Diagnostic Approach to Longitudinal Melanonychia

Clinical Assessment and Diagnostic Tests

• Fungal infection can present with dark discoloration, typically with additional features such as nail thickening, friability, and onycholysis, and requires KOH preparation and fungal culture for diagnosis, as 50% of dystrophic nails are non-fungal despite similar appearance 1, 2, 3, 4
• Bacterial infection, such as Green Nail Syndrome from Pseudomonas, produces green-black discoloration, not true longitudinal bands, and is associated with moisture exposure and paronychia 1, 3, 4

Diagnostic Criteria and Biopsy

• The British Journal of Dermatology recommends dermoscopic examination to reveal patterns suggestive of malignancy versus benign causes, and biopsy if high-risk features are present, such as abrupt onset after age 50, bandwidth >3 mm, Hutchinson sign, irregular borders, variegated pigmentation, rapid growth or darkening of pre-existing bands, associated nail plate dystrophy, or personal or family history of melanoma 1, 2