Periodic acid–Schiff (PAS) is astaining method used to detectpolysaccharides (such asglycogen) and mucosubstances (such asglycoproteins,glycolipids and mucins) in tissues. The reaction ofperiodic acid oxidizesvicinal diols in thesesugars, usually breaking up the bond between two adjacent carbons not involved in the glycosidic linkage or ring closure in the ring of monosaccharide units that are part of the long polysaccharides and creating a pair ofaldehydes at the two free tips of each brokenmonosaccharide ring. The oxidation condition has to be sufficiently regulated so as to not further oxidize the aldehydes. These aldehydes then react with theSchiff reagent to give a purple-magenta color. A suitable basic stain is often used as acounterstain.

• PAS diastase stain (PAS-D) is PAS stain used in combination withdiastase, anenzyme that breaks down glycogen.

• Alcian blue/periodic acid–Schiff (AB/PAS or AB-PAS) usesalcian blue before the PAS step.

PAS staining is mainly used for staining structures containing a high proportion ofcarbohydrate macromolecules (glycogen,glycoprotein,proteoglycans), typically found in e.g.connective tissues,mucus, theglycocalyx, andbasal laminae.

PAS staining can be used to assist in the diagnosis of several medical conditions:

• Glycogen storage disease (versus other storage disorders).
• Adenocarcinomas, which often secrete neutral mucins.
• Paget disease of the breast.^[1]
• Alveolar soft part sarcoma.^[2]
• Stainingmacrophages inWhipple's disease.^[3]
• It can be used to diagnoseα1-antitrypsin deficiency if periportal liver hepatocytes stain positive.
• Aggregates of PAS-positive lymphocytes are present in epidermis inMycosis fungoides andSézary syndrome, called Pautrier microabscesses.
• Ewing sarcoma
• Erythroleukemia, a leukemia of immature red blood cells. These cells stain a bright fuchsia.
• Pulmonary alveolar proteinosis.
• Fungalinfection, the cell walls of fungi stain magenta; this only works on living fungi.^{[citation needed]} In contrast,Grocott's methenamine silver stain (GMS) will stain both living and dead fungal organisms.
• It is used to identify glycogen in lung biopsy specimens of infants with pulmonary interstitial glycogenosis (PIG).
• It can be used to highlight super cross-linked lipids inclusions inceroid lipofuscinosis (NCL).

Presence of glycogen can be confirmed on a section of tissue by usingdiastase to digest the glycogen from a section, then comparing a diastase digested PAS section with a normal PAS section. The diastase negative slide will show a magenta staining where glycogen is present within a section of tissue. The slide that has been treated with diastase will lack any positive PAS staining in those locations on the slide

PAS staining is also used for stainingcellulose. One example would be looking for implanted medical devices composed of nonoxidized cellulose.

If the PAS stain will be performed on tissue, the recommendedfixative is10% neutral-buffered formalin orBouin solution. Forblood smears, the recommended fixative ismethanol.Glutaraldehyde is not recommended because freealdehyde groups may be available to react with theSchiff reagent, which may result infalse positive staining.^[4]

• PAS Reaction

• Biochemistry detection methods
• Carbohydrate methods
• Staining

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