Defining and Scoring for Multidrug-Resistant (MDR) Bacteria in Community-Acquired Pneumonia
Definition of MDR Organisms in CAP
MDR organisms in CAP are defined by specific healthcare exposures and risk factors, not by time of onset alone, and include pathogens resistant to multiple antibiotic classes including MRSA and Pseudomonas aeruginosa. 1
The key criteria for identifying patients at risk for MDR organisms include:
Healthcare-Associated Risk Factors (HCAP Criteria)
- Received intravenous antibiotics within 90 days 1
- Recent hospitalization for more than 48 hours within 90 days 1
- Residence in nursing home or long-term care facility with high prevalence of MDROs 1
- Received long-term hemodialysis within 30 days 1
- Received chemotherapy or chronic wound care 1
- Risk of aspiration and prior oropharyngeal colonization with MDROs 1
Important Caveat on HCAP Classification
The 2019 IDSA guidelines recommend abandoning the HCAP classification entirely and only covering empirically for MRSA or P. aeruginosa when locally validated risk factors for either specific pathogen are present 2. This represents a significant shift from older guidelines that broadly recommended empiric MDRO coverage for all HCAP patients 1.
Prevalence of MDR Organisms in CAP
The actual prevalence of MDR organisms varies significantly by institution:
- Overall prevalence ranges from 3.6% to 21.5% of all CAP patients with microbiological data 3, 4
- Among culture-positive CAP cases, MDR organisms account for 21.5% to 41.3% of isolates 3, 4
- The most common MDR pathogens identified are Acinetobacter baumannii (17.9%), Klebsiella pneumoniae (9.8%), and Pseudomonas aeruginosa (6.7%) 4
Risk Stratification Algorithm
High-Risk Patients Requiring Empiric MDRO Coverage
Only provide empiric MDRO coverage when locally validated risk factors are present, not based on HCAP criteria alone 2. The strongest validated risk factors include:
- Previous hospitalization within 90 days (strongest predictor) 3, 4
- Antibiotic use within 90 days 3
- Malignancy 4
- Cardiovascular disease 4
- Structural lung disease (COPD, bronchiectasis) 4
- Hospitalization for ≥5 days (for HAP/VAP specifically) 1
Low-Risk Patients NOT Requiring Empiric MDRO Coverage
Patients without the above risk factors should receive standard CAP therapy targeting typical pathogens (S. pneumoniae, H. influenzae, atypical organisms) 2, 5.
Clinical Implications and Management Approach
When to Cover for Specific MDR Pathogens
For MRSA:
- Add vancomycin or linezolid only when locally validated risk factors are present 2, 5
- Consider in patients with prior MRSA infection, recent hospitalization, or CAP following influenza 1, 5
For Pseudomonas aeruginosa:
- Cover empirically only with severe CAP requiring ICU admission plus specific risk factors 5
- Use antipseudomonal β-lactam (piperacillin-tazobactam, cefepime, or carbapenem) plus either ciprofloxacin/levofloxacin or an aminoglycoside 1
Critical Pitfall to Avoid
Do NOT provide broad-spectrum empiric MDRO coverage to all patients based solely on HCAP criteria, as this approach leads to unnecessary antibiotic exposure without mortality benefit 2, 6. The evidence shows that inappropriate empirical therapy (IEAT) for MDR organisms is not an independent risk factor for mortality when appropriate definitive therapy is provided once cultures return 6.
Recommended Approach
- Obtain blood and sputum cultures before initiating antibiotics in all hospitalized CAP patients 1, 2
- Start standard CAP therapy immediately (β-lactam plus macrolide or respiratory fluoroquinolone) 5
- Add MDRO coverage only if locally validated risk factors are present 2
- De-escalate therapy within 48-72 hours based on culture results and clinical response 2, 5
This targeted approach balances the need for prompt effective therapy against the risks of promoting further antimicrobial resistance 2, 3.