A blister is a small pocket of fluid that forms beneath the top layer of skin, usually resulting from intense friction, a burn, or localized trauma. This fluid-filled sac protects the underlying tissue while it heals. While most blisters resolve on their own, the protective skin layer can be compromised, allowing bacteria to enter and turn a minor injury into an infected wound. Recognizing the indicators of infection is important for preventing complications and ensuring faster healing.
What an Uninfected Blister Looks Like
A healthy, uninfected blister typically appears as a raised bubble filled with clear or light, watery yellow fluid. This fluid, known as serum, collects to shield the damaged skin layers below. A blood blister, which contains reddish or dark fluid due to broken blood vessels, is also considered uninfected if the outer skin remains intact.
The area around a normal blister may be tender or slightly uncomfortable, especially when touched. However, the pain should gradually lessen over several days. The blister will slowly decrease in size as the fluid is naturally reabsorbed by the body, typically healing completely within one to two weeks without intervention.
Localized Signs of Blister Infection
The first indications of a bacterial infection are usually visible at the wound site. A noticeable increase in redness and swelling surrounding the blister, extending beyond the initial injury margin, is a reliable sign of an inflammatory response. This spreading redness, medically termed erythema, indicates the infection is moving into adjacent skin tissues.
The blistered area will likely feel hotter than the surrounding skin, a sign of increased blood flow fighting the infection. Pain will also intensify and persist instead of improving, becoming sharper over time. The most definitive sign of localized infection is the presence of thick, cloudy discharge, often colored yellow or green, which is known as pus. A final indicator of bacterial activity is a foul odor emanating from the blister or the drainage.
Systemic Signs Requiring Immediate Care
If a localized infection is left untreated, the bacteria can spread, leading to a systemic infection that requires prompt medical intervention. The most common whole-body symptom is the development of a fever, generally defined as a body temperature above 100.4°F (38°C). This fever is often accompanied by chills as the body attempts to raise its core temperature to fight the widespread infection.
The appearance of thin red lines or streaks radiating outward from the blister is an especially concerning sign, indicating lymphangitis. This means the infection has entered the lymphatic vessels and is moving toward the torso. Swelling and tenderness in nearby lymph nodes, such as those in the armpit or groin depending on the blister’s location, are also signs that the body is struggling to contain the spreading bacteria.
Treatment and When to See a Doctor
For an uninfected blister, the primary treatment involves protecting the area with a padded dressing and keeping it clean and dry. If the blister has broken but shows no signs of infection, gently clean the area with mild soap and water, apply an over-the-counter antibiotic ointment like Bacitracin, and cover it with a sterile bandage. It is best to avoid intentionally popping a blister, as the intact skin acts as a natural barrier against microbial entry.
If you observe localized signs of infection, such as pus or worsening pain, consistent home care should be implemented immediately. This includes washing the area multiple times a day and consistently applying antibiotic ointment under a clean bandage. If localized signs worsen after a day or two of home treatment, or if the blister continues to produce a cloudy discharge, a doctor’s visit is necessary to assess the need for prescription oral antibiotics. The presence of any systemic signs, including fever, chills, or red streaks extending from the wound, mandates immediate medical attention, as the infection may have progressed to a dangerous stage like cellulitis.