A surgical site infection (SSI) is an infection that develops in or around a surgical wound after an operation. SSIs affect 2% to 4% of all patients undergoing inpatient surgical procedures, making them the most common preventable complication after surgery. They range from mild skin infections that clear up with antibiotics to serious deep infections that require additional surgery.
Three Types Based on Depth
Not all surgical site infections are the same. The CDC classifies them into three categories based on how deep the infection reaches.
Superficial incisional SSIs involve only the skin and the layer of fat just beneath it. These are the most common type. You might notice pus draining from the incision, redness, swelling, warmth, or increasing pain around the wound. Superficial infections must appear within 30 days of surgery to be classified as an SSI.
Deep incisional SSIs reach into the muscle and the tough connective tissue (fascia) beneath the fat layer. Signs include pus draining from deeper in the wound, fever above 100.4°F (38°C), worsening pain, or the wound opening on its own. In some cases, an abscess forms that shows up on imaging. These infections can develop within 30 or 90 days of surgery, depending on the procedure.
Organ/space SSIs are the most serious. They involve areas deeper than the muscle layer, affecting internal organs or the spaces around them. An abdominal surgery, for example, could lead to an infection inside the abdomen itself rather than just along the incision line. These also fall within the 30- or 90-day surveillance window.
When Symptoms Typically Appear
Most surgical site infections show up within 3 to 7 days after the procedure. That said, the official surveillance window extends to 30 days for most operations. For procedures involving implants or certain complex surgeries, the window stretches to 90 days. The list of procedures with a 90-day window includes joint replacements (hip and knee), cardiac surgery, coronary artery bypass grafts, spinal fusions, craniotomies, hernia repairs, pacemaker placements, and breast surgery.
This means an infection that develops two months after a knee replacement still counts as a surgical site infection, while the same timeline after an appendectomy would not.
Signs to Watch For
The hallmark signs of an SSI are straightforward to spot if you know what to look for:
- Redness spreading around the incision, especially if it’s getting worse rather than better over the first few days
- Increasing pain or tenderness at the surgical site, particularly if it was improving and then worsened
- Cloudy or foul-smelling drainage from the wound (clear or slightly blood-tinged drainage in the first day or two is normal, but thick, yellowish, or greenish fluid is not)
- Warmth or swelling around the incision
- Fever, especially above 100.4°F
Some redness and mild soreness right around the incision line is expected after any surgery. The difference with an infection is that these signs worsen over time or appear after a period of initial improvement.
What Raises Your Risk
Some factors that increase the chance of developing an SSI are within your control, and some are not. The major patient-related risk factors include obesity, diabetes (particularly when blood sugar is poorly managed), smoking, older age, low protein levels in the blood, and having an existing infection at the time of surgery. Conditions that reduce blood flow to tissues, such as vascular disease or prior radiation therapy to the surgical area, also raise risk because healing depends on good circulation.
On the surgical side, longer operations carry higher infection risk simply because the wound is open and exposed for more time. The type of surgery matters too. Procedures classified as “contaminated” or “dirty,” such as operations on the bowel or surgery performed on an already infected area, carry a naturally higher risk than a clean elective procedure like a knee replacement.
How SSIs Are Treated
Treatment depends on the type and severity of the infection. Superficial infections often respond to antibiotics chosen based on the likely bacteria involved and local resistance patterns. If the wound contains a pocket of pus, a surgeon may need to open part of the incision to drain it. Once opened, the wound is typically cleaned with sterile saline (or tap water after the first 48 hours) and allowed to heal gradually from the inside out, a process called healing by secondary intention. This requires regular dressing changes, sometimes with specialized wound dressings.
Deeper infections and organ/space infections are more complex. They often require a return to the operating room to drain abscesses, remove infected tissue, and thoroughly clean the area. Antibiotics alone usually aren’t enough for these cases. If an implant is involved, the device sometimes needs to be removed, treated, or replaced entirely.
Recovery from an SSI adds significant time to the healing process. Studies of patients who develop SSIs show hospital stays extending by roughly 11 days on average compared to patients without infections. In some cases, repeat procedures are necessary. About half of SSI cases in one study required a return to the operating room, with substantial additional costs.
The Financial Impact
SSIs are expensive for both healthcare systems and patients. The additional cost per case varies enormously depending on the country, the surgery, and how severe the infection becomes. In high-income countries, the extra cost ranges from as little as a few hundred dollars for a minor superficial infection to over $34,000 for a serious complication. In pediatric patients, one retrospective study found the average cost of SSIs reached nearly $137,000, with reentry procedures alone adding a median cost of over $116,000. Even in milder scenarios, overall healthcare costs for patients with SSIs run roughly 36% higher than for patients without them.
Reducing Your Risk Before and After Surgery
If you have an upcoming surgery, several steps can meaningfully lower your chances of infection. Quitting smoking before a procedure improves blood flow and immune function at the wound site. If you have diabetes, tighter blood sugar control in the weeks surrounding surgery reduces risk. Losing weight before an elective procedure helps if obesity is a factor, though this isn’t always practical depending on timing.
After surgery, proper wound care is essential. Keep the dressing clean and dry, and use a sterile technique (or clean hands at minimum) when changing it. You can shower 48 hours after most surgeries, but avoid submerging the wound in bathtubs, pools, or hot tubs until it’s fully closed. Follow your surgical team’s instructions on how to care for the incision, and pay close attention to the signs listed above. Catching an SSI early, while it’s still superficial, is far easier to treat than discovering a deep infection weeks later.