– The work up of a case of onychomycosis doesn’t end with the detection of fungal hyphae.

Trichophyton rubrum remains the most common cause of toenail fungus in the United States, but nondermatophyte molds – Scopulariopsis, Fusarium, and others – are on the rise, so it’s important to speciate, especially when patients have atypical presentations or fail to respond to the T. rubrum go-to treatment, terbinafine, according to Nathaniel Jellinek, MD, of the department of dermatology, Brown University, Providence, R.I.

Standard in-office potassium hydroxide (KOH) testing can’t distinguish one species of fungus from another, nor can pathology with Gomori methenamine silver (GMS) or Periodic acid-Schiff (PAS) staining. Both culture and polymerase chain reaction (PCR), however, do.

Since few hospitals are equipped to run those tests, Dr. Jellinek uses the Case Western Center for Medical Mycology, in Cleveland, for testing.

In an interview at the Hawaii Dermatology Seminar provided by the Global Academy for Medical Education/Skin Disease Education Foundation, Dr. Jellinek explained how to speciate, and the importance of doing so.

He also shared his tips on getting good nail clippings and good scrapings for debris for testing, and explained when KOH testing is enough – and when to opt for more advanced diagnostic methods, including PCR, which he said trumps all previous methods.

Terbinafine is still the best option for T. rubrum, but new topicals are better for nondermatophyte molds. There’s also a clever new dosing regimen for terbinafine, one that should put patients at ease about liver toxicity and other concerns. “If you tell them they’re getting 1 month off in the middle, it seems to go over a little easier,” Dr. Jellinek said.

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