| Nail bed | Underside of nail plate | P value | Combined results | P value* | |
|---|---|---|---|---|---|
| Positive KOH | 84 (79.2%) | 60 (56.65%) | .0007 | 92 (86.8%) | .143 |
| Positive culture | 93 (87.7%) | 76 (71.7%) | .0063 | 100 (94.3%) | .149 |
| *Differences between results from sampling the nail bed alone and results from combined nail bed and nail plate sampling were not statistically significant. DLSO, distal and lateral subungual onychomycosis; KOH, potassium hydroxide. | |||||
Discussion
In DLSO, most dermatophyte species invade the middle and ventral layers of the nail plate adjacent to the nail bed, where the keratin is soft and close to the living cells below. In fact, the nail bed is probably the primary invasion site of dermatophytes, and it acts as a reservoir for continual reinfection of the growing nail.7 Obtaining confirmation of fungal infection before initiating antifungal treatment is the gold standard in clinical practice, given that antifungal agents have potentially serious adverse effects, that treatment is expensive, and that medicolegal issues exist.8
The standard methods used to detect a fungal nail infection are direct microscopy with KOH preparation and fungal culture. The KOH test is the simplest, least expensive method used in the detection of fungi, but it cannot identify the specific pathogen. Fungal speciation is done by culture. More accurate histopathologic evaluation is possible with periodic acid-Schiff stain, immunofluorescent microscopy with calcofluor stain, or polymerase chain reaction, but these techniques are more expensive and less feasible in outpatient clinics.9
The diagnostic accuracy of the KOH test and fungal culture ranges from 50% to 70%, depending on the experience of the laboratory technician and the methods used to collect and prepare the sample.8-10 It is better to take samples from the most proximal infected area by curettage or drilling, but this technique is usually more difficult than a distal approach, should be performed by skilled personnel, and may cause discomfort to patients.3,5,6
Our recommendation for practice. Earlier studies suggested that nail specimens should be taken from the nail bed.11-13 We sampled the nail bed first in our study because, in trying to determine an optimal location for sampling, we wanted to avoid contaminating nail-bed specimens with debris from the underside of the nail. In practice, however, we suggest that, in cases of suspected DLSO, clinicians first obtain specimens from the distal underside of the nail, and then collect all remaining material from the distal part of the nail bed. This technique is simple and can easily be performed in an office setting. If test results are negative but DLSO remains clinically likely, test a second sample after a week or 2.
CORRESPONDENCE Boaz Amichai, MD, Department of Dermatology, Sheba Medical Center, Tel-Hashomer, Israel; boazam@clalit.org.il