[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Immunology](https://durgical.com/en/immunology/) › [Immunology lab](https://durgical.com/en/immunology/#a-lab) › Serology: IgM vs IgG, seroconversion, avidity [Immunology](https://durgical.com/en/immunology/) · [Immunology lab](https://durgical.com/en/immunology/#a-lab) # 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 ## Key points - IgM = recent; IgG = past or vaccine. - Seroconversion: fourfold rise in titer in paired sera. - Low avidity = recent infection. - Under 18 months: maternal IgG confuses HIV serology. - Dengue and other flaviviruses cross-react. ## 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. ## Memory aid IgM is the Messenger who arrives first; IgG is the Guard who stays. A Guard with a loose grip (low avidity) has only just arrived. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/immunology/serology-igm-vs-igg-seroconversion-avidity/) ## Related - [ELISA, western blot and rapid tests](https://durgical.com/en/immunology/elisa-western-blot-and-rapid-tests/) - [Passive immunization: immunoglobulins and antitoxins](https://durgical.com/en/immunology/passive-immunization-immunoglobulins-and-antitoxins/) - [Immunofluorescence and the ANA test](https://durgical.com/en/immunology/immunofluorescence-and-the-ana-test/) - [Antibody structure and isotypes (IgG, IgA, IgM, IgE, IgD)](https://durgical.com/en/immunology/antibody-structure-and-isotypes/) - [Primary vs secondary response and memory](https://durgical.com/en/immunology/primary-vs-secondary-response-and-memory/) - [B-cell activation, class switching and affinity maturation](https://durgical.com/en/immunology/b-cell-activation-class-switching-and-affinity-maturation/) ## Related conditions - [Dengue](https://durgical.com/en/conditions/dengue/) - [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/) - [Viral hepatitis](https://durgical.com/en/conditions/viral-hepatitis/) ## Microorganisms - [Toxoplasma gondii](https://durgical.com/en/microbiology/toxoplasma-gondii/) - [Hepatitis B virus](https://durgical.com/en/microbiology/hepatitis-b-virus/) - [Dengue virus](https://durgical.com/en/microbiology/dengue-virus/) - [Rubella virus](https://durgical.com/en/microbiology/rubella-virus/) - [Human immunodeficiency virus](https://durgical.com/en/microbiology/human-immunodeficiency-virus/) [← All of immunology](https://durgical.com/en/immunology/) 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](https://medlineplus.gov/lab-tests/antibody-serology-tests/) (MedlinePlus (U.S. National Library of Medicine), 2024); [Toxoplasma blood test](https://medlineplus.gov/ency/article/003514.htm) (MedlinePlus (U.S. National Library of Medicine), 2024); [ENZYME LINKED IMMUNOSORBENT ASSAYS REVISITED](https://pmc.ncbi.nlm.nih.gov/articles/PMC5530809/) (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. --- Source: https://durgical.com/en/immunology/serology-igm-vs-igg-seroconversion-avidity/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/immunology/serology-igm-vs-igg-seroconversion-avidity/