PAS stain
Summary
Periodic acid-Schiff (PAS) stain highlights carbohydrates such as glycogen, mucopolysaccharides, and glycoproteins, staining them magenta/pink. It is widely used to detect fungal cell walls, basement membrane thickening, and glycogen storage diseases.
Detail
PAS staining works by oxidizing vicinal diols in carbohydrates with periodic acid to form aldehydes, which then react with Schiff reagent to produce a magenta color. Clinically, PAS is used to: (1) Identify fungal organisms (e.g., Candida, Aspergillus, dermatophytes) since fungal cell walls contain polysaccharides—useful in tissue biopsies and skin scrapings. (2) Diagnose glycogen storage diseases (e.g., Pompe disease) by highlighting glycogen accumulation in tissues; diastase pretreatment (PAS-D) degrades glycogen, helping distinguish it from other PAS-positive substances like alpha-1 antitrypsin deficiency globules in hepatocytes (PAS-positive, diastase-resistant). (3) Evaluate renal pathology, particularly basement membrane thickening in diabetic nephropathy (diffuse/nodular glomerulosclerosis, Kimmelstiel-Wilson nodules) and membranous nephropathy (spike and dome pattern). (4) Detect Whipple disease, caused by Tropheryma whipplei, where PAS-positive macrophages are found in the lamina propria of the small intestine. (5) Identify mucin-producing tumors and glycoproteins in various tissues. PAS is often used alongside other special stains (e.g., GMS for fungi, Congo red for amyloid) to build a complete histological picture.
Sources
- Robbins and Cotran Pathologic Basis of Disease
- Rubin's Pathology
- First Aid for the USMLE Step 1
- Goljan Rapid Review Pathology
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