Diagnosis of T. gondii infection is made by a combination of tests, because no one procedure is completely accurate. CT scan and MRI can both be used to produce images of brain tissue, in which abnormalities such as encephalitis can be identified. MRI is a newer and more sensitive technique that is a better tool in diagnosis of toxoplasmosis. Encephalitis resulting from toxoplasmosis generally presents as one larger lesion surrounded by several of smaller size. CT scan and MRI can also be used to trace the progression of encephalitis in treatment.
Serologic testing is the most common, and least expensive, method to test for toxoplasmosis.
The Sabin-Feldman dye test, which relies on the complement system, is the original serological test developed for T. gondii . In this test, serum is incubated with dye causing parasitic cells to stain blue. In addition, another serum sample is exposed to IgG antibodies for T. gondii , and after incubation, parasitic organisms become disfigured, do not stain, and die because of antibody complementation lysis. With PCR, amplification of the B1 gene sequence of T. gondii has been 100 percent effective. Finally, biopsy of brain lesions discovered after CT scan or MRI can be performed.