Background. Fungal infection of the nail affects millions of people worldwide, and has an estimated prevalence of about 10% of the general population. Laboratory confirmation of fungal infection is currently accepted as a requirement before initiation of antifungal treatment in clinical practice. Aim. To examine the rationale for systemic treatment in cases of clinical onychomycosis with negative results on fungal examination (potassium hydroxide test and fungal culture). Methods. In total, 147 patients with suspected clinical toenail onychomycosis but with negative results on fungal examination underwent up to three consecutive fungal examinations of the affected nails. Patients who were negative after these examinations underwent a fourth set of investigations, including PCR. Results. Of the 147 cases initially thought to be negative, 138 (94%) were rated as positive after up to four consecutive sets of laboratory mycological investigations including PCR. Trichophyton rubrum was by far the commonest dermatophyte cultured from all samples. Conclusions. In the majority of cases of initially negative examinations, consecutive laboratory fungal tests will eventually produce positive results. These findings suggest that systemic antifungal treatment should be started in patients with suspected fungal infections, even if they have negative laboratory fungal examinations.
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