Background In onychomycosis, proper specimen collection is essential for an accurate diagnosis and initiation of appropriate therapy. Several techniques and locations have been suggested for specimen collection.
Objective To investigate the optimal technique of fungal sampling in onychomycosis.
Methods We reexamined 106 patients with distal and lateral subungual onychomycosis (DLSO) of the toenails. (The diagnosis had previously been confirmed by a laboratory mycological examination—both potassium hydroxide [KOH] test and fungal culture—of samples obtained by the proximal sampling approach.) We collected fungal specimens from the distal nail bed first, and later from the distal underside of the nail plate. The collected specimens underwent laboratory mycological examination.
Results KOH testing was positive in 84 (79.2%) specimens from the distal nail bed and only in 60 (56.6%) from the distal underside of the nail plate (P=.0007); cultures were positive in 93 (87.7%) and 76 (71.7%) specimens, respectively (P=.0063). Combining results from both locations showed positive KOH test results in 92 (86.8%) of the 106 patients and positive cultures in 100 (94.3%) patients.
Conclusions Based on our study, we suggest that in cases of suspected DLSO, material should be obtained by scraping nail material from the distal underside of the nail and then collecting all the material from the distal part of the nail bed.
When assessing possible onychomycosis, conventional practice is to take samples from the most proximal infected area. But this approach is usually technically difficult and may cause discomfort to patients.1-6 We therefore sought to determine the optimal location for fungal sampling from the distal part of the affected nail.
To assess the accuracy of distal sampling in diagnosing distal and lateral subungual onychomycosis (DLSO) of the toenails, we reevaluated 106 patients with DLSO previously confirmed by microscopic visualization of fungi in potassium hydroxide (KOH) solution and by fungal culture of specimens obtained using the proximal sampling approach.
Before we obtained our samples, we cleaned the nails with alcohol and pared the most distal part of the nails in an effort to eliminate contaminant molds and bacteria. Using a 1- or 2-mm curette, we took specimens first from the distal nail bed and, second, from the distal underside of the nail plate ( FIGURE ). We separated specimens for use in either direct KOH visualization or in fungal culture using Sabouraud’s Dextrose agar (Novamed; Jerusalem, Israel), which contains chloramphenicol or streptomycin and penicillin to prevent contamination.
Distal sampling for distal and lateral subungual onychomycosis
Using a 2-mm curette, we collected specimens from the distal nail bed first (A), and then from the distal underside of the nail plate (B). However, our recommendation for clinical practice is to reverse this order of sampling to collect all possible material.
We recorded sociodemographic characteristics and fungal culture results in basic descriptive (prevalence) tables. In univariate analysis, we used t-tests to compare the means of continuous variables (eg, age, duration of fungal infection). To assess the distribution of categorical parameters (eg, sex) and to gauge the efficacy of the different probing techniques, we used chi-square (χ2) tests. We analyzed coded data using SPSS (Chicago, IL) for Windows software, version 12.
We conducted the study according to the rules of the local Helsinki Committee.
We examined 106 patients with DLSO, of which 65 (61.3%) were male and 41 (38.7%) were female, ages 23 to 72 years (mean age, 44.6). The duration of fungal infection ranged from 3 to 30 years, with a mean of 14.9 years. In 70.8% of cases, the infection involved the first toenail. Duration of the fungal disease did not differ significantly between the sexes.
KOH test results were positive for 84 (79.2%) specimens from the distal nail bed, and for only 60 (56.6%) specimens from the distal underside of the nail plate (P=.0007); culture results were positive for 93 (87.7%) and 76 (71.7%) specimens, respectively (P=.0063). Combining results from both locations (all positive samples from the nail bed, plus positive samples from the nail underside when results from the nail bed were negative) yielded confirmation with KOH testing in 92 (86.8%) patients and with culture in 100 (94.3%) patients. There were no statistically significant differences between the combined results of both locations and the results from the distal nail bed alone (KOH, P=.143; culture, P=.149) ( TABLE ).
Accuracy of distal sampling in 106 patients with confirmed DLSO