Tinea unguium (Onychomycosis) is often confused with non-infected nail dystrophy due to skin disease, particularly psoriasis (also, lichen planus).

Onychomycosis may look similar to nail dystrophy due to psoriasis but those can be differentiated by presence of psoriasis in the surrounding skin with pitting nail or by circular area of discoloration involving the nail bed, known as the oil drop sign or by Onycholysis— typically detachment of nail plate from nail bed with a pink zone proximally or by Onychorrhexis -longitudinal nail ridge, split, or fissure and occasionally by Beau lines a transverse lines and ridges. 

Tinea unguium should be differentiate from lichen planus of nail that may present with clinical features like thins nail plate , nails may shed or stop growing altogether or ridging with roughness and onychorrhexis  - a longitudinal ridging and splitting of the nails or the cuticle is destroyed and may forms a scar on fold of skin connecting to nail called Pterygium. 

Fungal infection is quite common in damaged nails, so antifungal therapy does not always return the nail to normal even when culture is positive.