Potassium Hydroxide (KOH) preparation for nails is a rapid microscopic test designed to detect fungi in nail clippings or infected tissue. Samples from the affected nail (from the thickened, crumbly, or discolored part) are taken, placed on a microscope slide with KOH solution (10-20%). The solution dissolves the hard keratin of the nail, making fungal filaments (hyphae) visible under the microscope if an infection is present.