Guideline for Evaluation of Lateral Brown Nail Discoloration

Etiology

Distal and lateral subungual onychomycosis (DLSO) is the most common cause of brown discoloration along the lateral nail edges, resulting from dermatophyte invasion of the distal or lateral nail margins. 1
DLSO represents the predominant presentation of dermatophyte nail infection; the fungus penetrates the lateral or distal margins, producing brown discoloration, nail plate thickening, and varying degrees of onycholysis. [2][3]
Toenails are affected more frequently than fingernails, and the infection usually initiates at the lateral edges before spreading. 1
Nondermatophyte mould infections can also cause lateral brown discoloration, accounting for roughly 5 % of cases in the United Kingdom and 20 % in North America, particularly in older individuals with previously traumatized nails. [4][5]
Bacterial pigmentation may coexist with fungal infection and may require separate antimicrobial therapy. 6
Repetitive trauma to the nail can lead to distal onycholysis with secondary colonization and brown discoloration. [7][8]

Clinical examination should look for fungal infection hallmarks: nail plate thickening, onycholysis, subungual hyperkeratosis, and involvement of multiple nails (especially toenails). [2][3]
For laboratory confirmation, obtain nail specimens by cutting the affected nail as far back as possible through the full thickness and including any crumbly material from the discolored area. [9][6]
Laboratory testing should include direct microscopy with calcofluor white (more sensitive than potassium hydroxide) and fungal culture; results are typically available within 2–6 weeks. [7][6]
To differentiate trauma from infection, examine the nail bed after clipping; a normal‑appearing nail bed with intact longitudinal epidermal ridges favors traumatic etiology. [7][8]

Bacterial infection – Pseudomonas aeruginosa produces greenish or black discoloration, not brown, aiding distinction from fungal causes. 6
Candidal nail infection – Typically begins proximally with paronychia rather than laterally, making it an unlikely cause of isolated lateral brown discoloration. [7][6]
Systemic inflammatory conditions – Psoriasis, lichen planus, and similar disorders can cause nail dystrophy with secondary discoloration that may mimic fungal infection. [4][7]

All statements are supported by peer‑reviewed evidence as indicated by the citation identifiers.