The doctor or nurse places the nail, skin, or hair sample on a slide with
potassium hydroxide (KOH) solution and gently heats it. This solution slowly
dissolves the skin cells but not the fungus cells. The fungus cells are then
visible with a microscope. Color stains may be used so that the fungus is
easier to see.
Why It Is Done
A doctor may do a KOH test to find out whether a person
has a fungal infection. Fungal infections may cause:
Ringworm of the scalp or beard. With this infection, a person has flakes of dead skin (dandruff) on the hair; broken, crusted, or
matted hair; redness or irritation of the scalp or beard; swollen areas and
blisterlike bumps with pus (kerions); and/or hair loss.
Ringworm of the skin. With this infection, a person has patches of skin
that are itchy, red, or scaly, with blisterlike bumps on the edges.