Black Line Under Fingernail: Bruise, Pigment, or Subungual Melanoma?

Common harmless causes of dark nail lines:

Subungual hematoma (bruise under nail). Dark red, purple, or black. Develops after trauma — slamming a finger in a door, dropping a heavy object, repeated minor trauma from running shoes. Grows out with the nail over 6-9 months.

Longitudinal melanonychia (LM). A brown or black vertical stripe extending from the cuticle to the tip of the nail. Common in people of African, Asian, and Hispanic ancestry — often multiple nails affected, present from young adulthood, stable. Less common in light-skinned populations and a single-nail LM in this group warrants more attention.

Fungal nail infection (onychomycosis). Yellow, brown, or black discolouration with thickened or crumbly nail. Usually multiple nails affected.

Drug-induced pigmentation. Some chemotherapy drugs (hydroxyurea, doxorubicin) cause longitudinal nail pigmentation. Reversible after stopping medication.

Lichen planus. Inflammatory condition with longitudinal nail ridges and pigmentation.

Most LM in people of pigmented skin types is benign. The clinical question is whether a specific case fits the benign pattern or has features suggesting subungual melanoma.

Subungual melanoma is melanoma originating in the nail matrix (the cells that produce the nail). Features:

Usually presents as a longitudinal pigmented stripe (melanonychia striata).

More common in adults aged 50-80, but occurs in younger patients.

Most frequently affects thumb, index finger, or great toe.

Disproportionately affects people of Asian, African, and Hispanic ancestry — for these populations, subungual melanoma is one of the more common melanoma subtypes.

In light-skinned populations, subungual melanoma is rare overall but a single-nail LM is more likely to be melanoma than a multi-nail finding.

Why late diagnosis is common:

Mistaken for bruise. People remember bumping the finger and assume the stripe is a hematoma. But hematomas grow out; melanonychia stays put.

Mistaken for fungus. Especially toe lesions.

Not seen by most clinicians who don't specifically examine nails.

The most reliable warning sign: a stripe that doesn't grow out with the nail. Nails grow at about 0.1mm per day. A bruise stripe moves toward the tip and is replaced by clear nail at the cuticle over 6-9 months. A melanoma stripe stays in the same anatomical position because the pigment is being produced from the nail matrix, not deposited in the nail plate.

Hutchinson sign is pigmentation extending from under the nail onto the surrounding skin (cuticle, lateral nail folds, or fingertip). It is a strong predictor of subungual melanoma.

When present, the Hutchinson sign carries high specificity for melanoma — most lesions with this finding are biopsied promptly.

Look for:

Dark pigment on the cuticle, especially extending in line with the nail stripe.

Pigmentation on the lateral nail folds.

Pigmentation on the fingertip distal to the nail.

A pseudo-Hutchinson sign exists — pigmentation that's visible through translucent skin (especially in lighter-skinned people) without actual pigment cells in the surrounding tissue. Dermatologist examination distinguishes true from pseudo-Hutchinson reliably.

Any suspected Hutchinson sign warrants urgent (within 1-2 weeks) dermatology evaluation. This is one of the highest-priority skin cancer findings.

A modified ABCDE for subungual lesions, called ABCDEF:

A — Age. New nail pigmentation in adults aged 50-80 has higher prior probability of melanoma. Asian, Black, Hispanic ancestry.

B — Brown to black band, with breadth >3mm and irregular borders.

C — Change in size, colour, or features.

D — Digit involved (most commonly thumb, index finger, great toe).

E — Extension of pigment onto surrounding skin (Hutchinson sign).

F — Family history of melanoma or personal melanoma history.

Within 1-2 weeks if any:

New nail stripe in adult >50 of any ancestry.

Single-nail involvement in light-skinned individual.

Stripe wider than 3mm.

Irregular pigmentation within the stripe.

Hutchinson sign.

Nail dystrophy (nail splitting, breaking, or lifting from the nail bed at the affected stripe).

Within 4 weeks if any:

Multi-nail LM in pigmented skin type with no concerning features (often benign but baseline assessment helpful).

Gradual change in a previously stable LM.

For any nail stripe that prompts the question 'should I get this checked?' the answer is yes. Subungual melanoma is one of the cancers where late diagnosis is the biggest single predictor of bad outcomes; early biopsy is highly preferred.

Visual examination plus dermoscopy. The dermatoscope can examine nail pigmentation under magnification with polarised light, revealing patterns characteristic of benign vs malignant lesions.

If the dermatoscopic features are clearly benign (regular linear pigmentation, no irregular features), monitoring with photographs is reasonable.

If there is any concern, biopsy. The procedure for nail biopsy:

Nail matrix biopsy under local anaesthetic. The dermatologist accesses the nail matrix (root) by lifting the proximal nail fold or, in some cases, removing part of the nail plate.

A small sample is taken from the area producing the pigment.

The nail may have a partial defect during regrowth.

Results in 5-10 days.

This is a more involved procedure than skin biopsy elsewhere because of the nail anatomy. It's best done by dermatologists or hand surgeons experienced with nail procedures.

If melanoma is confirmed, treatment depends on stage. Early-stage subungual melanoma may be treated with wide local excision and possible nail unit reconstruction. More advanced cases may require digit amputation. Both have significantly better outcomes than late-diagnosed cases that would require more extensive surgery.

Monthly self-exam should include nails:

Look at all 20 nails (10 fingers, 10 toes) under bright light.

Note any pigmented stripes — colour, location, width, length.

Photograph anything notable with a coin for scale.

Reassess at the next monthly check.

For nail stripes specifically, the diagnostic test is whether the stripe grows out. Mark the stripe's position relative to the cuticle today, then check in 3 months. Bruise stripes will have moved toward the nail tip. Melanonychia and melanoma stripes stay in place because they're continuously produced from the nail matrix.

For people in higher-risk groups (Asian, Black, Hispanic; prior melanoma; family history), 6-monthly dermatology checks of nails as part of full-body exam is reasonable. Be explicit about wanting nail examination — many dermatology visits skip nails unless prompted.