What are the criteria and treatment approach for a patient suspected of having hospital-acquired pneumonia, considering their medical history and clinical presentation?

Hospital-Acquired Pneumonia Diagnostic Criteria

Hospital-acquired pneumonia (HAP) is defined as pneumonia occurring 48 hours or more after hospital admission that was not incubating at the time of admission, presenting with new lung infiltrate plus clinical evidence of infection including fever, purulent sputum, leukocytosis, and declining oxygenation. 1

Clinical Diagnostic Criteria

The diagnosis requires all of the following components:

• Timing: Pneumonia developing ≥48 hours after hospital admission 1
• Radiographic evidence: New or progressive lung infiltrate on chest imaging 1
• Clinical signs of infection including at least two of the following 1:
  • Fever >38°C or hypothermia <36°C
  • Leukocytosis (WBC >12,000/mm³) or leukopenia (WBC <4,000/mm³)
  • Purulent respiratory secretions
  • Declining oxygenation (increased oxygen requirements or worsening PaO2/FiO2 ratio)

Risk Stratification for Treatment Selection

The IDSA/ATS guidelines stratify HAP patients based on mortality risk and MRSA/multidrug-resistant gram-negative risk factors to guide empiric antibiotic selection. 1

High Mortality Risk Features

Patients with any of the following require aggressive dual gram-negative plus MRSA coverage 1:

• Need for mechanical ventilation due to pneumonia
• Septic shock requiring vasopressors at presentation
• IV antibiotic use within prior 90 days

MRSA Risk Factors

Add vancomycin or linezolid if any of the following are present 1:

• IV antibiotic use within prior 90 days
• Hospitalization in a unit where >20% of S. aureus isolates are methicillin-resistant
• MRSA prevalence unknown in your unit
• High risk of mortality (as defined above)
• Prior MRSA colonization or infection

Multidrug-Resistant Gram-Negative Risk Factors

Dual gram-negative coverage is required when 1:

• High risk of mortality is present (mechanical ventilation or septic shock)
• Receipt of IV antibiotics within prior 90 days
• Hospitalization ≥10 days prior to pneumonia onset 2
• Structural lung disease (bronchiectasis, cystic fibrosis)

Empiric Antibiotic Treatment Algorithm

Low-Risk Patients (No High Mortality Risk, No Recent Antibiotics)

Monotherapy with one of the following 1:

• Cefepime 2g IV every 8 hours
• Levofloxacin 750mg IV daily
• Imipenem 500mg IV every 6 hours
• Meropenem 1g IV every 8 hours

Moderate-Risk Patients (MRSA Risk Factors Present, No High Mortality Risk)

Gram-negative coverage PLUS MRSA coverage 1:

• Same gram-negative agent as above (cefepime, levofloxacin, imipenem, or meropenem)
• PLUS vancomycin 15mg/kg IV every 8-12 hours (target trough 15-20 mg/mL) OR linezolid 600mg IV every 12 hours

High-Risk Patients (High Mortality Risk or Recent IV Antibiotics)

Dual gram-negative coverage from different classes PLUS MRSA coverage 1:

• Antipseudomonal beta-lactam: Piperacillin-tazobactam 4.5g IV every 6 hours 3 OR cefepime 2g IV every 8 hours OR meropenem 1g IV every 8 hours
• PLUS second antipseudomonal agent: Ciprofloxacin 400mg IV every 8 hours OR levofloxacin 750mg IV daily OR amikacin 15-20mg/kg IV daily
• PLUS MRSA coverage: Vancomycin 15mg/kg IV every 8-12 hours OR linezolid 600mg IV every 12 hours

Critical note: The FDA label specifically recommends piperacillin-tazobactam 4.5g every 6 hours plus an aminoglycoside for nosocomial pneumonia 3

Renal Dose Adjustments

• For creatinine clearance ≤40 mL/min, reduce piperacillin-tazobactam dosing according to degree of renal impairment 1, 3
• Adjust vancomycin dosing based on renal function and therapeutic drug monitoring 1

Treatment Duration and De-escalation

De-escalate therapy at 48-72 hours based on culture results and clinical response 1:

• Discontinue MRSA coverage if cultures are negative for MRSA and patient is clinically improving
• Narrow gram-negative coverage based on susceptibility results
• Standard treatment duration is 7-8 days for responding patients 1

Monitoring Clinical Response

Clinical stability criteria include 1:

• Temperature ≤37.8°C
• Heart rate ≤100 bpm
• Respiratory rate ≤24 breaths/min
• Systolic BP ≥90 mmHg

Measure C-reactive protein on days 1 and 3-4 to assess response 1

If no improvement by 72 hours, consider 1:

• Complications (empyema, lung abscess)
• Resistant organisms not covered by initial regimen
• Alternative diagnoses (pulmonary embolism, heart failure, malignancy)
• Infection at another site

Critical Pitfalls to Avoid

• Do not use ciprofloxacin monotherapy for HAP due to poor activity against S. pneumoniae and lack of anaerobic coverage 1
• Do not routinely add anaerobic coverage unless lung abscess or empyema is suspected 1
• Do not delay antibiotics waiting for cultures, as this is consistently associated with increased mortality 1
• Do not assume all HAP requires MRSA coverage—use the risk stratification algorithm 1
• Avoid using two β-lactams together in combination regimens; select agents from different antibiotic classes 1
• Do not underdose in critically ill patients: Use piperacillin-tazobactam 4.5g every 6 hours (not 3.375g) for nosocomial pneumonia 3

Institution-Specific Considerations

Tailor empiric regimens to local antibiogram data, as the distribution of pathogens and antimicrobial susceptibilities varies significantly by institution 2. For example, if local data shows ciprofloxacin resistance >20% among gram-negative HAP isolates, an aminoglycoside should be preferentially selected as the second antipseudomonal agent 2.