In some cases of suspected nail fungus there is actually no fungal infection, but only nail deformity. A 2003 source gives a figure of 50% whereas a more recent source claims that fungus is present in 65 to 95 percent of cases. Avoiding use of oral antifungal therapy (e.g. terbinafine) in persons without a confirmed infection is a particular concern because of the possible side effects of that treatment. However, according to a 2015 study, the cost in the United States of testing with the periodic acid–Schiff stain (PAS) was about $148. Even if the cheaper KOH test is used first and the PAS test is used only if the KOH test is negative, there is a good chance that the PAS will be done (because of either a true or a false negative with the KOH test). But the terbinafine treatment only cost $10 (plus an additional $43 for liver function tests). In conclusion the authors say that terbinafine has a relatively benign adverse effect profile, with liver damage very rare, so it makes more sense cost-wise for the dermatologist to prescribe the treatment without doing the PAS test. (Another option would be to prescribe the treatment only if the potassium hydroxide test is positive, but it gives a false negative in about 20% of cases of fungal infection.) On the other hand, as of 2015 the price of topical (non-oral) treatment with efinaconazole was $2307 per nail, so testing is recommended before prescribing it.
Other causative pathogens include Candida and nondermatophytic molds, in particular members of the mold genus Scytalidium (name recently changed to Neoscytalidium), Scopulariopsis, and Aspergillus. Candida species mainly cause fingernail onychomycosis in people whose hands are often submerged in water. Scytalidium mainly affects people in the tropics, though it persists if they later move to areas of temperate climate.
Athlete's foot is a form of dermatophytosis (fungal infection of the skin), caused by dermatophytes, fungi (most of which are mold) which inhabit dead layers of skin and digests keratin. Dermatophytes are anthropophilic, meaning these parasitic fungi prefer human hosts. Athlete's foot is most commonly caused by the molds known as Trichophyton rubrum and T. mentagrophytes, but may also be caused by Epidermophyton floccosum. Most cases of athlete's foot in the general population are caused by T. rubrum; however, the majority of athlete's foot cases in athletes are caused by T. mentagrophytes.
Toenail fungus is an infection that gets in through cracks in your nail or cuts in your skin. It can make your toenail change color or get thicker. It can also hurt. Because toes are often warm and damp, fungus grows well there. Different kinds of fungi and sometimes yeast affect different parts of the nail. Left untreated, an infection could spread to other toenails, skin, or even your fingernails.