IgG vs IgM Antibody Test
Direct answer: IgM appears first after infection and indicates a recent or active immune response; IgG appears later, persists long-term and indicates past exposure or immunity. Together they help distinguish recent from past infection.
Comparison table
| IgM | IgG | |
|---|---|---|
| Appears | First (days after infection) | Later (weeks) |
| Persists | Weeks to months | Months to years |
| Indicates | Recent / active infection | Past exposure / immunity |
| Example use | Acute dengue, HAV IgM | Rubella immunity, past exposure |
What each marker indicates
- IgM positive: likely recent infection — but cross-reactivity can cause false positives.
- IgG positive: past exposure or immunity — may not indicate current infection.
Testing window
IgM is most useful in the acute phase; IgG becomes reliable later. Paired or combined IgM/IgG testing widens the window.
Strengths
- IgM: early indicator of recent infection.
- IgG: immunity assessment (e.g., rubella, varicella); persists for monitoring.
Limitations
- IgM: cross-reactivity, false positives.
- IgG: cannot confirm active infection alone; timing-dependent.
False positive / negative considerations
- False positive IgM: cross-reactivity with related pathogens.
- False negative IgG: testing before seroconversion.
Confirmatory testing
Serology is supportive; confirm active infection with antigen or molecular (PCR) testing where applicable.
Related pages
Which test to choose
Choose IgM to detect a recent infection; choose IgG for past-exposure or immunity assessment. Combined IgM+IgG covers both phases.
Last reviewed: 2026-09-12