Toxoplasma IgG: what the antibody result says about past infection

Toxoplasmosis is a common infection that can be caught from the poo of infected cats, from contaminated soil, or from infected meat. The NHS says it does not usually cause symptoms and most people do not know they have had it; some have flu-like symptoms such as high temperature, headache, sore throat, aching body and swollen glands. It is usually harmless but can cause serious problems in pregnancy or when the immune system is weakened.

After infection the body makes IgM antibodies first and IgG antibodies a little later. A positive IgG indicates that you have been exposed to Toxoplasma gondii at some time. Because IgG typically persists for life, a single positive result cannot say whether the infection was last week or many years ago.

Doctors may test if you are pregnant and suspected to have been exposed, if you have a weakened immune system, if you have eye symptoms or other features suggesting active infection, or when other causes of a glandular fever-type illness are being excluded. The NHS says a GP may do blood tests to see if you have been infected, and if you are pregnant and test positive the GP can refer you for more tests to see if your baby is infected, which it says is very rare.

The NHS advises an urgent GP appointment or NHS 111 if you are pregnant or have a weakened immune system and have any symptoms of toxoplasmosis, if you have blurred vision or difficulty walking, and 999 or A&E for sudden confusion or slurred speech.

No preparation is needed. A blood sample is taken from a vein. Reports show IgG with or without IgM; interpretation depends heavily on the timing of symptoms or exposure, so give your clinician that information. A single sample is sometimes followed by a second sample some weeks later to see whether antibody levels are rising.

An international review in the Journal of Clinical Microbiology explains that IgM is a sensitive indicator of primary toxoplasmosis but has low specificity, because natural IgM antibodies sometimes react with Toxoplasma antigens without infection, and IgM can persist for several months or years after infection. IgG avidity, the aggregate binding strength of IgG, matures gradually within about 6 months of primary infection. A high avidity index allows a recent infection (less than 4 months) to be excluded, whereas a low avidity index indicates a probable recent infection without excluding older ones.

The same review notes that the risk of passing the parasite to the baby is much greater when a woman is exposed to a primary infection after conception than when exposure was before conception, so classifying recent infection matters for pregnant women. This is why IgG alone is rarely enough in pregnancy.

Most people get better without treatment. The NHS says medicines, including antibiotics, are usually given if you are pregnant, have a weakened immune system or have eye symptoms, and newborns with toxoplasmosis are treated. Prevention for people who are pregnant or immunosuppressed includes wearing gloves in the garden, washing hands, thoroughly washing produce, cooking meat well, avoiding raw or cured meats and unpasteurised goats' milk, and avoiding cat litter or wearing gloves and washing hands afterwards. You cannot catch toxoplasmosis from stroking a cat.

A positive IgG with no recent exposure usually needs no action. If recent infection is suspected, a specialist infection or fetal medicine team arranges further testing and treatment. A negative result in pregnancy means you remain susceptible, so the prevention advice matters.