PAS Staining Variability in Fungal Infections of Skin

Abstract Introduction/Objective Developed in 1946 by J. F. A. McManus, the Periodic Acid Schiff (PAS) stain is among the most frequently used stains in histopathology as a cost-effective ancillary study for the detection of fungal organisms. Despite its wide use in dermatopathology, little research exists comparing PAS to its commonly used counterpart, Gomori methenamine silver (GMS), or to hematoxylin and eosin (H&E) alone. The variation in PAS staining between different fungi causing skin infections has been poorly studied. Methods/Case Report Eighteen cases of fungal skin infections from 1995-2023 were reviewed by a resident pathologist and attending medical mycologist. All eighteen cases possessed H&E, GMS, and PAS stains (staining preservation was confirmed). These cases were selected to represent the following categories of fungal skin infections: 1) superficial or cutaneous; 2) subcutaneous; and 3) systemic (including opportunistic mycoses and endemic dimorphic fungi). The following criteria were used to grade each specimen: ability of PAS to stain the organism’s cell wall; the demonstration of fungal morphology by PAS; and the ease of differentiating PAS-stained organisms from background compared to H&E alone. Results (if a Case Study enter NA) PAS allowed for ready visualization of fungal morphology for all organisms except for Histoplasma spp. and Blastomyces spp. in which organisms tended to blend in with background staining particularly in inflamed tissues. Both fungi were visualized better by GMS. The muriform bodies of chromoblastomycosis, while staining well with PAS, were easy to miss due to their tiny size. Conclusion Compared to H&E alone, PAS allowed for detection of organisms at a lower magnification (40X) for fungal infections except for those in chromoblastomycosis, histoplasmosis, and blastomycosis. PAS is a useful, cost- effective ancillary study for the detection of fungal organisms in dermatopathology; however, care must be taken when there is clinical suspicion for certain fungal organisms that are less well-visualized by PAS.