Blisters form when there is a separation in the upper layers of the skin. Fluid collects where the skin layers have separated, forming a bubble -- the blister. As the name implies, a blood blister has some blood within the skin bubble caused by injury to tiny blood vessels in the skin. Blood blisters usually form when the skin is pinched or crushed, but can develop with repeated friction. In most cases, blood blisters require no specific treatment and go away on their own. However, there are a few things you can to do to reduce your discomfort and protect the area until it heals.
Pinching your finger in a door or accidentally hitting your finger with a hand tool are common ways to get a blood blister. If the injury occurs at home or another location where you have access to ice, apply it to the area immediately. This might help stop bleeding into the blister by causing constriction of the injured blood vessels -- although your body's clotting system usually stops the bleeding very quickly even without ice. You have nothing to lose by trying some ice because it can help relieve the acute pain due to the injury even if it doesn't limit the size of the blood blister.
Protect An Intact Blister
Though you might be tempted to pop or puncture a blood blister, leave it be. This is particularly important if you have diabetes or another medical condition that weakens your immune system. The intact surface of a blister -- called the roof -- naturally protects the area from infection. So it's in your best interest to protect the blister from rupture if it's in a spot that will get bumped or rubbed. Depending on the size and location of the blood blister, you can use a loose adhesive bandage, a blister bandage or a moleskin "donut" with a hole cut in the center to leave the blister open to the air.
Clean and Protect An Open Blister
If the blood blister has already ruptured -- or is open because you punctured it -- clean the area with soap and water. Leave the roof skin that remains rather than trimming it off, as the skin beneath will be very sensitive and vulnerable to infection. Once you've cleaned the area, cover it loosely with a bandage to keep the skin clean and protected. Some doctors recommend use of an over-the-counter antibiotic ointment (Neosporin), although this medicine causes skin irritation in some people.
Let It Heal
A blood blister typically heals within a week or two. During this period, continue to keep the area clean and protected. Wash with soap and water as needed, and change the bandage or other protective covering regularly to keep the area clean and dry. If at some point the roof skin comes off, don't worry. Again, just keep the area clean, dry and protected. Avoid activities, shoes or clothing that might irritate the healing blister.
A blood blister caused by an injury or excess friction is typically not cause for concern. But large blood blisters and those near or under a fingernail or toenail can be quite painful. If you have such a blister, consult with your doctor about draining it. Additionally, an open blister can sometimes become infected. Watch for signs of infection, such as increased pain, spreading redness, fever or pus draining from the wound. Seek immediate medical attention if you have multiple, unexplained blisters as there are some serious medical conditions that cause this type of skin separation.