PASreaktio

PASreaktio, known in English as the periodic acid–Schiff (PAS) reaction, is a histochemical staining method used to detect polysaccharides and related carbohydrates in tissue sections. The method relies on oxidation of vicinal diols by periodic acid to generate reactive aldehyde groups, which then react with Schiff reagent to yield a magenta-colored deposit. The stain highlights glycogen, glycoproteins, mucopolysaccharides, basement membranes, and fungal cell walls, making it a versatile tool in diagnostic pathology.

A common variation, PAS-D, uses diastase digestion before staining to digest glycogen, allowing differentiation of glycogen from other carbohydrate-rich substances.

Applications of PAS include assessment of basement membrane integrity in kidney and other tissues, visualization of mucous substances in glands and tumors, detection of fungal elements in tissue sections, and general evaluation of glycoconjugates in both normal and diseased states. It is frequently employed in surgical pathology, dermatopathology, and gastroenterology to complement other stains.

Procedure and interpretation are typically carried out on formalin-fixed, paraffin-embedded sections. After deparaffinization and hydration, tissues are oxidized with periodic acid, treated with Schiff reagent, and counterstained (often with hematoxylin). Positive structures appear pink to magenta against a counterstained background. Limitations include incomplete specificity, possible background staining, and false positives with certain substances; appropriate controls and technique optimization are essential for reliable interpretation.