KOH (Potassium Hydroxide) Preparation

Exam Overview

Doctors use the KOH preparation to find out whether a fungal infection is present on the nails, skin, scalp, or beard.

A doctor or nurse takes a sample of skin by lightly scraping the infected area that is scaling or flaking.

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.
Athlete's foot . With this infection, a person has peeling, cracking, and scaling on the bottoms of the feet or between the toes , or blistered skin on the feet .