Surgical Site Infection

The Concept

A surgical site infection (SSI) is an infection that develops at or near a surgical incision within 30 days of the operation (or within 90 days if a prosthesis or implant is left in place). It is one of the commonest hospital-acquired infections, and it matters because it causes pain, prolongs hospital stay, can lead to wound breakdown or incisional hernia, and occasionally progresses to life-threatening sepsis. Almost everything about preventing SSI follows from one idea: infection occurs when the load and virulence of contaminating bacteria overwhelm the patient's local defences. Every preventive measure either reduces the bacterial load or strengthens the host — so if you understand that balance, you can reason out the whole management.

How It Develops

Every surgical wound is contaminated to some degree, but infection only follows when the bacterial inoculum is large enough (classically around 10⁵ organisms per gram of tissue) or when host resistance is lowered. A foreign body or dead tissue dramatically lowers the number of bacteria needed to establish infection, because it shelters organisms from the immune system — which is why implants, sutures and devitalised tissue are such important risk factors. The infection typically declares itself around the 5th–7th postoperative day, and the commonest organism is Staphylococcus aureus.

CDC Classification (BY Depth)

The CDC classifies SSI by the anatomical depth involved, which guides how it is managed:

Surgical Wound Classification (predicts Risk)

Wounds are graded by how contaminated they are at operation — and the expected infection rate rises accordingly, because a dirtier field means a higher bacterial load:

This classification is practically useful: it tells you which operations genuinely benefit from antibiotic prophylaxis and which patients need especially careful technique.

Risk Factors

Patient factors that lower resistance include diabetes, obesity (fat is poorly perfused and prone to infection), malnutrition, smoking, immunosuppression, and any distant untreated infection. Operative factors that raise the bacterial burden or reduce local defence include prolonged surgery, poor haemostasis or rough technique, contamination, a foreign body/implant, and intra-operative hypothermia (which impairs the neutrophils that fight infection).

Prevention — the Reasoning Behind Each Step

Because SSI is a battle between bacterial load and host defence, prevention attacks both sides:

• Optimise the patient — control blood glucose, correct malnutrition and anaemia, and stop smoking; each strengthens the host's ability to resist infection.
• Remove hair by clipping, not shaving — shaving with a razor causes microscopic cuts that quickly become colonised with bacteria, so shaved patients have higher infection rates than clipped ones; this is a classic, counter-intuitive exam point.
• Skin antisepsis — a chlorhexidine or povidone-iodine prep reduces the resident skin flora before incision.
• Antibiotic prophylaxis timed within 60 minutes before incision — the aim is a high tissue drug level at the moment of incision, when contamination occurs; given too early or after the incision it is far less effective. It is a single dose (re-dosed only for long operations or major blood loss), not a prolonged course, because the goal is to cover the operative window, not to treat an established infection.
• Maintain normothermia and good oxygenation intra-operatively — neutrophil bacterial killing is an oxygen-dependent process, so a warm, well-oxygenated patient fights contamination better.

Management of an Established Ssi

The principle mirrors that of any pus collection: open and drain the wound to release the pus and reduce the bacterial load, send a specimen for culture and sensitivity, and debride any dead tissue. The wound is then managed with regular dressings to heal by secondary intention. Antibiotics are given for spreading cellulitis or systemic sepsis, guided by culture — but, as with an abscess, they are an adjunct and cannot replace drainage of a collection.

• Prophylaxis — a single IV dose of an appropriate agent (e.g. A cephalosporin; add metronidazole for colorectal surgery) within 60 minutes before incision.
• Established infection — culture-guided antibiotics; empirical anti-staphylococcal cover pending sensitivities.

Timing of prophylactic antibiotic within 60 minutes of incision matters most.

• SSI = infection at the surgical site within 30 days (90 days with an implant); commonest organism S. Aureus, usually presents day 5–7.
• Infection = bacterial load/virulence overwhelming host defence; a foreign body/dead tissue lowers the inoculum needed.
• CDC depth: superficial incisional / deep incisional / organ-space. Wound classes clean→dirty predict rising risk.
• Prevent by attacking both sides: optimise patient (glucose, nutrition, smoking); clip don't shave; skin antisepsis; prophylactic antibiotic within 60 min; normothermia/oxygenation.
• Treat established SSI by opening/draining + culture-guided antibiotics; antibiotics can't replace drainage.