What test is most specific for diagnosing T. Gondii in a cat with clinical signs?

The immunoglobulin M (IgM) titer, particularly when found elevated in cerebrospinal fluid (CSF), is a specific marker for recent or acute infection. In the context of Toxoplasma gondii, a cat presenting with clinical signs consistent with an infection would most likely show elevated IgM antibodies as the immune system reacts to the presence of the pathogen. The significance of testing in the CSF adds an additional layer of specificity because it suggests that the infection may have neurological implications, which is a common concern with Toxoplasmosis in cats.

Other tests, such as the ELISA for IgG, measure antibodies that reflect past exposure to the organism rather than recent or ongoing infection, therefore providing less diagnostic specificity in determining current active disease. Blood culture is not typically employed for detecting T. gondii, as it is not a bacterium that can be cultured in this manner. Fecal flotation primarily identifies oocysts, which would indicate an environmental presence of the parasite but does not confirm active disease or the associated clinical signs in the individual cat. Thus, the elevation of IgM in CSF serves as a more definitive indicator of current T. gondii infection, making it the most specific test in this scenario.