Mycological diagnosis

Whenever fungal infection is suspected specimens should be taken before any treatment is commenced. Results of microscopy and culture may then be used to confirm the diagnosis, optimise treatment and identify the presence of mixed infection.

• specimens of superficial or proximal onychomycosis may be obtained by scraping the nail surface with a blunt scalpel.
• more distal and lateral areas of disease may also be sampled in a similar way by scraping or by taking chippings from subungal material and proximal part of the diseased nail (1). Subungual chippings should be taken as proximally as possible, though this may involve some slight discomfort.
• samples should be stored in either commercially available mycological packs or transported in a folded square of paper sealed with a paper clip.Samples should be kept at room temperature (1)

When to treat (2):

• a positive microscopy (fungal elements seen) is sufficient to start antifungals
• a positive dermatophyte culture with negative microscopy is still significant
• a negative microscopy or culture does not exclude fungal infection. In case of patients with a high clinical suspicion a repeat sample can be taken and treatment started (1,2)

Notes:

• many nail problems can look like fungal infections, eg psoriasis or injury. Always send samples before starting long-term treatment, as only 45% of dermatology samples received are positive for fungal infections (2)
• fungi of uncertain clinical significance (2)
  • non - dermatophyte moulds (eg Aspergillus spp., Scopulariopsis spp., Acremonium spp.) are very rare causes of nail infection
    • generally following nail trauma, immunosuppression, or underlying dermatophyte infection
    • discuss management with a local microbiologist or dermatologist
    • such a diagnosis requires positive direct microscopy, isolation of the organism in pure cultre, and ideally, on repeated occasions
    • repeat sample usually requested to confirm significance of non - dermatophyte moulds
• true nail infections with the yeasts C. albicans and C. parapsilosis are rare and are more likely to affect the finger nail or finger nail folds; other Candida spp. may very rarely cause paronychia

Reference:

• (1) Health Protection Agency 2009. Fungal nail & skin infections: diagnosis & laboratory investigation - Quick reference guide for primary care
• (2) Public Health England 2017. Fungal skin and nail infections: Diagnosis and laboratory investigation - Quick reference guide for primary care.