A blister under the skin: what to look for

A true blister usually has a visible fluid pocket beneath a thin surface. If the skin instead covers a firm, deep or very tender lump, avoid trying to puncture it because the cause may lie below the outermost skin layer.

Friction and burns can lift the surface and create clear fluid. Dyshidrotic eczema can present with small, deep-seated itchy blisters on the sides of fingers, palms or soles. Contact dermatitis can blister after an exposure, while shingles can present with painful grouped blisters in a band on one side. Chickenpox causes successive crops of itchy lesions. The location and overall pattern matter more than a single bump.

Remove pressure or friction from the site and keep the surface clean and dry. Do not insert a needle, cut the skin or squeeze a deep lump. Cover a rubbed blister with a non-stick dressing, changing it if wet or dirty. If the roof breaks naturally, let fluid drain without removing attached skin, then wash gently. Record exposure to heat, footwear, gloves, chemicals or an itchy rash elsewhere.

Seek timely assessment for a deep swelling that enlarges, becomes hot, develops pus, causes spreading redness or limits movement. Fever, severe pain, red streaks, facial or eye involvement, or blistering across a broad area warrants prompt care. Get urgent help if blisters accompany breathing trouble, mouth sores, marked weakness or widespread skin peeling.

Describe whether the lump is soft, firm, itchy, burning or painful and whether fluid is visible. Ask if it is a surface blister or a deeper skin problem, how it should be covered, whether a test is appropriate, and which changes should bring you back. Mention any one-sided band of pain or new lesions.