The polymerase chain reaction (.PCR) for detection of toxoplasma DNA in clinical samples is well established and successful. Two genes have been used for
amplification:
1. The B 1 gene for which no product is known and
2. The P30 surface protein gene.
Multiple copies of the BI gene are present in the genome, enhancing the sensitivity of the PCR. Studies have shown the B 1 - PCR is capable of detecting 1-10 organisms in samples
and is well conserved in many strains making it useful for testing clinical samples The majority of laboratories employ PCR methods targeting the B 1 gene.
The PCR is a powerful
diagnostic tool but it does have limitations. The very high sensitivity can give false positive results due to contamination from other samples or amplified products. It cannot
distinguish between tachyzoites and bradyzoites and therefore current or latent infection. It cannot distinguish between live or dead organisms. Failure to detect parasite DNA can be due to sampling
and therefore negative results do not always exclude toxoplasmosis. PCR results should not be used on their own but should be interpreted with specific toxoplasma serological results. In order
to make the best use of the PCR it is important that appropriate samples are referred for testing:
1. A wide range of clinical samples can be tested (buffy coat, biopsy, CSF, vitreous fluid. amniotic fluid).
2. Samples should be sent as quickly as possible to the laboratory to prevent degradation of the DNA.
3. The PCR is NOT a substitute for serological testing. All samples referred for PCR must
be accompanied by a serum sample for serology,
