Lyme Disease

If you have Lyme test results in hand, this page explains how the Western blot works, what reactive bands mean, and how to find the specific band result you are looking at.

How Lyme Disease Testing Works

Lyme disease testing in the US follows a two-step process recommended by the CDC:

Step 1 — ELISA or EIA (screening test). A blood test that measures the overall level of antibodies against Borrelia burgdorferi, the bacterium that causes Lyme disease. If this test is negative, no further testing is typically recommended. If it is positive or equivocal, a second confirmatory test is ordered.

Step 2 — Western blot (confirmatory test). This test separates Borrelia burgdorferi proteins by molecular weight and identifies which specific proteins your immune system has produced antibodies against. Results are reported band by band, with each band corresponding to a specific protein.

The Western blot is not ordered or interpretable in isolation — it is a confirmatory test, meaningful only in the context of a positive or equivocal first-tier ELISA.

IgG vs. IgM: Two Separate Panels

The Western blot reports results on two separate antibody panels:

IgG panel — 10 bands (18, 21, 28, 30, 39, 41, 45, 58, 66, 93 kD) A positive IgG Western blot requires 5 of 10 bands reactive. IgG antibodies develop 4–6 weeks after infection and can persist for years. A positive IgG result reflects past or longer-standing exposure rather than very recent infection.

IgM panel — 3 bands (23, 39, 41 kD) A positive IgM Western blot requires 2 of 3 bands reactive. IgM antibodies appear within 1–2 weeks of infection and typically decline after 8 weeks. A positive IgM result is most relevant in the context of early, acute infection.

An overall Western blot result is positive if either panel meets its threshold — 5/10 IgG bands or 2/3 IgM bands — in the context of a positive first-tier ELISA.

What Individual Band Results Mean

Each band on your Western blot report is listed as Reactive (or Present, Positive, Abnormal) or Non-reactive (or Absent, Negative). A reactive result on a single band does not make the overall Western blot positive — the total count of reactive bands across the panel determines the overall result.

Not all bands carry equal diagnostic weight. Some bands are highly specific for Borrelia burgdorferi — the 39 kD (BmpA) and 21 kD (OspC) bands are among the most Lyme-specific on the IgG panel. Others, particularly the 41 kD (flagellin/P41) band, are highly cross-reactive and commonly reactive in people without Lyme disease exposure.

Select the band result you are looking at from the panel below to read its specific interpretation.

Important Context for Interpreting Your Results

Lyme serology has known limitations that affect how results should be interpreted:

Timing matters. Antibodies take time to develop after infection. Testing too early — within the first 1–4 weeks — can produce false-negative results because the immune response has not yet reached detectable levels. If early Lyme disease is strongly suspected, repeat testing 2–4 weeks later is often appropriate.

Serology persists after treatment. IgG antibodies can remain detectable for years to decades after a successfully treated Lyme infection. A positive Western blot does not necessarily indicate active current infection — it may reflect past exposure.

Cross-reactivity causes false positives. Several conditions can produce false-positive Lyme Western blot results, including lupus (SLE), rheumatoid arthritis, Epstein-Barr virus (mononucleosis), and syphilis. A positive Western blot in a person with an autoimmune condition or recent viral illness should be interpreted with this in mind.

Diagnosis requires clinical context. CDC surveillance criteria for a positive Western blot are standardized testing thresholds — they are not a substitute for clinical evaluation. Lyme disease diagnosis requires interpretation alongside symptoms, tick exposure history, geographic risk, and clinical presentation. A positive Western blot in the absence of symptoms consistent with Lyme disease does not establish a diagnosis, and a negative Western blot does not exclude early Lyme disease when symptoms and exposure history are consistent.