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Do not take a pass on the PAS stain

DII small banner By Warren R. Heymann, MD
Nov. 7, 2016

My threshold for obtaining a Periodic acid-Schiff (PAS) on inflammatory lesions rivals the opinion of most Americans’ opinion of tomorrow’s election as reported in the New York Times — very low.

When I read the article by Elbendary et al I was hoping to find a clue when I could skip performing a PAS stain. The authors retrospectively reviewed 103 cases of tinea, confirmed by PAS staining. Concordance between pre-biopsy and histopathologic diagnosis was noted in 57.28% of cases, suggesting that the diagnosis is often not suspected clinically. Among the histopathologic features studied, a compact stratum corneum (either uniform or forming a layer beneath a basket weave stratum corneum), parakeratosis, mild spongiosis and neutrophils in the stratum corneum and within the blood vessels were the most frequent features noted. Interestingly, no cases showed a purely basket weave stratum corneum (1). In a study by Al-Amiri et al, however, 22% of tinea cases demonstrated basket weave keratin only.
Al-Amiri et al reviewed hematoxylin and eosin (H&E)-stained slides from 60 PAS-positive tinea cases. Two observers examined these slides histologically. Of the 60 cases of tinea, only 45% were diagnosed clinically. Histologic examination of H&E sections by the two observers disclosed the presence of hyphal elements in 68 and 45%, respectively. No significant histologic differences, except for the presence of hyphae, were observed between tinea and non-tinea cases. The authors concluded that the finding that only 57% of PAS-positive cases of tinea showed hyphal elements on H&E examination alone, together with no other differentiable histologic characteristics, lends strong support for the routine use of PAS-staining for inflammatory skin disorders (2).
Last week we had our CLIA inspection for our dermatopathology services. For the first time, the issue of how we handle KOH examinations was the focus of attention. I showed the inspector the entire process from start to finish, from scraping the scale, to placing the report in our electronic medical record.* Performing a KOH exam is such a basic part of dermatology, yet, I imagine that unfortunately, many have forgone providing this service rather than comply with CLIA. If that is the case, please make sure that you request a PAS stain on virtually every inflammatory lesion you biopsy, whether or not you are suspecting a tinea infection. While certain histologic features such as a compact stratum corneum or intracorneal neutrophils suggest the diagnosis of dermatophytosis, tinea may be present even with a dearth of histologic findings.

*Even though administrative burdens often bring me to tears, once in a while they induce a belly laugh. For years I would receive a specimen from a compliance-testing lab and actually perform a KOH. Recently I started receiving the test by Federal Express, with a notation on the envelope to refrigerate the envelope until opened. Of course I followed the directions. When I got to the test a couple of days later, I could not find the slide — it was a photograph of a positive KOH preparation with an answer sheet to mark my findings. Who knows what the result would have been had I not adequately preserved the image?

1. Elbendary A, et al. When to suspect tinea; a histopathologic study of 103 cases of PAS-positive tinea. J Cutan Pathol 2016; 43: 852-7.
2. Al-Amiri A, et al. The periodic acid-Schiff stain in diagnosing tinea: Should it be used routinely in inflammatory skin diseases? J Cutan Pathol 2003; 30: 611-5.

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