What should you know about a dark straight line on a nail?

A narrow brown or black band running from the base toward the tip is sometimes caused by pigment made in the nail unit. A knock, repeated pressure, or a splinter can also darken part of the nail. Fungal disease more often causes thickening, crumbling, or yellow-white discoloration, while nail psoriasis can create ridges, pits, lifting, or color change. One photo cannot distinguish these causes.

Notice whether the mark is on one nail or several, whether the color is even, and whether it moves outward as the nail grows. Record changes with dated photographs in consistent lighting.

Keep the nail short and avoid picking, tight footwear, aggressive manicures, or covering the area with opaque polish while you are observing it. Do not scrape or file a dark band to remove it. If the nail is thick, brittle, or lifting, a clinician may examine the nail and surrounding skin and may test material before choosing treatment. Related information includes Nail Fungus, Nail Psoriasis, and Discoloured Nails.

Arrange a dermatology assessment if the band is new, widening, uneven in color, spreading onto the nearby skin, or changing while the nail grows. Seek prompt care for bleeding, marked pain, swelling, pus, or a dark area after an injury that is not moving toward the tip. A clinician can decide whether monitoring, testing, or treatment such as Oral Antifungal Treatment is appropriate.

Ask whether the line appears to arise from the nail matrix, whether an injury or infection fits the pattern, and what changes should trigger review. Ask how to photograph and monitor it, and whether nail sampling or follow-up examination is needed.