EXPERT ANALYSIS FROM SDEF LAS VEGAS DERMATOLOGY SEMINAR
LAS VEGAS (FRONTLINE MEDICAL NEWS) – Until polymerase chain reaction (PCR) testing for diagnosing dermatophyte infections becomes available in the United States, the options remain the KOH test, periodic acid–Schiff (PAS) stain, and culture, Dr. Theodore Rosen said at the Skin Disease Education Foundation’s annual Las Vegas Dermatology Seminar.
PCR testing is both the most sensitive and most specific method to diagnose dermatophyte infections and eventually will become the gold standard for diagnosing such infections, noted Dr. Rosen , professor of dermatology, Baylor College of Medicine, Houston. A commercial PCR kit to diagnose dermatophytes, manufactured by a Danish company, is available outside of the United States but currently is not approved in this country.
A KOH test can take time and be difficult to interpret, he said. Referring to the mixed results with KOH, “sometimes you see fungus, and sometimes you don’t,” while “PAS positivity means there is fungus there that doesn’t belong.”
He listed several studies that compared these methods, including a retrospective trial that analyzed the reliability of different tests in verifying a clinical diagnosis of onychomycosis in 108 patients ( J Am Podiatr Med Assoc. 2015 Nov;105[6]:503-8 ). When toenail clippings from the study participants were tested, PAS produced the most consistent positive results (60%, compared with 43.5% for KOH and 39.8% for culture). Compared with the KOH test, PAS also had a higher sensitivity (0.79 vs. 0.64) for confirming fungal infection.
Dr. Rosen said that PCR testing should become available in the United States in the future. “PCR is not only good because we know that the fungus is there, but it can be very specific,” he said. A small percentage of onychomycosis cases are due to Candida, most often affecting the fingernails of people who have their hands in water frequently, such as bartenders and housekeepers.
“And then there’s another small percentage that are due to nondermatophyte molds, the kind of things that are everywhere in the environment,” said Dr. Rosen. Describing the appearance of these types of cases, he said, “sometimes they look different … they’re darker. The nails are a little more heavily affected.”
Dr. Rosen listed several PCR studies, including one that compared PCR with conventional diagnostic methods in 107 nail specimens of patients with clinically suspected onychomycosis. The study found that PCR use increased the diagnosis of specimens positive for dermatophytes by almost 40%. PCR was positive in 72% of the specimens, compared with 57% of the fungal cultures. In addition, PCR detected dermatophytes in 39 specimens that cultures missed ( Australas J Dermatol. 2013 May;54[2]:105-8 ).
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Dr. Rosen disclosed being a paid participant on the scientific advisory boards for Anacor and Valeant.
