Efforts to improve diagnostic performance of PCR assays cannot rely on improved PCR
detection limits alone because we have already achieved what approaches the maximum
analytical sensitivity with PCR technology, namely single molecule detection. Rather,
improvements in PCR assay performance will rest on a foundation of improvements in sample
manipulation, DNA extraction and target concentration methods prior to the PCR step. This
area of investigation has received little attention despite the great potential to impact diagnostic
performance. For example, a major limitation of fungal PCR is the small volume of DNA added
to each reaction, usually 1–20 μl, and the high proportion of human DNA, leading to limited
sampling of tissues for fungal nucleic acids. If a fungal organism is present at one to two
genomes per ml of blood then it is unlikely to be detected in most PCR assays, but this low
level of fungal DNA may be most informative diagnostically.
Most commercial extraction kits used for fungal PCR diagnostics are designed either for
mammalian cell lysis or for fungal cell lysis from pure cultures. The clinical specimen is
generally a combination of a few fungal cells or their nucleic acids entrapped in relatively large
volumes of tissue or body fluids. Therefore, an ideal extraction method would have high yields
to ensure minimal losses of DNA, the ability to concentrate fungal DNA from the large
background of human DNA, minimal copurification of PCR inhibitors, and reagents that are
compatible with some form of contamination control technology such as UV irradiation or
filtration.