Dermoscopy-guided sampling improves the diagnostic yield of fungal culture for onychomycosis: a comparative study

The diagnosis of onychomycosis relies on mycological examination, but the high false-negative rate of fungal culture, often due to improper sampling, remains a significant clinical challenge. Dermoscopy has emerged as a valuable tool for visualizing specific nail structures associated with fungal invasion. This study aimed to evaluate whether using dermoscopy to guide the site of specimen collection can improve the positive rate of fungal culture for onychomycosis compared to conventional sampling.

We conducted a comparative study involving 1,843 nail units from patients with clinically suspected onychomycosis. Samples were allocated to either a conventional sampling group (n = 791) or a dermoscopy-guided sampling group (n = 1,052). In the guided group, specimens were collected from sites exhibiting specific dermoscopic features indicative of fungal presence, such as spikes, ruin-like patterns, and longitudinal striations. The primary outcome was the fungal culture positive rate. Secondary outcomes included diagnostic performance metrics, the association between dermoscopic features and culture positivity, and subgroup analyses.

The dermoscopy-guided group demonstrated a significantly higher fungal culture positive rate than the conventional group (72.5% vs. 66.0%, P < 0.0001). Within the guided group, dermoscopic features like "spikes" (OR 5.31, 95% CI 4.23-6.66), "ruin-like" appearance (OR 4.31, 95% CI 2.59-7.18), and "longitudinal striations" (OR 2.21, 95% CI 1.76-2.78) were strong predictors of a positive culture (all P < 0.001). Multivariate logistic regression confirmed that dermoscopy-guided sampling was an independent predictor of a positive culture (Adjusted OR 1.63, 95% CI 1.30-2.04) after adjusting for confounders. The contamination rate in the guided group was low at 1.4%, and the predominant pathogen identified was Trichophyton rubrum (75.2% of positive cultures).

Dermoscopy-guided sampling significantly improves the diagnostic yield of fungal culture for onychomycosis by enabling precise targeting of viable fungal reservoirs. Integrating dermoscopy into the routine sampling procedure is a simple, non-invasive, and effective strategy to reduce false-negative results and enhance diagnostic accuracy.