Histopathological diagnosis of onychomycosis by periodic acid-Schiff-stained nail clippings

Background The current laboratory methods for diagnosing fungal infections of the nails are the potassium hydroxide (KOH) scraping technique and fungal culture. However, due to the long incubation period required for fungal culture and the reported rate of approximately 30% false negative results observed when using these methods, a quick and highly specific screening test for diagnosing onychomycosis is urgently needed. Objectives In a prospective study, to compare the traditional mycological diagnostics using culture medium and KOH preparation with the histopathological diagnosis of onychomycosis by periodic acid-Schiff (PAS)-stained nail clippings. Methods Material from 387 nails of 350 patients suspected of having onychomycosis was obtained and a KOH stain as well as two fungal cultures (Kimmig agar with and without cycloheximide) were prepared. In addition, the same specimen was histopathologically examined (PAS stain). Results Culture medium and KOH preparation respectively revealed 100 and 156 cases of onychomycosis, as compared with 182 cases by histological examination. Histological examination gave a significantly higher rate of positive results (P < 0.05). Considering the total number of positive results given by at least one of the three methods (total = 438), histological evaluation was found to give the highest rate of successful recognition of mycotic infection (41.6%). Conclusions The histopathological evaluation of PAS-stained nail clippings is very quick and easy to perform, and will increase the frequency of diagnosing onychomycotic disease above that achieved by culture and KOH preparation alone. However, because information concerning the vitality of the fungi and accurate identification of the specific pathogen is not available through this investigation alone, mycological culture continues to remain the indisputable 'gold standard' of mycological diagnostics.