Initial Approach to Pus vs Phlegmon
When pus is present or suspected, immediate surgical drainage is the priority ("don't let the sun set on pus"), whereas phlegmon (diffuse inflammation without discrete pus collection) is typically managed with antibiotics and close observation, with surgery reserved for failure of conservative management or development of abscess. 1
Immediate Assessment and Differentiation
Clinical Examination
- Palpate for fluctuance to distinguish abscess (pus collection) from phlegmon (indurated, non-fluctuant inflammation) 1
- Assess for systemic signs of sepsis including fever, tachycardia, hypotension, and altered mental status that would mandate urgent intervention 1
- Perform digital rectal examination if perianal/anorectal pathology is suspected 1
Imaging When Indicated
- In stable patients with unclear clinical findings, obtain CT scan to differentiate abscess from phlegmon and define extent of disease 1
- Imaging should never delay surgical intervention when pus is clinically evident or the patient is unstable 1
- In hemodynamically unstable patients after proper resuscitation, proceed directly to surgery without CT imaging 1
Management Algorithm
For Pus/Abscess (Discrete Fluid Collection)
- Obtain blood cultures immediately, then proceed urgently to incision and drainage within 24-48 hours 1, 2
- Collect purulent material for culture and Gram stain during drainage to guide antibiotic therapy 1, 2
- Base timing of surgery on presence and severity of sepsis—emergent drainage for septic patients, urgent drainage for stable patients 1
- Fit, immunocompetent patients with small perianal abscesses without systemic sepsis may be managed as outpatients 1
For Phlegmon (Diffuse Inflammation Without Pus)
- Initiate parenteral antibiotics covering likely pathogens based on anatomic location 2, 3
- For appendiceal phlegmon, initial conservative management with antibiotics is appropriate, with delayed appendectomy several weeks later if needed 1, 4
- Monitor closely for development of abscess requiring drainage (clinical deterioration, persistent fever, increasing inflammatory markers) 1
- If no clinical improvement occurs within 48-72 hours of antibiotic therapy, obtain repeat imaging to assess for abscess formation 1
Antibiotic Management
Empiric Coverage
- Administer broad-spectrum parenteral antibiotics immediately after obtaining cultures 2
- For anorectal infections, provide coverage in presence of sepsis, surrounding soft tissue infection, or immunocompromise 1
- Sample drained pus in high-risk patients or those with risk factors for multidrug-resistant organisms 1
Duration and Adjustment
- Continue antibiotics for 2 weeks for severe soft tissue infections without bone involvement 1
- Narrow antibiotic spectrum based on culture susceptibilities once available 1
- Discontinue antibiotics if cultures are negative and no recent antibiotic changes were made in the preceding 72 hours 1
Critical Pitfalls to Avoid
- Never delay surgical drainage of pus while waiting for imaging or antibiotic response—this increases morbidity and mortality 1
- Do not rely on superficial wound swabs for culture guidance, as they have high contamination rates with skin flora; obtain deep tissue or bone samples when possible 1
- Avoid probing for fistulas during acute abscess drainage to prevent iatrogenic complications 1
- Do not assume phlegmon will remain stable—serial clinical assessment is essential to detect progression to abscess 1
Special Considerations
Diabetic Foot Infections
- Evaluate for osteomyelitis with bone sampling if suspected, as this changes antibiotic duration and may require surgical debridement 1
- Assess peripheral perfusion and consider vascular surgery consultation, as impaired blood flow predicts poor healing 1