The Periodic Acid-Schiff (PAS) stain identifies glycogen and other tissue carbohydrates. Diastase is an enzyme that digests the glycogen. A PAS stain performed after diastase digestion will be negative.
Diastase is a mixture of alpha- and beta-amylase extracted from malt. It typically is used at a concentration from 0.1 to 1.0 percent.
PAS Without Diastase
PAS After Diastase
Interpretation
positive
negative
glycogen
positive
positive
non-glycogen tissue carbohydrate
Examination of the control slides is essential to make sure that everything worked like they were supposed to.
Causes for poor glycogen digestion (persistent glycogen after digestion):
(1) inactivation of diastase reagent (fresh solution works)
(2) failure to warm the solutions to 37°C prior to the digestion
(3) concentration of diastase too low
(4) digestion period too short
(5) failure to prepare in a phosphate buffer at pH 6.0
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