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Periodic Acid-Schiff Reaction and its Utility in Podiatric Medicine

By ProPath Staff


What is a Periodic Acid Schiff (PAS) reaction? A PAS reaction is a laboratory test whereby, through a series of oxidation-reduction reactions, polysaccharides, basement membrane material, and mucosubstances are stained bright red. Its utility in Dermatopathology is widespread, as it allows for the visualization of the lamina densa of the basement membrane zone (type IV collagen) in diseases where its alteration is of significance such as lupus erythematosus. In addition, because many normally colorless fungi possess carbohydrates within their cell walls or capsules, PAS reaction is an effective means to allow for their identification in tissue. PAS reaction may be performed on cutaneous biopsies, nail plate biopsies (Fig. 1), or biopsies from the deep tissue.

Why is PAS reaction the single best test for the diagnosis of onychomycosis?

In recent years, with the introduction of new effective medications for the treatment of onychomycosis and the expansion of the concept of evidence-based medicine, there has been increased demand for a sensitive and specific test for the diagnosis of onychomycosis. As an example, to curtail the expense of over-treatment with oral antifungals and topical lacquers, many health plans require a positive test for onychomycosis prior to approving payment for such medications. In addition, many clinicians now prefer to document the presence of onychomycosis prior to using routine debridement codes, thereby eliminating the possibility of future false over-utilization accusations.

With the newly arising need for accurate diagnosis in cases of onychomycosis, investigators have recently compared traditional testing methods such as KOH, dermatophyte testing media (DTM), and culture with newer procedures such as PAS reaction and calcofluor white. In 2001 Borkowski et al. compared KOH, DTM, culture, and PAS reactions performed on tissue taken from 50 persons with clinically suspected onychomycosis. In that series PAS reaction was found to be overwhelmingly superior to all other tests in terms of sensitivity with a p value of 0.001. KOH and culture were comparable in sensitivity. More recently, Weinberg et al. compared KOH, culture, PAS reaction. Again they found PAS to be more sensitive than culture (p=0.00002) and KOH (p=0.03). Of note is that, in contrast to the widely held belief that culture is infinitely more specific than PAS reaction, in this series of 105 patients, the specificities were comparable (PAS 72% versus Culture 82%). It must be stated that such comparisons likely over-estimate the specificity of culture. There is an inherent bias in that these comparisons make the false assumption that it is always the pathogenic organism which is isolated in culture. Obviously, such is not always the case. Many non- pathogenic fungi have the potential to contaminate the nail unit without necessarily causing disease. This false assumption has likely led to an over-estimation of the prevalence of non-dermatophytic onychomycosis and has the potential to result in medical mismanagement.

An unrelated benefit to PAS reaction is that specimens tested in this manner are reviewed histopathologically. This allows for the the use of histopathologic criteria for the diagnosis of alternate processes such as traumatic onycholysis, psoriasis, spongiotic disorders, and neoplastic processes that have been shown to mimic onychomycosis. In contrast, culture will only allow for the identification of fungi, and that in a decreased number of cases.

PAS reaction in the diagnosis and treatment of pedal skin disease 

Possibly the most under-utilized procedure in all of podiatric medicine is the simple cutaneous biopsy. Whether performed as a punch or curetting (11100, 11101), or a small shave (leg:11301/11302; foot: 11305/11306), these quick and easy techniques may be invaluable to clinicians for the definitive characterization of both neoplasms and various inflammatory conditions (dermatitis). Many podiatric clinicians have unknowingly omitted such biopsy-related CPT codes from their procedural superbills, a sine-qua-non of underutilization.

Amongst the most valuable uses for cutaneous biopsies in podiatric medicine is to differentiate between dermatophytosis and non-infectious inflammatory or neoplastic processes such as nummular (eczematous) dermatitis or lentigines (Fig. 2), respectively. Too commonly, patients are empirically treated with topical antifungals for protracted periods of time only to be lost in frustration to follow-up or to the office of an alternate clinician. Such frustrations may or may not ever become evident to the initial physician, rather they manifest only as slow practice growth or involution. In our Dermatopathology practice, we note roughly 2/3s of the cases sent as suspected tinea pedis are in actuality spongiotic processes such as “eczema” or allergic contact dermatitis, conditions that will readily respond to topical antiinflammatory agents. In this context, a punch biopsy with PAS reaction may be invaluable to assess for the presence of fungal elements resulting in better targeted patient care (Figs. 3a and 3b). PAS reactions may also be used on shave biopsies or scrapings. Although dermatophytes can be adequately identified in such superficial biopsies, these techniques may not allow for the specific diagnosis of alternate inflammatory conditions, some of which require the analysis of dermis for accurate diagnosis.


1. Weinberg JM, Koestenblatt EK, Najarian L. Comparison of diagnostic methods in the evaluation of onychomycosis. Am Acad Dermatol 2003; 49:193-197.

2. Borkowski P, Williams M, Holewinski J, Bakotic BW. Onychomycosis: An analysis of 50 cases and a comparison of diagnostic techniques. J Am Podiatr Med Assoc 2001; 91: 351-355.

Special thanks to Joseph Sindone, D.P.M. for the informative clinical image and insight.


Date of last revision: Fall 2003.


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