Comparative evaluation of potassium hydroxide mount, fungal culture, and histopathology of nail clipping with periodic acid–Schiff stain in the diagnosis of onychomycosis

Context: Onychomycosis (OM) is a commonly encountered problem in clinical practice, which needs to be differentiated from other causes of dystrophic nails as its treatment is long-term and may have potential side effects. Routinely used laboratory tests like direct microscopy with potassium hydroxide (KOH) and fungal culture (FC) show inconsistent sensitivity. Hence, newer methods of diagnosis are required. Aim: This study aims to compare the efficacy of KOH mount, fungal culture, and histopathology of nail clippings with periodic acid–Schiff stain for the diagnosis of onychomycosis. Settings and Design: This was a cross-sectional study conducted in the Dermatology Outpatient Department of a tertiary care center in Navi Mumbai from September 2013 to September 2014. Subjects and Methods: A total of 102 patients with clinically suspected OM were included in the study and evaluated for the following three methods: KOH mount, FC, and HP/PAS stain. To determine the efficacy and performance characteristics of each test, FC was chosen as the gold standard for statistical analysis. Statistical Analysis Used: Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated for each test. Association among the study group was assessed with the help of Chi-square test. Results: Of the 102 patients, direct microscopy with KOH mount was positive in 47, FC in 30, and HP/PAS in 71 patients. Ninety-one out of the 102 patients had at least one of the three diagnostic tests positive. Using this as the denominator, KOH mount, culture, and HP/PAS had sensitivities of 51.64%, 32.96%, and 78.02%, respectively. Diagnostic sensitivity increased to 89.38% when both KOH and HP/PAS were combined.For calculation of specificity of KOH and HP/PAS, we used FC as the gold standard because it gives precise identification of the pathogen. Specificity for KOH and HP/PAS was 59.7% and 34.7%, respectively. It increased to 73.7% if both KOH and HP/PAS were positive. Conclusion: Using PAS on nail scraping can yield higher diagnostic sensitivity with no loss of specificity and hence can be considered as the method of choice for the diagnosis of OM. Accuracy of diagnosis can be increased by combining the two tests (KOH and HP/PAS) instead of one test (HP/PAS).