Do pathogen-specific IgG or IgM alone prove active infection?

Pathogen-specific serology is most useful as contextual evidence of immune exposure, not as a stand-alone test for whether an infection is currently active. This is particularly important when results could prompt treatment, isolation, or further invasive testing.

Clinical interpretation

• IgG: Prior infection commonly produces persistent pathogen-specific IgG. Long-lived plasma cells can secrete antigen-specific IgG without renewed exposure; measles IgG, for example, has been detected >40 years after infection. A single positive IgG therefore usually indicates prior exposure (or, for some pathogens, vaccination), not active disease. CMV, Toxoplasma, and Lyme guidance similarly cautions that isolated IgG cannot date infection or distinguish past from active infection.
• IgM: A positive pathogen-specific IgM can support recent infection when it fits the symptom onset, exposure, and pathogen-specific serologic timeline. However, isolated IgM is not definitive: dengue IgM may persist for about 3 months, and Lyme guidance advises against relying on IgM after >30 days of symptoms.

Mechanistic and diagnostic considerations

• Persistent IgG is biologically expected from long-lived antibody-secreting plasma cells.
• IgM can remain detectable after recovery or be falsely reactive through cross-reactive antibodies, polyclonal immune activation, rheumatoid factor, or heterophile-antibody assay interference.
• When recent infection must be established serologically, IgG seroconversion or a substantial—often fourfold—rise between acute and convalescent samples is more informative than one IgG result. Direct pathogen detection (for example, PCR/NAAT) is often preferred at appropriate illness stages.

Bottom line

• The claim is well supported: persistent IgG generally records prior exposure, while IgM may suggest recent infection but can persist or be false-positive. Neither isolated persistent IgG nor isolated IgM reliably proves active infection; interpretation requires clinical timing, exposure risk, and pathogen-specific confirmatory testing.