Does a positive pathogen-specific IgG mean active infection?

Pathogen-specific IgG is best understood as evidence of immunologic contact with an antigen, not as a stand-alone marker of active disease. This distinction is particularly important when testing is performed in the absence of a new, compatible illness.

Clinical interpretation

• A positive pathogen-specific IgG generally indicates prior infection or other prior antigen exposure. Across EBV, CMV, Toxoplasma, Borrelia, and parvovirus B19 testing, IgG may remain detectable for months, years, or lifelong after the acute illness has resolved.
• A single IgG result ordinarily cannot establish when exposure occurred, distinguish remote from ongoing infection, or establish that symptoms are caused by that pathogen. CDC guidance cited for both SARS-CoV-2 and dengue does not support using isolated IgG to diagnose current acute infection.
• IgM can be supportive in selected pathogen-specific settings, but its value depends on the assay, timing of sampling, organism, and clinical presentation. An acute compatible syndrome raises suspicion but is not etiologically specific.
• For suspected current infection, appropriately targeted direct detection—such as NAAT/PCR or antigen testing—is the principal corroborating approach. Results still require interpretation by specimen type, timing, and clinical context.

Mechanistic context

• Long-lived plasma cells can continuously secrete antigen-specific IgG for years or longer after pathogen clearance. Persistent serum IgG therefore reflects durable humoral immunity rather than persistent pathogen presence.
• Memory B cells are distinct from circulating antibody: they enable a rapid response upon re-exposure, whereas plasma cells maintain measurable IgG.
• Depending on the antigen tested, vaccination can also yield positive IgG; SARS-CoV-2 spike antibodies are a prominent example.

Bottom line

• An isolated positive pathogen-specific IgG should usually be interpreted as evidence of prior immune exposure, not proof of active infection. When active infection would alter care—especially in an older adult—testing should be selected for the suspected pathogen and clinical syndrome, with direct detection or pathogen-specific serial serology where appropriate.