T. gondii may exist in a host as an inactive cyst that would likely evade detection.Serological testing can detect T. gondii antibodies in blood serum, using methods including the Sabin–Feldman dye test (DT), the indirect hemagglutination assay, the indirect fluorescent antibody assay (IFA), the direct agglutination test, the latex agglutination test (LAT), the enzyme-linked immunosorbent assay (ELISA), and the immunosorbent agglutination assay test (IAAT).The most commonly used tests to measure IgG antibody are the DT, the ELISA, the IFA, and the modified direct agglutination test. IgG antibodies usually appear within a week or two of infection, peak within one to two months, then decline at various rates. Toxoplasma IgG antibodies generally persist for life, and therefore may be present in the bloodstream as a result of either current or previous infection.To some extent, acute toxoplasmosis infections can be differentiated from chronic infections using an IgG avidity test, which is a variation on the ELISA. In the first response to infection, toxoplasma-specific IgG has a low affinity for the toxoplasma antigen; in the following weeks and month, IgG affinity for the antigen increases. Based on the IgG avidity test, if the IgG in the infected individual has a high affinity, it means that the infection began three to five months before testing. This is particularly useful in congenital infection, where pregnancy status and gestational age at time of infection determines treatment.In contrast to IgG, IgM antibodies can be used to detect acute infection but generally not chronic infection.
Some evidence suggests latent infection may subtly influence a range of human behaviors and tendencies, and infection may alter the susceptibility to or intensity of a number of psychiatric or neurological disorders.In most of the current studies where positive correlations have been found between T. gondii antibody titers and certain behavioral traits or neurological disorders, T. gondii seropositivity tests are conducted after the onset of the examined disease or behavioral trait; that is, it is often unclear whether infection with the parasite increases the chances of having a certain trait or disorder, or if having a certain trait or disorder increases the chances of becoming infected with the parasite. Groups of individuals with certain behavioral traits or neurological disorders may share certain behavioral tendencies that increase the likelihood of exposure to and infection with T. gondii; as a result, it is difficult to confirm causal relationships between T. gondii infections and associated neurological disorders or behavioral traits. Mental health
Some evidence links T. gondii to schizophrenia. Two 2012 meta-analyses found that the rates of antibodies to T. gondii in people with schizophrenia were 2.7 times higher than in controls. T. gondii antibody positivity was therefore considered an intermediate risk factor in relation to other known risk factors. Cautions noted include that the antibody tests do not detect toxoplasmosis directly, most people with schizophrenia do not have antibodies for toxoplasmosis, and publication bias might exist.