Image: Toxoplasma gondii tachyzoites (Photo courtesy of Ke Hu and John Murray).
A promising diagnostic tool has been developed to identify patients who are both immunocompromised and have the most common focal brain lesions.
The diagnostic accuracy, technical benefit, and clinical application of the duplex reverse transcription-polymerase chain reaction (duplex RT-PCR) assay have been assessed for the diagnosis toxoplasmic encephalitis (TE).
At the Mahidol University (Bangkok, Thailand) individuals with advanced human immunodeficiency virus (HIV) infection and central nervous system opportunistic infections were enrolled in a prospective study, performed from July 2007 to January 2009. Blood and cerebrospinal fluid samples were assayed by duplex RT-PCR to detect the genes of motile tachyzoite or the intracellular bradyzoite of Toxoplasma gondii.
The assay comprised three main steps: nucleic acid extraction, reverse transcription, and duplex polymerase chain reaction. Two sets of primers were used in this study, one specific to the tachyzoite surface antigen (SAG1) gene, and the other specific for the bradyzoite antigen (BAG1) gene. Originally, there were 70 HIV-infected patients eligible for the study, but only 61 could be analyzed. Altogether, there were eight confirmed TE cases, of which two were first episode and six were relapsed. Fifty-three were non-TE cases with other central nervous system opportunistic infections.
The duplex RT-PCR assay was performed on the MasterCycler-Gradient apparatus (Eppendorf; Hamburg, Germany) and showed high diagnostic accuracy, with 100% specificity and positive predictive value, as well as 87.5% sensitivity. Its efficacy reached 98.3%. This diagnostic method was rapid, needed only moderately skilled technicians, and was four times cheaper than previous standard procedures. It worked very well for blood samples, even after drug treatment had been started.
The authors concluded that the RT-PCR assay is also useful for the diagnosis of extracerebral toxoplasmosis in acquired immunodeficiency syndrome (AIDS) patients, in whom the clinical and radiological features are more difficult to interpret. Although serial specimens from AIDS patients are necessary for further evaluation of this technique, particularly from those who have TE with and without specific treatment, it can still be recommended for use as an alternative presumptive diagnostic procedure, especially in resource-poor settings. The study was published in the April 2012 issue of the International Journal of Infectious Diseases.