Infected wound

An infected wound is a break in skin or mucosa in which microorganisms have multiplied enough to cause local inflammation, purulence or systemic illness. It includes traumatic wounds, bite wounds, surgical site infections and infected chronic ulcers.

Clinical relevance. Surgical site infection complicates about 2 to 5% of clean operations and more after bowel surgery. Delay in recognising necrotising infection and retained foreign body drives most serious outcomes.

Mechanisms. Bacterial inoculation exceeds host defence when there is devitalised tissue, foreign material, haematoma, dead space, poor perfusion, high bacterial load or impaired immunity. Staphylococcus aureus and streptococci predominate in skin flora wounds. Enteric organisms and anaerobes follow bowel or perineal wounds. Pseudomonas aeruginosa follows water and burn exposure, and Pasteurella follows animal bites.

Classification.

• Superficial incisional: skin and subcutaneous tissue only, within 30 days of surgery.
• Deep incisional: fascia and muscle, within 30 days (or 90 days with an implant).
• Organ and space: abscess or infection in the operated cavity.
• Traumatic: clean, contaminated, dirty, bite, crush, puncture, agricultural or water exposure.
• Chronic wound infection: ulcer, pressure injury or diabetic foot with biofilm.
• Necrotising soft tissue infection: a surgical emergency, with rapidly spreading necrosis along fascial planes.

Temporal patterns. Streptococcal and clostridial infection can declare itself within 12 to 48 hours. Staphylococcal infection usually appears 4 to 10 days after injury or surgery. Late infection after months suggests foreign body, osteomyelitis, atypical mycobacteria or an implant.

Key distinctions. Colonisation is bacterial growth without host reaction and does not need antibiotics. Pain out of proportion to appearance, crepitus, bullae, skin anaesthesia and rapid spread mean necrotising infection until excluded.

History.

• Mechanism, time and place of injury, contamination (soil, faeces, fresh or sea water, farm), and animal or human bite (species, provoked or not, rabies risk in endemic regions).
• Time since surgery, type of operation, implants, drains, perioperative antibiotics and wound closure method.
• Fever, rigors, spreading redness, discharge, pain, swelling, stiffness or limb function loss.
• Diabetes, peripheral vascular disease, immunosuppression, HIV, malnutrition, smoking, obesity, corticosteroids, anticoagulants, chemotherapy, renal failure and liver disease.
• Tetanus immunisation status, allergies (especially penicillin), previous methicillin resistant Staphylococcus aureus and recent antibiotics.

Examination.

• Vital signs and signs of sepsis (temperature, heart rate, respiratory rate, blood pressure, mental state, urine output).
• Wound: size, depth, margins, erythema (mark the edge and time), warmth, induration, purulence, odour, necrosis, crepitus, bullae, grey or dusky skin, probing to bone or a foreign body.
• Surrounding area: lymphangitis, regional nodes, limb swelling, distal pulses, sensation and power, joint movement (septic arthritis), and tendon function in hand wounds.
• Dehiscence or sinus, drain output and fluctuance.
• Glucose, and examination of the feet in diabetes.

Red flags.

• Pain out of proportion, rapidly spreading erythema, bullae, skin necrosis, crepitus or anaesthesia over the area.
• Sepsis: temperature over 38.3 or under 36 degrees Celsius, heart rate over 100 beats/min, systolic pressure under 100 mmHg, respiratory rate over 22/min, confusion or lactate over 2 mmol/L.
• Deep infection of hand, face, joint, tendon sheath or bone (probe to bone, exposed hardware).
• Wound near a prosthesis or vascular graft.
• Trismus, spasms or autonomic instability (tetanus).
• Immunosuppression, diabetes with a foot wound, or neutropenia.
• Animal bite from a rabies endemic species.

Special groups. Diabetic foot wounds need vascular assessment and probing for bone. Children need weight based doses. Pregnancy limits tetracycline and fluoroquinolone use. Older adults may have blunted fever. Kidney impairment needs dose adjustment.

Superficial infection and its mimics.

• Impetigo, folliculitis, cellulitis, erysipelas, abscess, infected cyst, paronychia.
• Stitch abscess, suture sinus, hypertrophic scar, keloid.
• Contact dermatitis, stasis dermatitis, drug eruption, gout, haematoma, seroma, deep vein thrombosis, lymphoedema.

Deep and dangerous infections.

• Necrotising soft tissue infection (necrotising fasciitis, Fournier gangrene), clostridial myonecrosis (gas gangrene), pyomyositis, toxic shock syndrome.
• Septic arthritis, osteomyelitis, tenosynovitis, deep space hand infection, infected prosthesis or graft.
• Abscess in the operated cavity, anastomotic leak with wound extension.
• Tetanus, rabies, and sepsis from the wound.

Organism specific. Methicillin resistant Staphylococcus aureus, group A streptococcus, Pseudomonas aeruginosa, Pasteurella multocida and Capnocytophaga canimorsus (bites), Eikenella corrodens (human bites), Vibrio species and Aeromonas (water), Mycobacterium marinum and non-tuberculous mycobacteria (late or water exposure), Nocardia and fungal infection (immunosuppression), tuberculous infection.

Non-infectious. Pyoderma gangrenosum (pathergy, undermined violaceous border), vasculitis, calciphylaxis, retained foreign body, arterial or venous ulcer, malignant ulcer (Marjolin ulcer), factitious wound.

Step 1. Decide whether the infection is superficial, deep or necrotising, and whether the patient is septic. A deep or necrotising picture goes to surgery without waiting for imaging.

Step 2. First line tests: full blood count, CRP, urea, creatinine, electrolytes, glucose (and HbA1c in diabetes), lactate, liver tests and coagulation when septic. Blood cultures (two sets before antibiotics) for fever or sepsis.

Step 3. Microbiology. Clean the wound surface with saline and send deep tissue, pus or an aspirate for Gram stain and culture, because surface swabs reflect colonisation. Request anaerobic culture, and mycobacterial and fungal studies for late or immunosuppressed cases. Culture bite wounds that are over 8 hours old or show infection.

Step 4. Imaging.

• Plain radiograph for a foreign body (glass, metal, gravel), gas in the soft tissue, fracture and bone change.
• Ultrasound to find a collection or radiolucent foreign body.
• MRI for suspected osteomyelitis, deep space or tendon sheath infection (CT if MRI is not available).
• CT for suspected organ space infection after abdominal or thoracic surgery.

Step 5. Scoring. The LRINEC score (CRP, white cell count, haemoglobin, sodium, creatinine, glucose) of 6 or more raises concern for necrotising infection, but a low score does not exclude it, and clinical suspicion decides surgical exploration. Finger or bedside incision with a probe test (loss of resistance of fascia) when imaging is uncertain.

Interpretation. Raised CRP and white cells support infection but may reflect surgery or injury. A rising CRP after day 3 to 4 postoperatively suggests infection or collection.

Limitations. Swab cultures often grow colonisers. Imaging for gas has low sensitivity for necrotising infection. Normal blood tests do not exclude early infection.

Immediate priorities. Resuscitate sepsis (oxygen, IV access, crystalloid 30 mL/kg for hypotension or lactate of 4 mmol/L or more, blood cultures, and antibiotics within 1 hour). Call surgeons for suspected necrotising infection or deep collections. Control pain.

Local wound care (all patients).

• Irrigate with 500 mL to 1 L of warm sterile saline or clean tap water at low pressure. Remove foreign material and devitalised tissue under local anaesthesia.
• Open surgical site infections at the infected segment, drain collections and pack loosely. Leave contaminated traumatic and bite wounds open or delay closure at 3 to 5 days.
• Elevate and immobilise the part. Mark the erythema edge and review at 24 to 48 hours.

Tetanus.

• Clean minor wound: tetanus toxoid vaccine if fewer than 3 doses before or more than 10 years since the last dose.
• Tetanus prone wound (contaminated, devitalised, burns, over 6 hours old, bite or puncture): vaccine if more than 5 years since the last dose.
• Add human tetanus immunoglobulin 250 units IM at a separate site if immunisation is incomplete or unknown, or if the patient is immunocompromised. Give 500 units if the wound is over 24 hours old, heavily contaminated or a burn.
• Follow the local schedule for vaccine dose and interval in children and pregnancy.

Rabies prophylaxis (animal bite in an endemic region). Wash for 15 minutes with soap and water and apply an antiseptic (povidone iodine). Give post-exposure prophylaxis for category II and III exposures: rabies vaccine IM on days 0, 3, 7 and 14.

For category III exposure, also give human rabies immunoglobulin 20 IU/kg infiltrated into and around the wound (maximum volume that can be infiltrated, with the remainder given IM at a distant site). Do not suture bite wounds.

Antibiotics (superficial infection with cellulitis, or systemic signs).

• Flucloxacillin 500 mg to 1 g orally every 6 hours for 5 to 7 days (maximum 4 g daily; reduce dose in severe kidney impairment; monitor liver tests with courses over 14 days because of cholestatic jaundice).
• Cefalexin 500 mg orally every 6 hours for 5 to 7 days (maximum 4 g daily; 500 mg every 12 hours for CrCl 10 to 40 mL/min; use caution with penicillin allergy history of severe reaction).
• Penicillin allergy or MRSA suspected: clindamycin 300 to 450 mg orally every 6 hours for 5 to 7 days (maximum 1.8 g daily; stop for diarrhoea and check for Clostridioides difficile).
• Alternatively give co-trimoxazole 960 mg orally every 12 hours (maximum 1.92 g daily; halve the dose for CrCl 15 to 30 mL/min and avoid under 15 mL/min; monitor potassium and creatinine; avoid in pregnancy and sulphonamide allergy).
• Doxycycline 100 mg orally every 12 hours is an alternative outside pregnancy and childhood.
• Moderate or severe infection, or oral treatment impossible: cefazolin 1 to 2 g IV every 8 hours (maximum 6 g daily; for CrCl 10 to 34 mL/min give half the dose every 12 hours; monitor creatinine).
• Add vancomycin 25 mg/kg IV loading dose (maximum 3 g) then maintenance dosed by kidney function and levels when MRSA is likely; monitor creatinine and trough or area under the curve levels.
• Bite wounds: amoxicillin with clavulanic acid 500/125 mg orally every 8 hours for 3 days as prophylaxis (high risk wound, hand, delayed presentation, immunosuppression) or 5 to 7 days for established infection (maximum 1.5 g amoxicillin daily; 500/125 mg every 12 hours for CrCl 10 to 30 mL/min).
• Bite wounds with penicillin allergy: doxycycline 100 mg orally every 12 hours plus metronidazole 400 mg orally every 8 hours (maximum 1.2 g daily; avoid alcohol).
• Gut or perineal source: add anaerobic cover (metronidazole 500 mg IV or orally every 8 hours; maximum 1.5 g daily; monitor for neuropathy with prolonged use).
• Water exposed wound (fresh water): ciprofloxacin 500 mg orally every 12 hours (maximum 1.5 g daily; adjust dose when CrCl is under 30 mL/min; monitor QTc, tendon pain and glucose).
• Review at 48 to 72 hours, switch to oral treatment when improving, and narrow when cultures return.

Necrotising soft tissue infection. Urgent surgical debridement within hours and repeated until no necrosis remains.

Give piperacillin with tazobactam 4.5 g IV every 6 hours (maximum 18 g piperacillin daily; every 8 hours for CrCl 20 to 40 mL/min and every 12 hours under 20 mL/min).

Or give meropenem 1 g IV every 8 hours (1 g every 12 hours for CrCl 26 to 50 mL/min, 500 mg every 12 hours for CrCl 10 to 25 mL/min, 500 mg every 24 hours under 10 mL/min).

Add clindamycin 600 to 900 mg IV every 8 hours (maximum 2.7 g daily) to suppress toxin production. Add vancomycin 25 mg/kg IV loading dose (maximum 3 g) then maintenance dosed by kidney function and levels when MRSA is possible.

Intravenous immunoglobulin 1 to 2 g/kg once may be considered in streptococcal toxic shock (evidence is limited). Admit to critical care.

Analgesia. Paracetamol 1 g orally or IV every 6 hours (maximum 4 g daily; 3 g daily if weight under 50 kg, malnutrition, alcohol excess or liver disease). Avoid NSAIDs in sepsis and kidney impairment.

Monitoring. Temperature, heart rate and blood pressure every 4 hours for sepsis, wound reviewed daily with the erythema margin marked, CRP at 48 to 72 hours, and creatinine and drug levels with vancomycin, aminoglycosides or prolonged courses.

Referral and escalation. Surgery for any collection, necrosis, deep or hand infection, or dehiscence. Plastic surgery or hand surgery for hand, face and tendon wounds. Orthopaedics for bone or joint involvement. Vascular surgery for ischaemic wounds. Infectious diseases for resistant organisms, implants and persistent infection. Critical care for septic shock.

Admission criteria. Sepsis, necrotising infection, deep or hand infection, failure of oral antibiotics at 48 to 72 hours, immunosuppression, diabetic foot infection with systemic signs or deep tissue involvement, vascular or implant involvement, or social circumstances preventing follow up.

Discharge and safety netting. Discharge after drainage with dressings, antibiotics with the planned stop date, tetanus cover given, and review in 48 hours. Patients return for spreading redness, increasing pain, fever, pus, numbness, dusky skin or a failure to improve within 48 hours.