A dark or black line running along a nail can arise from several different processes. This page outlines common causes, features clinicians look for during assessment, when faster evaluation is advisable, and typical tests and referrals. This information is for general education and not a substitute for a personal medical assessment.
A longitudinal dark stripe may come from subungual bleeding after trauma (subungual hematoma), increased melanin production by melanocytes (melanonychia), a benign nevus or lentigo, fungal infections, systemic pigmentary changes, medication or chemical exposure, or—rarely—subungual melanoma. The underlying mechanism may be blood, pigment deposition, or a growth within the nail unit.
Clinicians note whether the line affects one nail or several, its width, color uniformity and edge regularity, any disruption of the nail plate, visible mass, nail bed bleeding, and extension onto adjacent skin (for example a Hutchinson sign). A detailed history—recent trauma, new medications, other systemic symptoms, or family history of skin cancer—helps interpretation.
Seek prompt medical review if a dark line appears suddenly and is rapidly widening, if the nail plate becomes disrupted or bleeds, or if there is a painful mass or spreading pigment. These features may need rapid dermatologic assessment to exclude malignancy. Severe pain or heavy bleeding warrants emergency care.
Initial evaluation may be by a family physician or a dermatologist. Tools and tests can include dermatoscopic (nailfold) examination, nail clipping or scraping for microscopy and culture, photographic follow‑up, and—when suspicion persists—biopsy of the nail unit for histology. Test selection depends on clinical findings and specialist judgment.
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