Identification of Toxoplasma gondii in formalin-fixed, paraffin-embedded tissue by polymerase chain reaction.

We investigated the use of polymerase chain reaction (PCR) for diagnosis of toxoplasmosis in formalin-fixed, paraffin-embedded tissue, using seven cases in which the diagnosis was certain based on histologic examination and eight cases in which histologic findings only suggested Toxoplasma gondii infection. After amplification the PCR products were subjected to agarose gel electrophoresis followed by Southern blotting with a 32P-labeled oligonucleotide probe. In six of seven proven cases of toxoplasmosis, organisms were detected using DNA extracted from 1/50 to 3/50 of a 6-microns tissue section. In addition, organisms were detected in one of eight cases in which infection was suspected, but not proven by identification of organisms. We conclude on the basis of this study that PCR may be useful for detection of T. gondii in a few cases in which routine histologic examination or immunoperoxidase staining is not definitive. Since PCR has 1000-fold higher sensitivity when applied to fresh, rather than paraffin-embedded, tissues, we expect PCR to be more sensitive in fresh tissue.