The PAS (Periodic acid–Schiff) stain is a histochemical technique that highlights carbohydrates and carbohydrate-rich structures in tissues. Periodic acid oxidizes vicinal diols to aldehydes, which react with Schiff reagent to produce a magenta color. It detects glycogen, neutral mucins, basement membranes, fungal cell walls, and certain pathogens, aiding diagnosis of glycogen storage diseases, mucinous tumors, and fungal infections.

A positive PAS (Periodic acid–Schiff) stain indicates abundant carbohydrate-containing substances in tissue — glycogen, neutral mucins, glycoproteins, and basement membranes. It highlights fungal cell walls and some microorganisms, mucin-producing tumors, and glycogen storage. PAS positivity helps diagnose fungal infections, certain adenocarcinomas, storage diseases, and assess basement membrane integrity. Results are interpreted with other stains and clinical findings for accurate diagnosis.

PAS (Periodic Acid–Schiff) is a histochemical stain used on tissue biopsies to detect fungal infection. Periodic acid oxidizes carbohydrate-rich fungal cell walls to aldehydes; Schiff reagent then binds, producing a magenta color that highlights yeasts, hyphae and mucopolysaccharides. It helps diagnose fungal pathogens (eg, Candida, Aspergillus, Pneumocystis) and complements culture and other stains in pathology.

In liver tissue, the Periodic Acid–Schiff (PAS) stain highlights glycogen within hepatocytes and other PAS-positive carbohydrates such as mucopolysaccharides and glycoproteins. It stains basement membranes and intracellular inclusions (e.g., alpha‑1‑antitrypsin globules) magenta and can detect fungal cell walls. PAS with diastase digestion distinguishes glycogen (diastase-sensitive) from other PAS-positive material.

PAS (Periodic acid–Schiff) stains carbohydrates and highlights glycogen, mucins, and fungal organisms. Conditions that are PAS‑positive include Whipple disease (Tropheryma whipplei macrophages), mucin‑producing adenocarcinomas, fungal and some protozoal infections (e.g., Candida, Histoplasma), glycogen storage diseases, alpha‑1 antitrypsin inclusion bodies, mucopolysaccharidoses, and thickened basement membranes (e.g., diabetic nephropathy). Glycogen is diastase‑sensitive while alpha‑1 antitrypsin inclusions and many mucins are diastase‑resistant.

PAS stands for physician-assisted suicide. It describes a physician providing a competent, terminally ill patient with the means or information to self-administer a lethal medication to intentionally end their life. It is distinct from euthanasia, where a clinician directly administers the fatal dose. Legal status, eligibility, safeguards and ethical considerations differ widely across jurisdictions.