How do you assess the severity of an infection?

Assessing Infection Severity

Use the APACHE II scoring system within 24 hours of hospital admission to stratify infection severity, as it demonstrates the highest discriminative power (AUC 0.81) for predicting mortality and guides treatment intensity. 1, 2

Primary Severity Assessment Tool

APACHE II Score is the gold standard for severity assessment in hospitalized patients with serious infections, particularly complicated intra-abdominal infections and critically ill patients. 1, 2

APACHE II Components and Calculation

The score incorporates three key domains: 1, 2

• 12 physiologic parameters (temperature, mean arterial pressure, heart rate, respiratory rate, oxygenation, arterial pH, serum sodium, potassium, creatinine, hematocrit, white blood cell count, Glasgow Coma Scale)
• Age (increasing points for older patients)
• Chronic health conditions (history of severe organ insufficiency or immunocompromise)

Risk Stratification Using APACHE II

Categorize patients into three risk groups based on APACHE II scores: 1

• Low risk: 0-10 points (baseline mortality risk)
• Intermediate risk: 11-15 points (predicted mortality 20-30%)
• High risk: >15 points (predicted mortality ~50%)

Clinical Actions Based on APACHE II Score

For APACHE II ≥8: Implement enhanced monitoring protocols with 83.3% sensitivity and 91% specificity for mortality prediction. 2

For APACHE II ≥15-17: Consider continuous or extended infusion of beta-lactam antibiotics rather than intermittent dosing, which improves clinical cure rates and reduces mortality. 2

For APACHE II ≥20: Continuous beta-lactam administration shows reduced mortality (RR 0.73) compared to intermittent dosing, particularly for anti-pseudomonal therapy. 2

Site-Specific Severity Assessment

For Diabetic Foot Infections

Use the IDSA classification system based on clinical signs: 1

Uninfected (Grade 1): No purulence or inflammation present. 1

Mild (Grade 2): 1

• Local infection involving only skin and subcutaneous tissue
• Erythema >0.5 cm to ≤2 cm around the wound
• No systemic signs

Moderate (Grade 3): 1

• Erythema >2 cm around the wound, OR
• Infection involving deeper structures (abscess, osteomyelitis, septic arthritis, fasciitis)
• No systemic inflammatory response

Severe (Grade 4): 1

• Local infection PLUS systemic inflammatory response syndrome (SIRS)
• SIRS defined as ≥2 of: temperature >38°C or <36°C, heart rate >90 bpm, respiratory rate >20 breaths/min, WBC >12,000 or <4,000 cells/μL or ≥10% bands

For Complicated Intra-Abdominal Infections

Beyond APACHE II, identify additional independent mortality risk factors: 1

Non-modifiable factors: Advanced age, presence of malignancy, degree of peritoneal involvement. 1

Potentially modifiable factors (prioritize these): 1

• Renal dysfunction (10 studies identified this)
• Cardiovascular dysfunction (5 studies)
• Respiratory dysfunction (4 studies)
• Sepsis or shock (3-5 studies)
• Inadequate source control (3 studies)
• Delayed source control >24 hours (5 studies)

Alternative Scoring Systems

SOFA Score

Use for sequential monitoring in ICU patients with sepsis, but NOT for initial severity stratification. 1, 3, 4

• Evaluates 6 organ systems (respiratory, cardiovascular, neurologic, renal, hepatic, coagulation)
• Does NOT include age or comorbidities (major limitation)
• Lower discriminative power than APACHE II (AUC 0.75 vs 0.81)
• Not suitable for categorizing low-moderate severity patients without organ failure in first 24 hours

SOFA ≥2 points increase from baseline with documented infection defines sepsis per Sepsis-3 criteria. 4

When APACHE II is Not Feasible

For emergency department triage: Use modified National Early Warning Score (mNEWS) ≥5, which has 91.18% sensitivity for 30-day mortality prediction. 5

For community-acquired pneumonia: Use PSI (Pneumonia Severity Index) with highest AUC 0.81, or CURB65 (AUC 0.80) for simpler bedside assessment. 6

Serial Monitoring Strategy

Recalculate severity scores every 48-72 hours to track disease trajectory: 2, 3, 4

• Decreasing scores indicate treatment response
• Static or increasing scores signal treatment failure requiring escalation
• Pattern changes may indicate complications like sepsis

Critical Pitfalls to Avoid

Do not rely on inflammatory markers alone (temperature, WBC, ESR, CRP) as they are absent in up to 50% of patients with severe infections, though when present they predict worse outcomes. 1

Do not use SOFA for initial risk stratification in non-ICU patients or those without organ failure—it was designed for sequential assessment, not initial categorization. 1, 3

Do not delay severity assessment—calculate within 24 hours of admission to guide appropriate therapy intensity and urgency of source control. 1

Do not ignore clinical context—scoring systems augment but do not replace clinical judgment, particularly regarding patient-specific factors like immunosuppression or critical ischemia. 1