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Diagnosing Toenail Fungus

If your nails show the tell-tale signs of a fungal nail infection, such as discoloration, thickening nails, nail separation from the nail bed or itchy nails, then it is recommended that you see a doctor to determine whether a fungal infection is present and what fungus is the cause. There are several reasons why a formal diagnosis by a doctor is a valuable step in treating suspected toenail fungus.

First, there are several other conditions of the nail that can mimic the symptoms of a toenail fungal infection. Nail trauma, nail psoriasis, lichen planus, contact dermatitis, nail bed tumor and yellow nail syndrome, among others, may exhibit symptoms similar to those that would be found with a typical toenail fungus. Treating the wrong conditions can not only be a waste of the patient's resources but also delays treatment of the true underlying disease or condition.

Second, even if the condition is a fungal infection, it could be caused by dermatophytes, Candida or other nondermatophyte molds (NDM). To determine the best course of treatment, the physician must know which fungus is the cause of the nail infection. For example, itraconazole is effective against Candida, but terbinafine is not, or at least much less so. If the prescribed treatment is not geared to the correct fungus, once again resources are wasted and the time delay may allow the fungus to take a stronger hold in the nails.


A doctor will first visually examine the infected nail. A physical examination of the nail itself may provide clues to help narrow the range of potential conditions.

Next, to confirm the suspected diagnosis, your doctor may scrape some debris from under your toenail for further analysis. Doctors commonly employ three methods to test scrapings of a nail they believe to be infected with a nail fungus:

  • potassium hydroxide (KOH) smear
  • culture
  • examining the scrapings under a microscope (histological examinations)

Recent results show that the most accurate diagnostic approach is a combination of a direct KOH smear and histological examination.

A biopsy can be useful if a diagnosis of psoriasis or another dermatosis is being considered. In that instance, the toenail and underlying tissue should be biopsied.

It should be noted that the presence of a fungus in a dystrophic nail does not necessarily mean that the fungus was the cause of the problem. An existing problem in the nail may enable the colonization of the fungus. As a result, treating the fungus may not resolve the underlying problem.

Likewise negative test results do not necessarily mean that there is no fungus. Particularly in the case of nondermatophyte molds (NDM), several tests may be needed to identify the fungus.