Which test is the most sensitive for diagnosing Toxoplasma in cats with clinical signs?

The most sensitive test for diagnosing Toxoplasma in cats with clinical signs is the measurement of IgM antibodies in cerebrospinal fluid (CSF). When cats are infected with Toxoplasma gondii, particularly in cases that show clinical signs of neurologic involvement, the presence of IgM in the CSF indicates an active or recent infection that may be contributing to the clinical symptoms.

IgM antibodies are typically the first antibodies to be produced in response to an acute infection, making their detection particularly relevant in the setting of recent infection. In cases where neurological signs are present, testing the CSF provides localized information directly relating to central nervous system involvement, which is critical since Toxoplasma can cause significant neurological disease in cats.

While serum tests for IgG and IgM can provide useful information, they may not be as sensitive as CSF testing in the context of active central nervous system infection. IgG antibodies often indicate past exposure and may not reflect current infection status. A direct smear can detect the organism but is less sensitive and less specific, especially if the sample quality is suboptimal or if there are low parasite loads. Therefore, CSF analysis for IgM is the preferred method in this scenario.