in the first 4 to 6 weeks, antibody can persist for months to years and cannot establish cure, and cross-reactivity occurs with relapsing fever, syphilis, rheumatoid arthritis and EBV infection. Reports differ in how they label bands and whether they include an indeterminate category.
LOINC 6320-6 (primary), 6321-4, 18203-0, 62342-1, 60342-3, 60343-1, 13502-0, 13503-8. Specimen: serum.
Sources
- Recommendations for Test Performance and Interpretation from the Second National Conference on Serologic Diagnosis of Lyme Disease, MMWR Morbidity and Mortality Weekly Report (CDC) (1995)
- Clinical Testing and Diagnosis for Lyme Disease, CDC
- Updated CDC Recommendation for Serologic Diagnosis of Lyme Disease, MMWR Morbidity and Mortality Weekly Report (CDC) (2019)
- Lyme Antibody Modified 2-Tier with Reflex, Serum (test SLYME), Mayo Clinic Laboratories (Extended Catalog)
- Lyme Disease Tests, MedlinePlus (NIH)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.