Neglect of the histological diagnostics of onychomycosis – the best would be so easy

up to 50 % of all nail diseases are due to mycotic infection [1]. The prevalence of onychomycosis has been increasing since 1950 and also shows an increasing trend with age [2–4]. For these reasons, diagnostics of nail mycosis are of considerable importance. They lay the foundation for a successful therapy and thus also of disease containment. Currently, the combination of microscopic KOH preparations and mycological culture is defined as the diagnostic gold standard [5]. In addition to these two methods, molecular biological techniques (PCR) and direct microscopic evaluation of stained sample preparations are used for the diagnosis of nail mycoses. In this retrospective study, we compared the three commonly used diagnostic procedures (microscopic KOH preparations, mycological culture, histology) with respect to their diagnostic value. A total of 1359 nail samples from fingernails and toenails, with clinical suspicion of onychomycosis and which were examined between 2013 and 2017 by means of at least one of the three diagnostic procedures at the Clinic for Dermatology in Homburg/Saar, were included. Two hundred and five of these nail samples were examined by all three diagnostic procedures and could thus be used to calculate sensitivity and negative predictive value (NPV). The clinically remarkable samples were all collected as nail clippings. In each case, for both native preparation and culture, subungual curettage material was used for analysis. For histological examination, the preparations were fixated in formalin according to standard procedure, stained with hematoxylin-eosin (HE) and periodic acid-Schiff (PAS) reaction and analyzed by microscopy [6]. For culture purposes, a Sabouraud glucose agar containing 4 % glucose was inoculated with the nail material and processed according to a standardized procedure [7]. Examples from all three methods are shown in Figure 3 phase contrast microscopy of a KOH preparation (Figure 3a), microscopy of a nail shard after PAS reaction (Figure 3b), and culture on Sabouraud glucose agar (Figure 3c). All nail specimens that showed positive fungal detection in at least one diagnostic procedure in addition to clinical suspicion were considered positive for fungal infection. This definition was used as the gold standard for calculating the statistical parameters sensitivity and NPV [8–10]. In this study, all nail samples collected during the stated period were included, regardless of any possible previous antifungal therapy. Of 1359 nail specimens, 544 were positive in at least one diagnostic procedure, resulting in 40 % of cases being diagnosed with onychomycosis. Of the 205 nail specimens examined by all three methods, clinical suspicion of onychomycosis was confirmed by at least one diagnostic method in 83 cases (40 %). Of the three diagnostic procedures, histologic examination was performed least frequently. It was used in only 347 of the 1359 (26 %) cases, whereas the procedures of the previous gold standard, microscopic KOH preparation and culture, were performed in 1222 cases (90 %) and 1184 cases (87 %), respectively. The combination of KOH preparations and evaluation of histological specimens by microscopy resulted in the highest sensitivity (94 %). Comparing sensitivities from a combination of two methods, the previous gold standard came in third at 86.7 %. Histological examination, the procedure with the highest sensitivity for individual application, scored only slightly lower at 80.7 %. Individual sensitivity of the KOH preparation followed at 72.3 %, with culture techniques scoring 53 % (Figure 1). The combination of histology and KOH preparation (96.1 %) also ranked first in terms of NPV. The best individually applied procedure was again histology (88.4 %) (Figure 2). It can be seen from the number of times each procedure was performed that histological examination of nail shards was not standard procedure during the observed period. Nevertheless, in our study, histology is the procedure with the highest sensitivity (80.7 %) and NPV (88.4 %). For combinations of two procedures, we found that both combinations involving histology had higher sensitivity DOI: 10.1111/ddg.14382 Neglect of the histological diagnostics of onychomycosis – the best would be so easy Clinical Letter