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Fungal skin and nail infections: diagnosis and laboratory investigation

  • 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
  • Microscopy detects 91% of positives, and provides the most rapid diagnosis
  • Culture distinguishes dermatophyte from non-dermatophyte moulds, which is important as this may alter treatment

When should I take dermatological samples for fungi?

  • Samples are not needed for:
    • uncomplicated Athlete’s foot (tinea pedis)
    • mild infections of the groin; if samples are not taken, treat as suspected Candida or Erythrasma with topical imidazole
    • mild skin ringworm
  • Take samples for fungi:
    • when oral treatment is being considered (scalp ringworm or nail disease)
    • in severe or extensive skin fungal infections, eg moccasin-type Athlete’s foot
    • skin infections refractory to initial treatment, as occasionally gram negative bacterial infections cause interdigital cracking that looks like tinea pedis
    • when the diagnosis is uncertain
  • Ensure clinical details are stated, including treatment, animal contact, and overseas travel

How should I take samples for fungal investigation?

  • Swabs are of little value for dermatophytes, unless there is insufficient material obtained by scraping
  • Wipe off any treatment creams before sampling
  • Keep any samples at room temperature. Do not refrigerate as dermatophytes are inhibited at low temperatures, and humidity facilitates the growth of contaminants
  • Samples should be collected into folded dark paper squares. Secure dark paper squares with a paper clip and place in a plastic bag, or use commercially available fungal packets
  • Skin scrapings:
    • scrape skin from the advancing edge of lesion; use a blunt scalpel blade or similar
    • 5 mm2 of skin flakes are needed for microscopy and culture
  • Nail samples (better taken by clinicians):
    • most viable fungi are usually found in the most proximal part of diseased nail; sample with chiropody scissors
    • include full thickness clippings of the diseased nail
    • sample as far back from nail tip as possible, as this is where fungi are usually found; also sample debris from under the diseased part of the nail
    • in superficial infections, scrape surface of diseased nail plate with scalpel blade
  • Hair samples:
    • take scalp scrapings, as this often pulls out infected hair stumps, which are critical for successful culture and microscopy; hair plucking does not produce the best samples
    • a soft toothbrush can be used if scrapings are not possible
  • When to treat:
    • 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 rule out fungal infection, particularly with kerion and nail infections; if clinical appearance very suggestive of fungal infection, repeat sample and start treatment
  • Significant fungi isolated and reported:
    • the most common dermatophytes from foot or trunk infections are T. rubrum (80%) and T. interdigitale (15%)
    • Epidermophyton floccosum and Microsporum species are also encountered
    • T. tonsurans and T. violaceum cause 80% of scalp infections in the UK
    • Scytalidium spp. are the most common non-dermatophyte moulds that can cause both skin and nail infections
    • 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
  • Fungi of uncertain clinical significance:
    • non-dermatophyte moulds (eg Aspergillus spp., Scopulariopsis spp., Acremonium spp.) are very rare causes of nail infection, usually 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 culture, and ideally, on repeated occasions
    • repeat sample usually requested to confirm significance of non-dermatophyte moulds
  • Antifungal susceptibilities:
    • susceptibility testing of dermatophytes is not required, as antifungal resistance is rare, and there is no known correlation between antifungal susceptibilities and outcome

Treating fungal skin and nail infections

  • For non-dermatophyte moulds other than Candida spp. seek the advice of a microbiologist or dermatologist
  • Dermatophyte and Candida infection of the fingernail or toenail:
    • treat only if infection confirmed by laboratory; only use topical treatment if superficial infection of the top surface of nail plate; 5% amorolfine nail lacquer; 1–2 times weekly; 6 months on fingers; 12 months on toes
    • for infections with dermatophytes use oral terbinafine; 250 mg o.d.; 6–12 weeks on fingers; 3–6 months on toes; or itraconazole; 200 mg b.d.; 2 courses of 7 days a month for fingers; 3 courses of 7 days a month for toes
    • for infections with Candida or non-dermatophyte moulds use oral itraconazole
    • idiosyncratic liver and other severe reactions occur very rarely with terbinafine and itraconazole
    • for children, seek specialist advice
  • Dermatophyte infection of the skin:
    • take skin scrapings for culture
    • as terbinafine is fungicidal, one week is as effective as 4 weeks azole which is fungistatic; topical 1% terbinafine; 1-2 times daily; 1 week
    • if intractable, consider oral terbinafine
    • discuss scalp infections with specialist
    • use a 1% azole cream for groin infections; 1–2 times daily; 4–6 weeks
    • topical undecenoic acid or 1% azole; 1–2 times daily; 4–6 weeks
  • Candida infection of skin:
    • confirm by laboratory
    • treat with 1% azole cream; use lotion if treating paronychia; 1–2 times daily; 1 week, or in case of paronychia, until swelling goes
    • seek advice for nail infection
  • Pityriasis versicolor:
    • scratching the surface of the lesion should demonstrate mild scaling
    • 1% azole cream; 1% terbinafine or shampoo containing ketoconazole; 1–2 times daily; usually 1 week
  • Follow-up: unless there is underlying disease, eg psoriasis, eradication of the fungus generally restores the nail to its pre-infection state. Siblings of children with scalp ringworm should be screened by scalp brushing


full guideline available from…


Public Health England. Fungal skin and nail infections: diagnosis and laboratory investigation. Quick reference guide for primary care: for consultation and local adaptation. London: Public Health England. June 2017.