Abscess

An abscess is a walled-off collection of pus. It is worth understanding that it is not simply an infection that got worse. It is the body’s containment strategy, and a reasonably sophisticated one.

When bacteria establish themselves in tissue, white blood cells arrive and fight them. In the process both die, along with the surrounding tissue, and that debris liquefies. The body then builds a wall around the whole thing out of fibrin and granulation tissue to keep it from spreading. What is inside that wall is pus: dead white cells, bacteria, and liquefied tissue.

This is the same phenomenon described in the appendicitis section, where the omentum and intestines move to wall off a perforation. The body is quite good at containment. The cost of containment is that it creates a space with no blood supply.

Why antibiotics alone usually do not work

This is the most useful thing on this page and the thing patients most often push back on.

Antibiotics reach infection through the bloodstream. The inside of an abscess cavity has no blood supply — that is what defines it. Drug levels inside are therefore a fraction of what they are in surrounding tissue. On top of that, the contents are acidic, which inactivates some antibiotic classes outright, and the bacterial density is enormous with many organisms in a dormant metabolic state that antibiotics are poor at killing.

Surgeons have known this for a very long time. The old aphorism is ubi pus, ibi evacua — where there is pus, drain it. The Latin is old because the principle is old and it has not been improved upon.

So when I tell you that an abscess needs to be drained rather than treated with another course of antibiotics, it is not surgical enthusiasm. Antibiotics have a role — treating the surrounding cellulitis, protecting immunocompromised patients, covering systemic illness — but they are an adjunct to drainage, not a substitute for it.

How abscesses get drained

Skin and soft tissue abscesses are drained by incision. The cavity is opened, the pus is evacuated, and any internal partitions are broken up so that the whole space can drain as one.

A word about the discomfort, because patients deserve an explanation rather than a surprise. Local anesthetic works poorly in infected tissue. Local anesthetics are weak bases and require a neutral pH to cross into nerve cells and function, and abscess tissue is acidic. The anesthetic is being partially neutralized before it can work. This is why an incision and drainage can hurt despite numbing medication, and it is a chemistry problem rather than a technique problem. For larger or deeper abscesses this is a reasonable argument for doing the procedure under sedation or in an operating room instead.

Whether to pack the cavity afterward is debated more than you would expect. For a large cavity, packing keeps it open and draining. For a small simple abscess, the evidence that packing improves anything is weak, and it commits the patient to painful daily dressing changes.

Abscesses deep inside the abdomen are usually drained by an interventional radiologist, who places a catheter through the skin under CT or ultrasound guidance. This is generally the right first move when the collection is above roughly three centimeters and there is a safe window to reach it without crossing bowel or major vessels. It is far less invasive than an operation, and it allows the acute infection to be controlled so that any definitive surgery, if needed, can be done later on a calm abdomen rather than an inflamed one.

Drainage is not always the same as fixing the problem

This distinction matters and gets lost.

Draining an abscess controls the infection. It does not necessarily address whatever created it. A diverticular abscess can be drained beautifully while the diseased sigmoid colon that perforated remains exactly where it was. A perianal abscess can be drained while the tiny internal tract that fed it stays open.

So the question after any drainage is always: what made this, and does that thing need to be dealt with separately? Sometimes the answer is no. When the answer is yes, that is a second, planned, elective conversation rather than an emergency.

Perianal abscesses

These deserve specific mention because they are common, miserable, and frequently mismanaged.

They arise from small glands that sit between the muscles of the anal canal. When one of those glands becomes obstructed and infected, the infection tracks outward into the surrounding spaces. Patients describe severe constant pain, worse with sitting and with bowel movements, and often a visible or palpable swelling near the anus.

The two things to know: a perianal abscess is drained, not treated with antibiotics alone, and I have seen patients suffer for a week on a prescription that was never going to work. And roughly a third of them will subsequently develop a fistula in ano — a persistent small tunnel from the anal canal to the skin that continues to drain intermittently. That is not a sign the drainage failed. It reflects the fact that the original tract from the gland stayed open. Fistulas are treated separately and electively, and there are several approaches depending on how much sphincter muscle the tract involves.

The thing that is not an abscess

There is one diagnosis that must be separated out, because the management is entirely different and the time frame is measured in hours.

A necrotizing soft tissue infection is not a walled-off collection. It is infection spreading rapidly through the tissue planes, destroying them as it goes. The warning features are pain far out of proportion to what the skin looks like, rapid progression over hours, systemic illness that seems disproportionate, skin that becomes dusky or blistered or anesthetic, and crackling under the skin from gas in the tissue.

This is a surgical emergency requiring immediate and often extensive operative debridement. If any of that describes what is happening, it is an emergency department visit immediately, not an appointment.

Living with a drain

If a percutaneous drain is placed, expect it to stay for a while — typically one to several weeks. You will be asked to record the output daily, and that number is what determines when it comes out, generally once it falls to a small volume and repeat imaging shows the cavity has collapsed.

Occasionally a drain keeps producing more than it should, and that usually means it is draining something ongoing rather than a static collection — most often a communication with the bowel. In that case a small study is done where contrast is injected through the drain to see where it goes.

If it keeps coming back

An abscess that recurs in the same location is telling you something. Recurrence at the same site should prompt a search for an underlying cause rather than a third round of the same treatment. Common culprits are hidradenitis suppurativa, a pilonidal disease process, an unrecognized fistula, Crohn’s disease, a retained foreign body, or poorly controlled diabetes.

When to call after a drainage

Increasing rather than decreasing pain after the first day or two, fever, spreading redness, a wound that closes over on the surface while the area underneath becomes firm and tender again, or drainage that stops abruptly and is followed by worsening pain. That last pattern usually means the opening has sealed while the cavity is still producing, and it needs to be reopened.