Fingernail Onychomycosis: A Laboratory-Based Retrospective Study with Species Profiling and Antifungal Susceptibility...

Background/Objectives: Fingernail onychomycosis differs etiologically and epidemiologically from toenail infections and is frequently complicated by colonization and mixed growth. Reliable interpretation of microscopy-culture correlations is essential for avoiding overdiagnosis and guiding therapy. This study aimed to characterize the diagnostic structure, species distribution, and antifungal susceptibility patterns of fingernail onychomycosis in a large routine-laboratory cohort, and to evaluate the performance of a five-tier operational classification integrating microscopy and semi-quantitative culture. Methods: Laboratory records from 1075 patients with clinically suspected fingernail onychomycosis (including nail and periungual samples) were analyzed retrospectively (2017-2024). Direct microscopy with calcofluor white, semi-quantitative culture, and MALDI-TOF MS identification were performed. Cases were categorized based on predefined criteria combining microscopic elements with colony quantity and purity. Species distribution, age-sex patterns, diagnostic concordance between microscopy and culture, and results of EUCAST broth microdilution testing for selected yeasts were assessed. Results: The overall proportion of mycologically positive cases was similar in women and men, although age-dependent patterns differed. Microscopic findings correlated with culture outcomes, with hyphae predicting dermatophytes, yeast cells predicting ascomycetous yeasts, and negative slides aligning with the absence of growth. Yeasts predominated (Candida parapsilosis 30.9%, C. albicans 18.5%), dermatophytes were mainly Trichophyton rubrum, and molds were uncommon. Periungual swabs showed species distributions closely matching those from nail samples and demonstrated high analytical concordance. EUCAST MICs revealed species-dependent variation, including elevated amorolfine MICs in C. parapsilosis and reduced fluconazole activity in Wickerhamomyces pararugosa. Conclusions: Fingernail onychomycosis in this cohort was predominantly yeast-associated, with predictable microscopy-culture relationships and distinct age-sex patterns. The five-tier operational framework improved classification of infection versus colonization, and is proposed as a preliminary tool requiring clinical validation, while contemporary MIC data highlighted clinically relevant interspecies differences. The absence of clinical correlation data (symptoms, severity, treatment history) remains the primary limitation, preventing definitive distinction between infection and colonization in all cases.