Correlation between histopathologic features and likelihood of identifying superficial dermatophytosis with periodic acid Schiff-diastase staining: a cohort study

Background: Periodic acid Schiff-diastase (PAS-D) staining is used routinely in dermatopathology. Conventionally, parakeratosis or intracorneal neutrophils justify use of PAS-D stain to identify dermatophytosis. We aimed to investigate the histopathological features of superficial dermatophytosis. Methods: Skin biopsies with PAS-D stain were retrospectively reviewed by a blinded dermatopathologist. Histopathologic findings in cases with dermatophytosis were compared with those without. Results: We studied 110 cases, of which 63 were PAS-D positive for dermatophytes. Dermatophytes were identified on hematoxylin and eosin (H&E) staining in 49 (77.8%) of the PAS-D-positive cases. Diffuse compact orthokeratosis (79.37%, p < 0.001) and focal parakeratosis (65.08%, p = 0.03) were significantly more frequent in dermatophytosis. Intracorneal neutrophils and spongiosis were not significant findings. Fungal elements were frequently observed within compact orthokeratosis (62.0%) and parakeratosis (31.8%) but rarely within intracorneal pustules (12.7%). In some dermatophyte cases, intracorneal neutrophils (47.6%) or parakeratosis (17.5%) were absent. Conclusions: Compact orthokeratosis or focal parakeratosis justifies PAS-D staining, whereas the absence of intracorneal neutrophils or parakeratosis is insufficient to exclude dermatophytosis. Results indicate that having a low threshold to perform PAS-D stain is justified. PAS-D stain may be deferred until H&E slides have been evaluated.