Community-Acquired Pneumonia
System: Respiratory Infections · Parent MOC: MOC - Diseases by System
One-Sentence Definition
Acute infection of the pulmonary parenchyma acquired outside the hospital, ranging from mild outpatient disease to life-threatening sepsis.
Clinical Presentation
- Cough, fever, dyspnea, pleuritic pain; focal crackles / consolidation
- Elderly: may present with falls, delirium, fewer respiratory complaints
- Severity tools (e.g., CURB-65 / clinical judgment) guide site of care
Pathogens
| Context | Organism | Notes |
|---|---|---|
| Classic lobar | Streptococcus pneumoniae | Still top bacterial CAP |
| Alcoholism / aspiration associations | Klebsiella pneumoniae | Currant-jelly lore; mucoid Capsule |
| Post-viral / severe | Staphylococcus aureus | Incl. MRSA in selected risks |
| Atypicals | Mycoplasma, Legionella, Chlamydia (TBD) | Often PCR/urine Ag for Legionella |
| Viral | Influenza, SARS-CoV-2, RSV (TBD) | MOC - Virology |
Pathogenesis
- Inhalation/aspiration of organisms past airway defenses → alveolar inflammation
- Bacteremic pneumococcal disease → link Bloodstream and Sepsis
Diagnosis
| Step | Test | Note |
|---|---|---|
| Imaging | CXR / CT | Infiltrate supports diagnosis |
| Micro (inpatient / severe) | Blood culture, sputum Gram Stain/culture | Quality sputum matters |
| Adjunct | Urine pneumococcal / Legionella Ag | Syndrome-dependent |
| Molecular | Respiratory PCR panel | Colonization ≠ etiology for some bacteria |
Treatment Principles
- Empiric: cover pneumococcus ± atypicals per guideline/local resistance
- Escalate for MRSA/Pseudomonas only with risk factors (those → think HAP spectrum)
- Definitive: narrow with culture/AST; short courses when stable
- Supportive O₂, fluids; steroids in select severe cases per evidence/guidelines
Prevention
- Pneumococcal + influenza (+ COVID) vaccination
- Smoking cessation
Differential / Pitfalls
- PE, CHF, aspiration pneumonitis without infection, TB, cancer
- Treating nasal PCR positives as pneumonia without infiltrate/clinical fit
Learning Aids
Diagrams
Clinical Example
Example
Case: 45M, acute fever, rusty sputum, lobar consolidation; sputum Gram: GPC pairs.
Question: Likely pathogen and key prevention message after recovery?
Answer: Streptococcus pneumoniae. Ensure pneumococcal/influenza vaccine status addressed.
Related
- Respiratory Infections · Hospital-Acquired Pneumonia
- Streptococcus pneumoniae · Klebsiella pneumoniae
- MOC - Antimicrobials · PCR
Active Recall
- Most common typical bacterial cause of CAP?
- When to get blood cultures in CAP?
- How does colonization confound respiratory PCR?