• Suspect fungal infection whenever the nail looks abnormal — the colour is the most important clue to diagnosis.
• A variety of typical presentations have been described (and recognizing them can help guide treatment):
• White or yellow opaque streaks appear along one side of the nail (lateral onychomycosis).
• Scaling occurs under the distal nail; the nail is discoloured, opaque, and thickened (subungual hyperkeratosis).
• The end of the nail lifts up, and the free edge erodes (distal onycholysis).
• Small flaky white patches and pits appear on the top of the nail plate; the nail becomes roughened and crumbles easily (superficial white onychomycosis).
• White or yellow spots appear in the half-moon (lunula), the proximal growing end of the nail (proximal onychomycosis). This should not be confused with the common white spots caused by injury.
• The nail is completely destroyed (total dystrophic onychomycosis).
• Images of these presentations of fungal nail infection are available at www.dermnetnz.org/fungal/onychomycosis.html.
• Suspect Candida nail infection when an abnormal nail is associated with paronychia. There is redness and swelling next to the nail, and there may be a collection of pus.
• Remember that fungal nail infection commonly coexists with other nail disorders such as psoriasis, lichen planus, and eczema.

Classification of onychomycosis according to presentation

• This classification of fungal nail infection is widely used by dermatologists, and is described in detail in UK guidelines and reviews [Roberts et al, 2003; DTB, 2008; olde Hartman and van Rijswijk, 2008].

Conditions that affect the nails and that can be confused with fungal nail infection include:

• Psoriasis:
• Affected nails are pitted with shallow or deep holes. The nail may be deformed, thickened, discoloured (brownish yellow), and separated from the nail bed.
• The characteristic skin rash is dry, red skin plaques with silver scales; lesions are most often found on the elbow, knee, scalp, or lower back.
• Lichen planus:
• Affected nails are thin (although they may thicken) and become grooved and ridged; they may be discoloured and separate from the nail bed. The cuticle can be destroyed, leaving a scar. The nails may shed, stop growing altogether, or rarely, completely disappear.
• Skin symptoms: lichen planus has many forms, and affects the skin and mucous membranes. The skin rash of the classical form has shiny, flat-topped, firm papules varying from pin point to larger than a centimetre in diameter; lesions are purple and often crossed by fine white lines (Wickham's striae).
• Eczema (atopic or seborrhoeic):
• Affected nails are ridged and thickened.
• The skin is dry and reddened, and itchy or painful.
• Bacterial infection, especially Pseudomonas aeruginosa — the nail is black or green.
• Onychogryphosis — the nail is thickened with scaling under the nail; it is common in the elderly.
• Onycholysis — the nail separates painlessly from the nail bed; it can be spontaneous or due to physical trauma.
• Viral warts — periungual warts grow at the sides or under the nails and can distort nail growth.
• Subungual melanoma — pigmentation extends onto the nail fold, and there may be bleeding from the mass.
• Alopecia areata:
• Affected nails can have pits (most common), longitudinal ridging, a spoon-shape (koilonychia), signs of brittleness (vertical splits, peeling), spotting of the lunula, separation of the nail plate from the underlying nail bed (proximally or distally), and redness of the skin around the nail.
• The nail changes associated with alopecia areata usually accompany the hair loss, but may occasionally precede or follow the onset of alopecia by months or years.
• For more information see the CKS topic on Alopecia areata.

The differential diagnosis of fungal nail infection reflects expert opinion and is discussed in more detail in clinical reviews [Lynde, 2001; olde Hartman and van Rijswijk, 2008].

• To confirm the diagnosis, send specimens of nail clippings or scrapings for fungal microscopy and culture.
• Testing for infection is not needed if treatment would not be given — see Decision to treat.
• Microscopy results should be available within a few days, but culture results may take 4–6 weeks.
• The results are regarded as positive:
• For dermatophytes, if either microscopy or culture is positive.
• For Candida species, if both microscopy and culture are positive.
• For non-dermatophytes if both microscopy and culture are positive on at least two samples taken at different times.
• Non-dermatophyte moulds are rare causes of nail infection. They usually colonize nails as a secondary infection following trauma or an underlying dermatophyte infection.
• False-negative rates are high (about 30%). Therefore:
• A negative test cannot definitively exclude fungal nail infection.
• Repeat the test if the result is negative, and there is high clinical suspicion that the nail is infected.
• Testing for antifungal susceptibilities is not required.

To take samples of the abnormal nail, and any debris between the nail and nail bed:

• Wipe off any treatment creams, lotions, or powders with 70% alcohol before sampling.
• If superficial infection of the nail is suspected:
• Use a scalpel to obtain scrapings of the surface of the nail.
• If deeper infection of the nail is suspected:
• Use chiropody scissors to sample the diseased part of nail.
• When clipping the nail, include the full thickness of the nail and extend as far back from the nail tip as possible; viable fungi are most likely to be found in the most proximal part of diseased nail.
• Include scrapings of debris from the area between the nail and nail bed.
• Collect the samples into folded dark paper squares, secure with a paper clip, and place in a plastic bag. Alternatively, use a commercially available fungal specimen packet. Label the sample clearly.
• Keep the samples at room temperature; do not refrigerate them (dermatophytes die at low temperatures).

These recommendations reflect expert opinion in reviews and the British Association of Dermatology guidelines, which are based on the best available evidence [Roberts et al, 2003; DTB, 2008].

Collection of specimens

• A study found that culture for fungi was more likely to be positive the nearer the sample site was to the cuticle of the nail affected by onychomycosis [Shemer et al, 2008].

Diagnosis by microscopy and culture

• Candida infection:
• Because Candida yeasts are commonly isolated from normal nails, Candida nail infection should be diagnosed only if both microscopy and culture are positive [Johnson, Personal Communication, 2009]. Distal nail infection with Candida yeasts is uncommon and virtually all people who have distal Candida nail infection also have Raynaud's phenomenon or some other form of vascular insufficiency [Roberts et al, 2003].
• Non-dermatophyte infection:
• Non-dermatophyte moulds are of uncertain significance. A confident diagnosis of non-dermatophyte mould infection therefore requires positive direct microscopy and isolation of the organism in pure culture, ideally, on repeated occasions [Roberts et al, 2003].