Serology: IgM vs IgG, seroconversion, avidity !
- Topic
Serology looks for antibodies in the blood. Their type and amount suggest whether an infection is recent, past, or whether there was vaccination.
Updated 2026-11-01
What it is
Serological tests detect specific antibodies (or antigens) in serum. IgM appears first and suggests recent infection; IgG comes later, persists and marks past infection, vaccination or immunity. Reading them requires knowing the time since exposure, cross-reactivity, and maternal antibodies in infants.
Key facts
- IgM: first to appear after a primary infection; fades over weeks or months, though it sometimes persists.
- IgG: appears later and lasts years; on its own it cannot tell past infection from vaccination.
- Seroconversion: going from negative to positive, or at least a fourfold rise in titer between an acute and a convalescent sample.
- IgG avidity: low in recent infections and high in old ones; it helps date toxoplasmosis or cytomegalovirus in pregnancy.
- Titer: the highest serum dilution that is still positive (for example 1:160).
Typical serological patterns
| IgM | IgG | Usual reading |
|---|---|---|
| Positive | Negative | Early acute infection (or false-positive IgM) |
| Positive | Positive, low avidity | Recent infection |
| Negative | Positive, high avidity | Past infection or vaccination |
| Negative | Negative | Susceptible, or sample taken in the window period |
In the lab
- Window period: days or weeks pass after infection before antibodies are detectable; an early test can be falsely negative.
- Infants keep maternal IgG up to 18 months, so HIV in them is diagnosed with tests for the virus, not antibodies.
- Cross-reactivity: flaviviruses (dengue, Zika, yellow fever) give false positives with each other.
- The tests alone do not say how long protection lasts.
Test yourself
Think of the answer, then tap the question to see it.
What does a positive IgM with a negative IgG suggest?
Early acute infection, although a false-positive IgM is possible and repeating or confirming is wise.
What is seroconversion?
Antibodies going from negative to positive, or at least a fourfold rise in titer between the acute and convalescent phases.
What is the IgG avidity test for in a pregnant woman with positive Toxoplasma IgG and IgM?
To date the infection: high avidity means infection several months earlier, probably before pregnancy; low avidity suggests recent infection.
Why does a positive HIV antibody test in a 6-month-old infant not confirm infection?
It may reflect maternal IgG that crossed the placenta; under 18 months, tests that detect the virus are used.
Related
- ELISA, western blot and rapid tests
- Passive immunization: immunoglobulins and antitoxins
- Immunofluorescence and the ANA test
- Antibody structure and isotypes (IgG, IgA, IgM, IgE, IgD)
- Primary vs secondary response and memory
- B-cell activation, class switching and affinity maturation
Related conditions
Microorganisms
Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.
Sources: Antibody Serology Tests (MedlinePlus (U.S. National Library of Medicine), 2024); Toxoplasma blood test (MedlinePlus (U.S. National Library of Medicine), 2024); ENZYME LINKED IMMUNOSORBENT ASSAYS REVISITED (Medical Journal Armed Forces India (PubMed Central), 1997).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.