Hospital-Acquired Pneumonia vs Community-Acquired Pneumonia: Inpatient Treatment
Hospital-acquired pneumonia (HAP) requires broader-spectrum antibiotics targeting resistant organisms including MRSA and Pseudomonas, while community-acquired pneumonia (CAP) typically needs narrower coverage focused on typical respiratory pathogens—the key difference is HAP's mandatory Staphylococcus aureus coverage and risk-stratified approach to multidrug-resistant organisms.
Hospital-Acquired Pneumonia Treatment Algorithm
HAP treatment is fundamentally driven by mortality risk stratification and MRSA risk factors 1.
Low Mortality Risk, No MRSA Risk Factors
For patients without high mortality risk (no ventilatory support needed, no septic shock) and no MRSA risk factors, use monotherapy with MSSA coverage 1:
- Piperacillin-tazobactam 4.5 g IV q6h, OR
- Cefepime 2 g IV q8h, OR
- Levofloxacin 750 mg IV daily, OR
- Imipenem 500 mg IV q6h, OR
- Meropenem 1 g IV q8h
MRSA Risk Factors Present (But Low Mortality Risk)
MRSA risk factors include: IV antibiotics within 90 days, unit MRSA prevalence >20% or unknown, or prior MRSA detection 1.
Use the same monotherapy options as above PLUS add MRSA coverage 1:
- Vancomycin 15 mg/kg IV q8-12h (target trough 15-20 mg/mL; consider 25-30 mg/kg loading dose for severe illness), OR
- Linezolid 600 mg IV q12h
High Mortality Risk or Recent IV Antibiotics
High mortality risk = ventilatory support needed for pneumonia or septic shock 1.
Use dual antipseudomonal therapy (avoiding two β-lactams) PLUS MRSA coverage 1:
Choose TWO from different classes:
- Piperacillin-tazobactam 4.5 g IV q6h
- Cefepime or ceftazidime 2 g IV q8h
- Levofloxacin 750 mg IV daily or ciprofloxacin 400 mg IV q8h
- Imipenem 500 mg IV q6h or meropenem 1 g IV q8h
- Aminoglycoside: amikacin 15-20 mg/kg IV daily, gentamicin 5-7 mg/kg IV daily, or tobramycin 5-7 mg/kg IV daily
- Aztreonam 2 g IV q8h
PLUS MRSA coverage:
- Vancomycin (dosed as above) OR linezolid 600 mg IV q12h
Critical caveat: If structural lung disease (bronchiectasis, cystic fibrosis) is present, always use dual antipseudomonal coverage regardless of other risk factors 1.
Community-Acquired Pneumonia Treatment Algorithm
CAP treatment is stratified by severity and drug-resistance risk factors 2.
Nonsevere CAP (No ICU Criteria)
Standard regimen without resistance risk factors 2:
- β-lactam PLUS macrolide: Ampicillin-sulbactam 1.5-3 g q6h, cefotaxime 1-2 g q8h, ceftriaxone 1-2 g daily, or ceftaroline 600 mg q12h PLUS azithromycin 500 mg daily or clarithromycin 500 mg twice daily, OR
- Respiratory fluoroquinolone monotherapy: Levofloxacin 750 mg daily or moxifloxacin 400 mg daily
If MRSA risk factors present (recent hospitalization with parenteral antibiotics): Add vancomycin or linezolid AND obtain nasal PCR/cultures to allow de-escalation 2.
If Pseudomonas risk factors present (prior respiratory isolation of Pseudomonas): Add antipseudomonal coverage per HAP guidelines and obtain cultures 2.
Severe CAP (ICU Admission Required)
Use β-lactam PLUS macrolide OR β-lactam PLUS fluoroquinolone 2:
- Same β-lactam options as nonsevere CAP
- PLUS azithromycin 500 mg daily OR levofloxacin 750 mg daily or moxifloxacin 400 mg daily
Add MRSA coverage if risk factors present (vancomycin or linezolid) 2.
Add Pseudomonas coverage if risk factors present using antipseudomonal agents from HAP regimens 2.
Key Differences Between HAP and CAP
Pathogen Coverage
HAP mandates Staphylococcus aureus coverage in all cases (either MSSA or MRSA depending on risk factors), while CAP focuses on typical respiratory pathogens (Streptococcus pneumoniae, Haemophilus influenzae, atypical organisms) 1, 2.
HAP has higher rates of multidrug-resistant gram-negative organisms requiring broader-spectrum β-lactams and consideration of dual antipseudomonal therapy 1. CAP rarely requires antipseudomonal coverage unless specific risk factors exist 2.
Risk Stratification Approach
HAP uses mortality risk (ventilatory support, septic shock) and antibiotic exposure history (IV antibiotics within 90 days) as primary stratification 1.
CAP uses severity criteria (ICU admission need) and drug-resistance risk factors (recent hospitalization with parenteral antibiotics, prior organism isolation) 2.
Antibiotic Selection Philosophy
HAP empiric therapy is inherently broader because nosocomial pathogens are more resistant. Even "low-risk" HAP receives agents like piperacillin-tazobactam or cefepime that would be reserved for severe CAP 1, 2.
CAP allows narrower initial therapy in nonsevere cases, with macrolide monotherapy or fluoroquinolone monotherapy being acceptable for outpatients, whereas HAP never uses such narrow coverage 2.
Common Pitfalls
Do not use CAP regimens for HAP—the microbiology is fundamentally different, and inadequate coverage increases mortality 1.
Always assess local antibiograms—the 20% MRSA threshold is institutional-specific and should guide your empiric MRSA coverage decisions 1.
De-escalate based on cultures—both HAP and CAP guidelines emphasize obtaining cultures before antibiotics in appropriate patients to allow narrowing of therapy 1, 2.
Recent antibiotic exposure is critical—IV antibiotics within 90 days dramatically increases resistance risk in both HAP and CAP, mandating broader coverage 1, 2.
Severe penicillin allergy requires special consideration—if using aztreonam, you must add separate MSSA coverage since aztreonam lacks gram-positive activity 1.