Classification of Moderate vs Severe Purulent SSTIs
Moderate purulent SSTIs are defined by the presence of systemic signs of infection (fever >38°C, tachycardia >90 bpm, tachypnea >24 breaths/min, or abnormal WBC <12,000 or >12,000 cells/μL), while severe purulent SSTIs include patients who have failed incision and drainage plus oral antibiotics, exhibit these same systemic signs, or are immunocompromised. 1
Key Distinguishing Features
Moderate Purulent SSTI Criteria
The IDSA 2014 guidelines establish that moderate purulent infections require the presence of any of the following systemic inflammatory markers 1:
- Temperature >38°C (100.4°F) 1
- Heart rate >90 beats per minute 1
- Respiratory rate >24 breaths per minute 1
- White blood cell count <12,000 or >12,000 cells/μL 1
These patients have purulent collections (abscesses, furuncles, carbuncles) but demonstrate systemic involvement beyond the local infection. 1
Severe Purulent SSTI Criteria
Severe infections are identified by any of the following 1:
- Treatment failure: Patient has already undergone incision and drainage PLUS received oral antibiotics without improvement 1
- Systemic signs of infection: Same criteria as moderate (temperature >38°C, HR >90, RR >24, abnormal WBC) 1
- Immunocompromised status: Any condition causing impaired host defenses 1, 2
- Clinical signs of deeper infection: Bullae, skin sloughing, hypotension, or evidence of organ dysfunction 1
Algorithmic Approach to Classification
Step 1: Confirm purulent nature - Look for abscess, furuncle, carbuncle, or visible pus 1
Step 2: Assess for systemic signs - Check vital signs and laboratory values for the four cardinal markers listed above 1
Step 3: Evaluate treatment history - Determine if patient has failed prior I&D plus oral antibiotics 1
Step 4: Assess immune status - Identify immunocompromising conditions (diabetes, HIV, chemotherapy, chronic steroids, etc.) 1, 2
Step 5: Look for deep tissue involvement - Examine for bullae, necrosis, skin sloughing, disproportionate pain, or rapid progression 1, 3
If only systemic signs are present → Moderate
If treatment failure, immunocompromise, OR signs of deep infection are present → Severe
Critical Clinical Pitfalls
Do not underestimate immunocompromised patients: Even without overt systemic signs, these patients automatically qualify as severe due to unpredictable progression and higher mortality risk. 1, 2 This includes diabetics, elderly patients with multiple comorbidities, and those on immunosuppressive therapy. 1, 2
Recognize that "moderate" still requires systemic antibiotics: Unlike mild purulent SSTIs where I&D alone may suffice, moderate infections mandate antibiotic therapy in addition to drainage. 1 The presence of any systemic sign indicates bacteremia risk and necessitates antimicrobial coverage.
Watch for rapid progression: Severe infections can evolve quickly, particularly with signs like violaceous bullae, disproportionate pain relative to physical findings, or rapid spread of erythema beyond 5 cm. 3, 4 These suggest necrotizing infection and require immediate surgical consultation. 1
Treatment failure is a severity marker: If a patient returns after appropriate I&D and oral antibiotics without improvement, this automatically upgrades them to severe regardless of other findings. 1 This indicates either resistant organisms, inadequate source control, or deeper infection requiring IV therapy and possible hospitalization.
Additional Severity Considerations
The Eron classification system provides complementary context, where Class 3 patients (appearing toxic with fever, tachycardia, tachypnea, or hypotension) and Class 4 patients (sepsis syndrome or necrotizing infection) align with severe disease. 1 This reinforces that systemic toxicity, regardless of local findings, drives severity classification.
Abnormal white blood cell counts matter in both directions: Both leukocytosis (>12,000) and leukopenia (<12,000, particularly <4,000) indicate systemic involvement and qualify as moderate or severe depending on other factors. 1 Leukopenia may suggest overwhelming sepsis and warrants aggressive management.