Can persistent or isolated Borrelia IgM occur without active Lyme infection?

Persistent or recurrent Borrelia IgM should not be interpreted as proof of ongoing Lyme borreliosis, particularly when it is isolated from IgG reactivity or occurs long after symptom onset.

Clinical interpretation

• CDC and IDSA/AAN/ACR guidance state that serologic antibodies can remain detectable for months to years and cannot distinguish active from resolved infection or assess treatment response.
• In long-term follow-up 10–20 years after treated Lyme borreliosis, B. burgdorferi–specific IgM remained detectable in 10% of people with prior early disease and 15% with prior Lyme arthritis—direct evidence that IgM persistence can occur without active infection.
• Under standard two-tier testing, IgM is clinically interpretable only during the first 30 days of compatible illness. After this, isolated IgM with negative IgG should be disregarded because persistent or false-positive reactivity is common enough to undermine its diagnostic value.

Cross-reactivity and mechanisms

• Isolated IgM can result from nonspecific binding or polyclonal immune activation, rather than Borrelia infection. Recognized settings include Epstein–Barr virus infection, syphilis and other spirochetal infections, rheumatoid arthritis/rheumatoid factor, and related autoimmune or infectious states.
• Viral-infection studies report frequent IgM reactivity to OspC and flagellin. Persistent OspC IgM may reflect recognition of its terminal PKKP motif, shared with human and microbial proteins. IgM specificity also varies by antigen construct, including lower specificity reported for some VlsE assays.

Practical implications

• Interpretation should integrate compatible manifestations, tick exposure risk, illness duration, and validated two-tier testing—not an isolated antibody result. PCR must likewise be matched to the clinical syndrome and specimen; neither a negative result nor post-treatment positivity independently establishes disease activity.

Bottom line

• The claim is strongly supported: persistent/recurrent Borrelia IgM may reflect past immune reactivity, and isolated IgM may be cross-reactive rather than evidence of active infection.