PAS (Periodic Acid Shiff) stain is the most versatile and widely used technique for carbohydrate visualization.

From a diagnostic point of view, PAS is one of the most useful and used special stains in the pathology laboratory. It can help on the one hand to the differential diagnosis of tumors and on the other hand to detect fungal diseases.

PAS stain showed mucins (brush-edged epithelium), basement membranes, glycogen, and mycelial spores and filaments. The reaction with the periodic acid of Schiff corresponds to the oxidation of certain polysaccharides by the periodic acid, revealed by a red coloration (fixation of the Schiff leucofuchsin). PAS is commonly associated with diastase (PAS-diastase) which digests glycogen but leaves the pink stain of intra- or extracellular mucins persistent. In undifferentiated tumors, this staining is useful for guiding the diagnosis to a glandular origin (adenocarcinoma). It is required in the diagnosis of pathologies of the liver and the kidney.