Harvey's Garden

Periodic acid–Schiff reaction

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  • Mechanism
    • Stains carbohydrates and carbohydrate-rich macromolecules (e.g., glycogen, mucopolysaccharides, glycoproteins).
    • Periodic acid oxidizes carbon-carbon bonds in carbohydrates, forming aldehydes.
    • The Schiff reagent then binds to these aldehydes, creating a distinct magenta or purple-red color.
  • High-Yield PAS-Positive Structures/Diseases
    • Glycogen: Found in liver (hepatocytes), skeletal muscle, and certain tumors.
      • Clinical Correlation: Glycogen storage diseases (e.g., McArdle, Pompe).
      • Clinical Correlation: Ewing sarcoma (tumor cells are rich in glycogen).
    • Mucin: Secreted by goblet cells and glandular epithelium.
      • Clinical Correlation: Used to identify mucus-producing adenocarcinomas.
      • Clinical Correlation: Stains goblet cells in the GI tract (e.g., in Barrett esophagus).
    • Fungal Cell Walls: The polysaccharides in fungal walls stain bright magenta.
      • Clinical Correlation: Helps identify fungi like Candida, Aspergillus, Cryptococcus.
    • Basement Membranes: Stains the GBM in the kidney and other epithelial BMs.
    • Whipple Disease: The hallmark finding is PAS-positive macrophages containing Tropheryma whipplei within the lamina propria of the small intestine.
    • Alpha-1 Antitrypsin (A1AT) Deficiency: Characterized by PAS-positive, eosinophilic globules in hepatocytes. These represent the misfolded A1AT protein.
    • Paget Disease of the Breast/Vulva: The malignant intraepithelial Paget cells are PAS-positive.
  • The Crucial Role of Diastase (PAS-D Stain)
    • Diastase is an enzyme (amylase) that digests glycogen.
    • Performing the PAS stain both with and without prior diastase digestion is a key diagnostic step.
      • PAS-positive, Diastase-sensitive (-): The magenta color disappears after diastase treatment. This confirms the stained material is glycogen.
        • Example: Glycogen in a liver biopsy for a glycogen storage disease.
      • PAS-positive, Diastase-resistant (+): The magenta color persists after diastase treatment. This indicates the material is not glycogen.
        • Classic Example: The protein globules in A1AT deficiency. They are PAS-positive but are not digested by diastase. This is pathognomonic.

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