Diagnostic accuracy of fluorescence microscopy of hematoxylin-eosin-stained nail sections for onychomycosis.

BACKGROUND: Onychomycosis requires laboratory confirmation before prolonged oral antifungal therapy is initiated. Periodic acid-Schiff (PAS) staining is highly sensitive for the histopathologic diagnosis of onychomycosis but increases cost and turnaround time. This study evaluated the diagnostic accuracy of fluorescence microscopy of routine hematoxylin-eosin (H&E)-stained nail sections compared with PAS-stained nail sections. METHODS: A diagnostic accuracy study was performed using archived nail clipping slides from patients evaluated between 2019 and 2023. Fluorescence microscopy of H&E-stained nail clippings served as the index test, while PAS-stained nail clippings served as the reference standard. Slides were randomly selected from de-identified records and independently assessed. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated. RESULTS: A total of 84 slides were included, comprising 42 PAS-positive onychomycosis slides and 42 PAS-negative non-onychomycosis slides. Fluorescence microscopy showed a sensitivity of 80.95% (95% confidence interval [CI], 65.88-91.40%) and a specificity of 97.62% (95% CI, 87.43-99.94%). The PPV and NPV were 97.14% (95% CI, 85.08-99.93%) and 83.67% (95% CI, 70.34-92.68%), respectively. False-negative results were primarily associated with sparse fungal elements and yeast forms lacking pseudohyphae, while the single false-positive result was associated with fluorescent plasma globules and neutrophils. CONCLUSION: Fluorescence microscopy of H&E-stained nail sections exhibits high specificity and may serve as a rapid, lower-cost adjunctive diagnostic method for confirming onychomycosis, particularly in resource-limited settings where fluorescence microscopy is available.