What is the role of immunoglobulin testing in the diagnosis of toxoplasmosis?

Updated: Mar 08, 2019
  • Author: Murat Hökelek, MD, PhD; Chief Editor: Michael Stuart Bronze, MD  more...
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Acute systemic toxoplasmosis has traditionally been diagnosed by seroconversion. Anti-Toxoplasma immunoglobulin G (IgG) titers present a 4-fold increase that peak 6-8 weeks following infection and then decline over the next 2 years, although they remain detectable for life. Anti-Toxoplasma IgM appears in the first week of the infection and then declines in the next few months. The presence of anti-Toxoplasma IgA has also been shown to be detectable in acute infection; however, since the titers can last for more than 1 year, its value in helping to diagnose an acute phase is limited.

Detection of IgG is possible within 2 weeks of infection using the ELISA test, the IgG avidity test, and the agglutination and differential agglutination tests. The presence of IgG indicates a likely past infection, while the presence of IgM usually indicates acute infection (particularly in the absence of IgG). However, IgM has, in some cases, been documented to persist for months or years.

Lack of IgG and IgM may exclude infection. IgM alone that then transitions to IgG without IgM or both IgG and IgM indicates likely acute infection. There is a significant rate of false IgM positivity. The sensitivities and specificities of the commercially available IgM and IgG tests vary substantially.

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