Nail Streaks: What Lines in Your Nails Mean

Types of nail streaks

Brown or black streaks: longitudinal melanonychia

Melanonychia is a brown or black band running from the cuticle to the free edge. It forms when pigment cells (melanocytes) in the nail matrix, where the nail grows, become active.

Common harmless causes:

• Ethnic (racial) melanonychia: very common in people with Black, Asian and Hispanic heritage. Often several nails have bands, which may be present for years.
• Repeated friction or injury: nail biting, tight shoes rubbing the toenails.
• Pregnancy.
• Medicines: some chemotherapy drugs, hydroxyurea, zidovudine, minocycline and antimalarials.
• Medical conditions: Addison's disease, vitamin B12 deficiency, HIV.
• Nevus (mole) in the nail matrix, especially in children.
• Fungal or bacterial infection, which may cause dark or green-black discoloration.

Nail melanoma warning signs (ABCDEF)

Melanoma under the nail (subungual melanoma) is rare but often diagnosed late. Dermatologists use the ABCDEF rule:

• Age 50 to 70 and African American, Asian or Native American heritage (where it makes up a larger share of melanomas)
• Band: brown to black, Breadth 3 mm or more, Blurred borders
• Change in the band, or no improvement with treatment
• Digit: the thumb, big toe and index finger are most often affected; usually one nail only
• Extension of pigment onto the cuticle or surrounding skin (Hutchinson's sign)
• Family or personal history of melanoma

Blood under the nail

After an injury, blood under the nail (subungual hematoma) appears as a purple-black patch. It does not form a neat band from the cuticle, and it grows out with the nail over months. A dark area that does not move forward over time should be checked.

Splinter hemorrhages

These are tiny, thin, dark red or brown lines running lengthwise under the nail, looking like splinters. They come from small bleeds in the nail bed capillaries. Most are from minor trauma, especially in manual workers, and appear in one or two nails. They are also seen in psoriasis and fungal infection. Multiple splinter hemorrhages, especially near the base of the nail, with fever, a heart murmur or feeling unwell, can be a sign of endocarditis, an infection of the heart valves.

Red streaks (longitudinal erythronychia)

A red line in a single nail can come from a benign growth such as a glomus tumor (which is very painful with cold or pressure) or an onychopapilloma. Red lines in several nails can occur with lichen planus or Darier disease. Rarely, a single red streak is a sign of squamous cell carcinoma or amelanotic melanoma.

White lines

• Small white spots or lines (leukonychia): usually minor injury to the base of the nail.
• Paired white bands across the nail that do not move (Muehrcke's lines): low blood albumin, from kidney or liver disease or malnutrition.
• Single white bands across all nails that grow out (Mees' lines): rare, linked to arsenic poisoning, chemotherapy or severe illness.
• Half white, half brown nails: kidney disease.

How nail streaks are diagnosed

A dermatologist examines all nails and the surrounding skin, often with a dermatoscope, a magnifier that shows the pattern and color of the band. They will ask how long it has been there, whether it has changed, and about injury, medicines and family history. Photographs help monitor change. A nail matrix biopsy is done if melanoma cannot be ruled out. Clippings can be tested for fungus.

Treatment

• Harmless pigment needs no treatment, only monitoring for changes.
• Injury-related changes grow out over 4 to 6 months in fingernails and 12 to 18 months in toenails.
• Fungal infections are treated with antifungal medicine.
• Psoriasis and lichen planus are treated with steroid injections or medicines that treat the skin condition.
• Medicine-related streaks may fade after the medicine is stopped, if your doctor agrees.
• Nail melanoma is treated by surgery, often removal of the end of the digit, and further treatment depending on stage.
• Endocarditis is treated in hospital with IV antibiotics.

Frequently asked questions

Is a dark line on my fingernail cancer?

Usually not. Most dark lines are harmless pigment, especially if several nails are affected and the lines are stable. A single new, widening or irregular band, especially with pigment on the surrounding skin, should be checked by a dermatologist to rule out nail melanoma.

What causes a vertical black line on a nail?

Common causes include harmless pigment (very common in darker skin), injury, friction, some medicines, fungal infection and a mole in the nail matrix. Rarely, it is melanoma. A dermatologist can examine it with a dermatoscope.

What do splinter hemorrhages mean?

Splinter hemorrhages are small lines of blood under the nail. Most come from minor injury. In several nails, with fever or a heart condition, they can be a sign of endocarditis and need urgent assessment.

What do white lines on nails mean?

Small white spots or lines usually come from minor injury and grow out. White bands across several nails can be linked to low protein levels, kidney or liver disease, chemotherapy or, rarely, poisoning, and are worth discussing with a doctor.