Non‐dermatophytic onychomycosis diagnostic criteria: an unresolved question

Abstract

Non-dermatophytic moulds (NDMs) have been increasingly recognised as causative agents of onychomycosis. The diagnosis of onychomycosis is most often obtained by microscopic observation of nail specimens where fungal elements can be detected and cultured by standard mycological techniques. Direct microscopic examination does not always result positive in NDM onychomycosis; therefore to perform a correct diagnosis, a proper mycological culture is often required. The purpose of our study was to evaluate the role of direct microscopic examination in the NDM onychomycosis diagnosis. The results show that only 57.2% of the specimens from onychomycosis patients could be properly diagnosed showing positivity to both direct microscopic examination and NDMs culture isolation in two or more subsequent inoculations, while 42.8% of analysed specimens with a negative direct microscopic examination, showed NDMs growth after three or more subsequent inoculations. The large proportion of false negatives (more than 42%) could be related to the duration of the infection and/or to the experience and skills of the personnel dedicated to specimen collection. We point out the need for thoroughly evaluating all specimens showing cultural growth in at least three subsequent medium inoculations, whatever the result of the microscopic examination, in order to reduce false-negative rates. This strategy would allow for more accurate diagnosis of this mycosis.

Onychomycosis: An Updated Review

“…The remaining dermatophyte infections are caused by Epidermophyton floccosum , Microsporum species, Trichophyton verrucosum , Trichophyton tonsurans , Trichophyton violaceum , Trichophyton soundanense , Trichophyton krajdenii , Trichophyton equinum , and Arthroderma species [ 13 - 17 ]. Non-dermatophyte molds that can cause onychomycosis include Aspergillus species, Scopulariopsis species, Fusarium species, Acremonium species, Syncephalastrum species, Scytalidium species, Paecilomyces species, Neoscytalidium species, Chaetomium species, Onychocola species, and Alternaria species [ 11 , 17 , 18 - 31 ]. Non-dermatophyte molds account for approximately 10% of onychomycosis cases globally [ 6 , 32 ].…”

Onychomycosis in the 21st Century: An Update on Diagnosis, Epidemiology, and Treatment

“…17 In an additional study, 42.8% of nondermatophyte moulds (NDM) onychomycosis patients (N = 35) were negative for microscopy but positive in 3 or more subsequent cultures. 18 This large proportion of false negatives could be the result of disease duration differences or the skill level of the technician. 18 The sensitivity of direct microscopy can be enhanced when combined with culture or histology testing, increasing sensitivity rates from 48% to 74% and 89%, respectively.…”

“…18 This large proportion of false negatives could be the result of disease duration differences or the skill level of the technician. 18 The sensitivity of direct microscopy can be enhanced when combined with culture or histology testing, increasing sensitivity rates from 48% to 74% and 89%, respectively. 17 It is therefore recommended that microscopic examination is used with repeated isolations and identification across several subsequent cultures.…”

Cutaneous mycology: a six-year experience in a tertiary hospital in Malaysia

“…Interestingly, the direct microscopic examination of non-dermatophyte mold onychomycosis has been shown to yield a high false negative result of more than 42%. 3 In addition, a recent meta-analysis concluded that the accuracy of KOH and cultures was lower compared to nail biopsy with Periodic Acid-Schiff (PAS)-staining, 4 something worth considering in patients with recalcitrant onychomycosis.…”