Background@#Onychomycosis, a common fungal nail infection, particularly affects the elderly. The diagnostic accuracy of a potassium hydroxide (KOH) smear, a primary diagnostic method, can vary based on the examiner’s proficiency. @*Objective@#To evaluate the diagnostic utility of KOH smears in a real-world setting and the influence of clinical and practical factors on onychomycosis. @*Methods@#This study included 381 patients who underwent KOH smear and histopathological staining for nail diseases at Seoul National University Hospital between March 2020 and February 2023. The study evaluated the diagnostic utility of the KOH smear, including its sensitivity, specificity, positive predictive value, negative predictive value, and diagnostic odds ratio. We investigated the influence of factors such as training duration of examiners, prior history of antifungal treatment, and lesion location on diagnostic utility. @*Results@#Of the 381 patients, 139 were male (36.5%) and 242 were female (63.5%). Onychomycosis was the most common diagnosis (59.3%), with 34.1% of patients reporting prior antifungal therapy within the past 6 months. The sensitivity of the monthly KOH smear varied from 16.7% to 70.0% with an annual sensitivity of 48.1%. The specificity ranged from 83.3% to 100.0%, with an annual specificity of 90.3%. While prior antifungal therapy decreased the sensitivity and specificity, no statistical significance was observed. Examiner's experience, measured by training duration, was not associated with sensitivity or specificity. Among patients with a history of antifungal treatment, an increasing number of KOH smear tests showed reduced sensitivity. @*Conclusion@#Variations in the diagnostic utility of the KOH smear test, influenced by various factors, highlight the necessity for cautious interpretation in clinical settings.