Advancing molecular testing
Back to Cover Story
Molecular testing historically has been implemented when traditional
testing hasn’t worked well or in the absence of a traditional assay,
noted David Hillyard, MD, director of molecular pathology and associate
medical director for infectious disease testing, ARUP Laboratories,
Salt Lake City. "There is no better example than HIV," Dr. Hillyard
told CAP TODAY.
RT-PCR and bDNA tests for HIV viral load "compete for the majority
of the U.S. market. Each has undergone fairly rapid generational
improvement," Dr. Hillyard said. RT-PCR has the disadvantage of
limited dynamic range but is highly specific and is the only FDA-approved
assay. New versions of continuously monitored PCR tests with extended
dynamic range eventually will be available.
Dr. Hillyard agreed that bDNA has some false positives, perhaps
three to five percent, he said, but "In our experience, those false
positives have been less than 200 copies." In clinical settings,
this should not be a problem provided physicians understand these
limitations at the very low end. Finally, bDNA has higher throughput.
Dr. Hillyard is using tests for cytomegalovirus largely to assist
bone marrow transplant surgeons. "There is controversy about whether
one should be using antigenemia or quantitative PCR," Dr. Hillyard
said. Almost everyone gets infected with CMV, so a too-sensitive
test would pick up CMV in so many people that it wouldn’t be useful.
He uses antigenemia and a qualitative assay that has been adjusted
so it is not too sensitive. "We will move toward antigenemia pending
the releases of improved quantitative PCR assays," he stated.
"It is very important to do molecular testing on CSF for HSV and
varicella-zoster in the clinical setting of encephalitis," he explained.
"This has been a real success story for molecular testing," which
can detect more than 95 percent of HSV infections, compared to about
five percent for culture. Unfortunately, surveys document tremendous
variability in home-brew assays for HSV.
"This is a general theme for all PCR tests," Dr. Hillyard said.
"One of our greatest challenges is to bring more uniform quality
to PCR tests so they are accurate and resistant to artifacts of
For HCV, qualitative molecular testing helps confirm the diagnosis
and document successful endpoints in therapy. And because data link
HCV viral load to prognosis and response rates, many clinicians
order a quantitative test and follow viral load during therapy.
Others get a pretherapy viral load and order a qualitative test
at the end of therapy to see whether virus is undetectable. "There
is nowhere near the consensus about how HCV tests should be used
as there is for HIV," commented Dr. Hillyard.
William Check, PhD