Non-dermatophytic onychomycosis diagnostic criteria: an unresolved question

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.