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Black Nail Streaks: Causes and When to Worry

Black streaks on fingernails may be indicative of other health problems

Published on 07/20/2018
NailsDiagnosis and DetailsMelanoma Skin CancerNail HealthWesternHair and Nails
Longutidinal melanonychia on the thumb

Longitudinal melanonychia, otherwise known as a black nail streak,[1] may be quite intimidating, as it raises fear about nail cancer. There are certain features that clinicians look for when assessing the possibility of malignancy, like melanoma of the nail. Here are some underlying conditions and external factors that may cause benign dark streaks to appear on or in the nail.

In a normal, healthy nail, the cells that produce skin pigment (known as melanocytes) are not overly activated, which is why nails are clear and natural colored in appearance. The brown to black color that may show on the nail in the form of a streak is due to either activation of melanocytes or dividing melanocytic cells, and there are many non-malignant reasons for these processes to occur. Here are a few factors that can affect if you will have a black streak on your nail.


Studies have shown that people with ethnically darker skin have a higher chance of developing a benign dark streak on the nail. One study conducted with a population of African Americans concluded that close to 90% of these patients will have a benign nail streak before the age of 50.[2] Nails streaks seen in darker pigmented individuals are more common on the thumb, index finger, and middle finger.[3] A study in 57 Mexican patients determined that nearly 74% of the black streaks were benign and due to ethnic factors,[4]  20% were due to injury or infection, and only 6% were due to skin cancer. The least common cause of a black streak was nail melanoma. However, it is not to say that a black streak on someone of ethnic descent should not be concerning, as many other factors, such as family history of melanoma and age may be important. Any new black streak should be evaluated by a dermatologist.


There are many biological changes that take place in a woman’s body during pregnancy. Skin darkening (known as hyperpigmentation) is common in pregnancy and benign in nature. Some of the areas affected by hyperpigmentation include the face, which is known as melasma,[5] the abdomen, known as the linea nigra,[5] and the nail bed.

Why does this happen? Estrogen, progesterone, and melanocyte-stimulating hormone are all increased during pregnancy and cause darker pigmentation to form in select areas of the body.[6]


Repeated trauma or injury to the nail can lead to permanent damages. Poorly fitted shoes, lacerations, and any physical injury to the nail can cause a black nail streak[7,8].


A series of chemotherapeutic agents, antibiotics, and steroids are associated with longitudinal melanonychia[7].

Other Conditions

Endocrinology diseases can cause an alteration in pigmentation of the nail. Examples are Addison’s disease, pituitary tumor, Cushing syndrome, and hyperthyroidism.[7,8] Furthermore, autoimmune conditions like psoriasis, lichen planus, amyloidosis, lupus, and scleroderma have also been linked to LM.[7,8]

Nevus of the nail

A nevus, more commonly known as a mole, of the nail unit is seen more commonly in children and young adults.[3]

Fungal infection of the nail

In some cases, a fungus can infect the nail and lead to black pigmentation along the nail bed.[9,10] There are over 21 types of fungus that can cause black pigmentation once they infect the nail.

Melanoma of the nail

One way to check if a dark streak on the nail has a higher chance of being a nail melanoma skin cancer is to review the ABCDEF’s[11] of nail melanoma.

  • Age: usually affects individuals in the 50-70 age group
  • Band: band greater than 3mm
  • Change: rapid increase in size or no change at all after anti-fungal treatment
  • Digit: thumb, big toe, and index finger more commonly affected
  • Extension: streak extension past the border or nail tip
  • Family history: any family members with a history of melanoma or moles. 

The ABCDEF’s of melanoma were created to help people understand when a black nail streak is more concerning for a nail melanoma. The idea of the mnemonic is that the more letters that are positive, the more important that the nail is evaluated by a dermatologist. An additional finding that increases the risk for melanoma is named Hutchinson’s sign[12] and this is when the nail discoloration extends onto the skin behind the nail. Ultimately a biopsy is the only true diagnostic tool that will confirm a nail melanoma. However, nail melanoma is often misdiagnosed or diagnosed late, which can lead to a poor prognosis for the patient. One study noted that 50% of patients die before 5 years of a nail melanoma diagnosis and it is due to late diagnosis, which limits treatment options.[12] The ABCDEF’s of nail melanoma can decrease mortality by creating awareness of nail changes and seeking early treatment if needed.

When in Doubt…

It is important to know that there is a possibility that a benign presentation may be masking a malignancy; therefore it is important to be aware of any changes over time.  


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