Positive IgM: what next?
Class M antibodies appear in the first one to two weeks after Toxoplasma infection and usually disappear within a few months. Their detection indicates a fresh process, and during pregnancy, such a result requires immediate clarification rather than observation.
The difficulty is that IgM to Toxoplasma can persist in the blood for an unusually long time - sometimes more than a year after the infection. In addition, false-positive results occur due to cross-reactions and the presence of rheumatoid factor. Therefore, positive IgM in isolation is never considered evidence of acute toxoplasmosis.
The standard clarification algorithm includes determination of IgG, re-examination of both classes after two weeks to assess dynamics and, most informatively, determination of IgG avidity. High avidity virtually eliminates infection in the last three to four months and eliminates concerns even with positive IgM. Only the totality of this data allows us to talk about a fresh infection and decide on treatment.
What do IgM and IgG mean?
The immune system produces different classes of antibodies at different times. Immunoglobulins M appear first, usually in the first or second week after infection, last for several weeks or months and then disappear. Their detection indicates an acute or recent process.
Immunoglobulins G appear later, after a few weeks, and persist for years, and often for life. They mean that a meeting with the pathogen took place in the past, and in many cases indicate formed immunity. Therefore, positive IgG in itself is not a reason for treatment.
Why dynamics are important, not just a number
A single result is often ambiguous. Antibody levels vary from person to person, with borderline levels falling into what is known as a gray area. A reliable sign of active infection is a fourfold increase in IgG titer in paired sera taken at intervals of 10–14 days.
This is why serology is rarely prescribed as a single test. The doctor either combines IgM and IgG, or repeats the test, or confirms the result with a direct method - PCR, which detects the DNA or RNA of the pathogen itself.
Limitations of the method
Antibodies do not appear immediately: in the first days of the disease, serology can be negative if the infection has already begun - this is the period of the serological window. In case of immunodeficiency and against the background of immunosuppressive therapy, antibodies may not be produced at all.
Cross-reactions with related pathogens are also possible, giving a false-positive result. Therefore, treatment is never prescribed “based on analysis” - only on the basis of a combination of clinical data and research results.