Do Borrelia antibodies prove active infection?

Borrelia serology is valuable for documenting prior immune exposure, but it cannot independently determine whether infection is currently active. This distinction is especially important when evaluating nonspecific or persistent symptoms in an older adult, where alternative explanations may also be common.

Clinical interpretation

• Detection of Borrelia-specific IgM or IgG shows antigen-specific humoral immune reactivity: B-cell responses generate antibodies to borrelial proteins including OspC, flagellin, and Dbp proteins.
• This result does not by itself date the infection, establish viable organisms, demonstrate treatment failure, or prove an active Lyme manifestation. Results should be interpreted with the clinical syndrome, objective examination findings, exposure history, prior Lyme history, and validated standard or modified two-tier testing.
• In prolonged illness, isolated IgM reactivity is particularly unreliable as evidence of active Lyme borreliosis. Later untreated manifestations would generally be expected to have a substantial IgG response; suspected Lyme arthritis with IgM-only or limited IgG reactivity therefore warrants caution.

Persistence and treatment monitoring

• Long-term seropositivity is well documented. In a follow-up study of 79 individuals tested 10–20 years after Lyme disease without current signs or symptoms, persistent IgM/IgG reactivity occurred in 10%/25% after early disease and 15%/62% after prior Lyme arthritis.
• Thus, antibody tests should not be used to confirm bacterial eradication or to monitor response to antibiotics: IgM and IgG may remain detectable for years or decades after successful treatment.

Timing and direct testing

• Early active infection may be seronegative before antibodies develop, so a negative early serology does not invariably exclude infection.
• PCR is not a universal arbiter of activity: blood PCR has low or variable sensitivity, and PCR positivity in selected specimens can reflect residual bacterial DNA rather than viable organisms.

Bottom line

• Borrelia antibodies demonstrate immune reactivity and can persist for many years; a positive result alone cannot establish active infection and requires clinical correlation.