Periodic acid-Schiff (PAS) is a staining method used to detect glycogen and other polysaccharides in tissues. The reaction of periodic acid oxidizes the diol functional groups in glucose and other sugars, creating aldehydes that react with the Schiff reagent to give a purple-magenta color. A suitable basic stain is often used as a counterstain.
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PAS staining is mainly used for staining structures containing a high proportion of carbohydrate macromolecules (glycogen, glycoprotein, proteoglycans), typically found in e.g. connective tissues, mucus, the glycocalyx, and basal laminae.
PAS staining can be used to assist in the diagnosis of several medical conditions:
Presence of glycogen can be confirmed on a section of tissue by using diastase 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 staining cellulose. One example would be looking for implanted medical devices composed of nonoxidized cellulose.
If the PAS stain will be performed on tissue, the recommended fixative is 10% neutral-buffered formalin or Bouin solution. For blood smears, the recommended fixative is methanol. Glutaraldehyde is not recommended because free aldehyde groups may be available to react with the Schiff reagent, which may result in false positive staining.[5]
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