Pneumonia Risk Groups and Associated Etiology

Patient Stratification

The American Thoracic Society recommends that patients be stratified according to age, cardiopulmonary comorbidities, residence in long-term care facilities, and risk factors for resistant pathogens, as these determine bacterial etiology and prognosis more than age alone 1, 2
The Infectious Diseases Society of America suggests that Streptococcus pneumoniae is the most common pathogen in all age groups, accounting for 48% of cases in adults 3
Mycoplasma pneumoniae represents 10-13% of cases, especially in patients aged 5-25 years 5
Chlamydia pneumoniae is detected in 12-17% of outpatients 3
Respiratory viruses account for 10% of cases 3

The American College of Chest Physicians identifies risk factors for drug-resistant Streptococcus pneumoniae (DRSP), including age ≥65 years, recent β-lactam therapy, alcoholism, multiple medical comorbidities, and immunosuppression 1
The Society for Healthcare Epidemiology of America notes that risk factors for Pseudomonas aeruginosa include structural lung disease, corticosteroid therapy, recent broad-spectrum antibiotic use, and malnutrition 1, 2

Post-influenza pneumonia is associated with Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, and Streptococcus pyogenes group A β-hemolytic 4, 5
Community-acquired aspiration pneumonia is associated with normal oropharyngeal flora, including aerobic and anaerobic bacteria 5
Nosocomial or long-term care facility-acquired aspiration pneumonia is associated with gram-negative bacilli and Staphylococcus aureus 5

The American Thoracic Society recommends a stepwise approach to patient stratification, including determination of treatment location, identification of cardiopulmonary disease, and evaluation of modifying factors such as risk factors for DRSP and gram-negative bacteria 1, 2
The Infectious Diseases Society of America suggests adjusting empirical antibiotic coverage according to the identified patient group 1, 2