A black fingernail is known as a subungual hematoma and is caused by a build-up of blood under a fingernail usually resulting from an impact or trauma to the finger.
Symptoms of a subungual hematoma
Pain on the finger nail at the time of impact and afterwards. The pain is often described as a throbbing pain. Blood quickly bleeds and gathers underneath the fingernail increasing a feeling of pressure under the nail. Over time the nail will appear black.
Treatment of a black fingernail
Most subungual hematomas can be treated at home and do not require medical attention. If the total area of blood is no more than 25% of the nail then the following guidelines may apply.
Apply ice (wrapped in a cloth) or cold therapy for 10 minutes at a time, every hour to reduce bleeding and swelling. Elevate the hand to reduce bleeding and swelling. This aids the tissue fluids to flow away from the site of injury using gravity to assist.
Anti-inflammatory medication such as Ibuprofen may be recommended to ease pain and inflammation. The pain and pressure will ease over a couple of days and the blood will drain on its own.
If the bleeding covers more than 25% of the nail then seek medical attention as the blood under the nail may need draining. A local anaesthetic may be injected into the finger so it is numb and a hole can then be made in the nail so the blood can drain through. This may be done using a cautery to burn a hole, a needle to drill a hole or a sterilized paper clip. No follow-up treatment is usually necessary. However, occasionally, the whole nail is removed.