For PAS reaction, which fixative is NOT recommended?

• A. Formalin
• B. Bouin's solution
• C. Carnoy's solution
• D. Glutaraldehyde

Answer: (D)

Periodic acid–Schiff staining relies on oxidizing carbohydrate groups to form aldehydes that react with Schiff reagent to give a magenta color. For this to happen, the carbohydrate residues in the tissue must remain accessible after fixation. Fixatives that cross-link proteins too heavily can mask these carbohydrate sites and hinder the oxidation step, reducing or eliminating the staining.

Glutaraldehyde is a strong cross-linking fixative that creates dense, extensive protein networks, which obscures carbohydrate moieties and impedes the periodic acid oxidation and Schiff reaction. That’s why it’s not recommended for PAS. Formalin, Bouin’s solution, and even Carnoy’s solution can preserve tissue while maintaining enough accessibility of carbohydrates for the PAS reaction (though each has its own caveats).