To determine the cause of a nail problem and diagnose a fungal nail infection, your doctor:
- Will ask about your medical history, including any previous symptoms of nail damage or fungal nail infections.
- Will look at the skin and nails on your hands and feet.
- May take a sample of the bits of skin and nail fragments (debris) from under the infected nail. If a sample of debris can't be taken easily, a nail sample may be taken by lightly scraping the nail near the infected area or by using a small blade to shave off a piece of nail.
Tests used to examine nail and debris samples include:
- KOH (potassium hydroxide) preparation. This test can find out if the nail or skin condition is caused by fungi. This test can be done in a clinic or doctor's office.
- A fungal culture. This can show which type of fungus is present. Fungi typically grow slowly, so it can take several weeks for a culture to produce test results.
Almost all fungal toenail infections are caused by a type of fungus called a dermatophyte. Because of this, if the KOH test shows that there's a fungus present, your doctor may assume that the infection is caused by a dermatophyte and prescribe treatment. But because one medicine may work better than another medicine against certain types of fungus, your doctor may want to do a fungal culture.
Your doctor may remove a small piece of nail and look at it under a microscope (nail biopsy) if the KOH preparation and fungal culture do not show the presence of fungi but a fungal infection is still suspected.