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.