The Journal of Medical Microbiology, Vol 46, Issue 1 92-96, Copyright © 1997 by Society for General Microbiology
The effect of sample storage on polymerase chain reaction-based detection of Toxoplasma gondii in amniotic fluids
A. W. Joss and D. O. Ho-Yen
Scottish Toxoplasma Reference Laboratory, Raigmore Hospital NHS Trust, Inverness.
The effects of sample storage, target preparation and annealing temperature
on a nested polymerase chain reaction (PCR) test for Toxoplasma gondii DNA
were investigated with experimentally seeded amniotic fluids to simulate
congenital infection. Aliquots of 17 amniotic fluid samples remaining after
investigation for conditions other than toxoplasmosis and seeded to contain
small numbers of T. gondii tachyzoites, were tested after storage for up to
2 weeks. There was no deterioration in test sensitivity on samples stored
for 1-2 weeks at room temperature; thus, samples sent by post to reference
laboratories are acceptable for examination. There was no significant
difference in the results from two target preparation methods or two
different annealing temperatures. One-to-ten parasites were detectable in
13 of 17 test samples. The significant feature in the remaining four
samples was the use of washed stored parasites to seed the amniotic fluids
rather than any feature of the amniotic fluids used or the test. False
positive results were found in up to 15% of unseeded amniotic fluid
samples, which contrasts with only 1.9% for the same PCR test in other
routine or negative control samples tested as part of the laboratory's
reference diagnostic work. The difference illustrates the increased risk of
cross-contamination in PCR when the majority of specimens tested are
positive. The results suggest that PCR techniques are likely to be
sensitive and effective tools for the routine diagnosis of congenital
toxoplasma infection.
