SFMS Laboratory Practice Test

Sudan III is used to detect fats in urine sediment. It is a lipid-soluble dye that dissolves in neutral fats and lipid droplets, giving them an orange color. This makes lipiduria visible—often seen in conditions like nephrotic syndrome. Other components such as proteins, nucleic acids, or carbohydrates don’t take up Sudan III in the same way, so they won’t appear orange with this stain. So the orange-stained droplets you see indicate fat in the urine.