Does pathogen-specific IgG with a negative PCR usually indicate prior exposure rather than active infection?

Pathogen-specific IgG and PCR address different biological signals: immune memory versus detectable microbial nucleic acid in a particular specimen at a particular time. In a 77-year-old, as at other ages, this distinction is central to avoiding treatment decisions based on isolated serology.

Interpretation of IgG-positive/PCR-negative results

• Pathogen-specific IgG generally supports prior immune recognition or exposure, often without defining when infection occurred. IgG may remain detectable long after clearance; in Lyme disease, for example, antibodies can persist for months to years after successful treatment and are not a test of cure.
• Comparable interpretation applies across examples cited: isolated Chlamydia pneumoniae IgG can reflect infection at an indeterminate prior time; Mycoplasma pneumoniae IgG principally indicates previous exposure; and Toxoplasma gondii IgG commonly indicates chronic or latent infection.
• Stronger evidence of recent infection may require pathogen-specific methods such as paired sera showing a fourfold antibody rise, seroconversion, or an appropriately selected nucleic-acid test.

Why PCR and IgG can be discordant

• A negative PCR argues against detectable pathogen nucleic acid in the tested specimen and window, not necessarily infection in every tissue or disease stage. Diagnostic yield depends on pathogen burden, timing, target, and specimen selection.
• In suspected Lyme neuroborreliosis, blood and CSF PCR have inadequate sensitivity, whereas serum antibody testing is preferred. Conversely, PCR in synovial fluid or tissue may add evidence in Lyme arthritis but can remain positive after therapy, so it also does not independently prove ongoing infection.

Clinical implications

• An IgG-positive/PCR-negative pattern should not alone diagnose active disease or trigger treatment. Symptoms, exposure history, alternative diagnoses, and syndrome-specific confirmatory testing determine its significance.
• Bottom line: This pattern usually indicates prior immune recognition rather than demonstrable active infection, but neither positive IgG nor negative PCR should be interpreted as a universal, pathogen-independent answer to whether infection is currently active.