Seroma

What it means

A seroma is a build-up of serum — the clear, pale-yellow fluid part of blood, without the red blood cells — in a space where tissue was disturbed, most commonly after surgery. When skin and deeper tissue layers are separated during an operation, small lymphatic and blood vessels are cut, and as the body heals it can leak fluid into the empty space left behind. That fluid collects into a soft, fluctuant pocket, which shows up on imaging as a well-defined, fluid-filled area with a thin wall and no solid internal contents.

Why it appears on a CT or MRI report

Radiologists note a seroma when they see a smooth-walled collection of simple fluid, usually at or near a surgical bed, a scar, or a site of prior trauma. The report typically describes its size, location relative to the surgery, and — importantly — whether the fluid looks simple (uniform and dark on CT, or with a clean fluid signal on MRI) or complex, with debris, a thick or enhancing wall, internal septations, or gas. That distinction matters because simple fluid is the hallmark of an uncomplicated seroma, while a thickened or enhancing wall raises the possibility of infection turning it into an abscess.

What it usually means

Seromas are extremely common after surgery, particularly operations that involve removing tissue or creating a large space under the skin, such as breast, abdominal, or hernia surgery. Most are small, cause no symptoms beyond a soft swelling near the incision, and resolve on their own over several weeks as the body reabsorbs the fluid. Larger ones can cause discomfort, a feeling of fullness, or pressure on the wound, and may occasionally need to be drained with a needle for comfort or to help the surgical site heal properly. The main concern patients and reports are watching for is whether a seroma has become infected — which would then be treated as an abscess — so a report noting a "simple" seroma with no worrying features is generally reassuring news rather than a cause for alarm.

When to follow up

A small, simple seroma found on a routine post-operative scan usually just needs monitoring, and your surgical team will likely check it at a follow-up visit. Contact your doctor sooner if the area becomes increasingly red, warm, or painful, if you develop a fever, if fluid is leaking from the incision, or if the swelling is growing quickly rather than staying the same or shrinking — these can be signs that a simple seroma is becoming infected. Very large or persistent seromas that are not improving over several weeks may need to be drained by your surgeon, which is usually a quick and low-risk procedure.

A plain-language way to picture it

Picture a water balloon forming naturally under the skin after surgery has created a small empty pocket in the tissue — the body simply fills that gap with clear fluid the way a low spot in a garden fills with rainwater after it rains. As the surrounding tissue knits back together over the following weeks, the "low spot" gradually closes up and the fluid gets reabsorbed, much like a puddle drying out once the ground around it settles. It is only a problem if that puddle turns cloudy or the surrounding ground gets hot and inflamed, which would point toward infection instead.