Sudan stains and Sudan dyes are synthetic organic compounds that are used as dyes for various plastics (plastic colorants) and are also used to stain sudanophilic biological samples, usually lipids.
Sudan dyes have high affinity to fats, therefore they are used to demonstrate triglycerides, lipids, and lipoproteins.
Sudan stain test is often used to determine the level of fecal fat to diagnose steatorrhea.
A small sample is dissolved in water or saline, glacial acetic acid is added to hydrolyze the insoluble salts of fatty acids, a few drops of alcoholic solution of Sudan III are added, the sample is spread on a microscopic slide, and heated twice to boil.
Normally a stool sample should show only a few drops of red-orange stained fat under the microscope.