Community-Acquired Pneumonia (Adult)

1. Overview

Community-Acquired Pneumonia (CAP) is an acute infection of the lung parenchyma (alveoli and respiratory bronchioles) in a patient who has not been hospitalised or resided in a long-term care facility for ≥14 days before the onset of symptoms. [1] It remains the leading infectious cause of death worldwide and a major reason for hospitalisation in the elderly.

The most common pathogen is Streptococcus pneumoniae (Pneumococcus), although the spectrum of causative organisms is changing with the rise of viral pathogens (Influenza, COVID-19) and the recognition of "Atypical" organisms. [2]

Management is driven by clinical risk stratification using the CURB-65 score, which determines the site of care (home vs. hospital vs. ICU) and the choice of empirical antibiotics. A key update in 2024-2025 is the validated use of adjunctive corticosteroids in severe CAP to reduce mortality and the need for mechanical ventilation. [3]

2. Epidemiology

Global Burden

• Incidence: 5-11 per 1,000 adults annually. This rises significantly in the winter months and in those > 65 years.
• Mortality: Low-risk CAP (less than 1%), Moderate-risk (8-10%), Severe-risk (> 30%).

Specific Pathogen Associations

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3. Aetiology & Pathophysiology

⚠️ THE 7-STEP MOLECULAR MECHANISM

1. Microaspiration & Entry: Pathogens enter the lower respiratory tract, primarily via silent microaspiration of oropharyngeal flora.
2. Alveolar Invasion: Bacteria bypass the mucociliary escalator and adhere to alveolar Type I pneumocytes.
3. Cytokine Cascade: Alveolar macrophages detect Pathogen-Associated Molecular Patterns (PAMPs) and release TNF-α, IL-1, and IL-8.
4. Congestion Phase (24h): IL-8 recruits neutrophils. Capillary permeability increases, leading to a protein-rich exudate (edema) filling the alveoli.
5. Red Hepatisation (Day 2-4): Massive confluence of neutrophils, erythrocytes, and fibrin fills the alveolar spaces. The lung becomes firm, airless, and resembles the liver ("hepatisation").
6. Grey Hepatisation (Day 4-8): Erythrocytes disintegrate. Fibrinopurulent exudate persists, but the red tint fades. This is the peak of V/Q mismatch and Type 1 Respiratory Failure.
7. Resolution or Complication: Macrophages clear the debris. Lung architecture is typically preserved (unlike TB), but failure to clear can lead to Empyema or Abscess formation. [4, 5]

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4. Clinical Presentation

Symptoms

• Productive Cough: Green/yellow sputum. (Rusty = Pneumococcal).
• Fever & Rigors: Sudden onset.
• Pleuritic Chest Pain: Suggests involvement of the parietal pleura.

Physical Signs

1. Bronchial Breathing: Harsh breath sounds with an expiratory gap (pathognomonic for consolidation).
2. Dullness to Percussion: Over the affected lobe.
3. Increased Vocal Resonance: "99" sounds clearer over the consolidated area.
4. Coarse Inspiratory Crackles: Due to fluid-filled alveoli popping open.

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5. Investigations

Bedside

• CURB-65 Score:
  • Confusion (AMTS less than 8).
  • Urea (> 7 mmol/L).
  • Respiratory Rate (≥30/min).
  • Blood Pressure (Sys less than 90 or Dia ≤60).
  • 65 (Age ≥65).

Imaging & Laboratory

• CXR: Mandatory. Look for Air Bronchograms (visible air-filled bronchi against opaque consolidated lung).
• Urinary Antigens: For Legionella and Pneumococcus. Fast and unaffected by early antibiotics.
• Blood Cultures: Essential in CURB ≥2.
• Sputum Culture: High yield only if good quality (few epithelial cells).

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6. Management: The Clinical Algorithm

1. Antibiotic Selection (Empirical)

• CURB 0-1 (Mild): Amoxicillin 500mg TDS PO (5 days).
• CURB 2 (Moderate): Amoxicillin + Clarithromycin (to cover atypicals).
• CURB 3-5 (Severe): Co-amoxiclav IV + Clarithromycin IV. [6]

2. The "CAPE COD" Revolution: Corticosteroids

The CAPE COD trial (2023) proved that Hydrocortisone (200mg/day) started early in severe CAP reduces 28-day mortality by ~50% compared to placebo. It is now a Class IIa recommendation for severe CAP. [7]

3. Oxygen & Fluids

• Target SpO2 94-98%.
• Judicious fluid resuscitation; avoid over-hydration in the elderly (risk of HF).

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7. Complications

1. Parapneumonic Effusion: Simple fluid in the pleural space.
2. Empyema: Infected pleural space (pH less than 7.2, low glucose, high LDH). Requires drainage.
3. Lung Abscess: Cavitating lesion on CXR. Common with S. aureus or Klebsiella.
4. Sepsis & MODS: Systemic spread of the inflammatory cascade.

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8. Evidence: Landmark Trials

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9. Single Best Answer (SBA) Questions

Question 1

A 72-year-old male presents with confusion, RR 32, BP 88/54, and Urea 9.2 mmol/L. CXR shows right lower lobe consolidation. What is his CURB-65 score and appropriate management?

• A) CURB 2; Outpatient Amoxicillin
• B) CURB 3; Hospital admission, IV Co-amoxiclav + Clarithromycin
• C) CURB 4; Urgent ICU review, IV Co-amoxiclav + Clarithromycin + Steroids
• D) CURB 5; Palliative care
• E) CURB 3; IV Amoxicillin only
• Answer: C. The patient scores 4 (Confusion, Urea, RR, BP). Age is already ≥65. This is severe CAP requiring ICU review and, per newer evidence, adjunctive steroids.

Question 2

A 30-year-old traveler presents with pneumonia, hyponatraemia (Na 128), and deranged LFTs. What is the most appropriate diagnostic test and treatment?

• A) Sputum culture; Amoxicillin
• B) Cold agglutinins; Doxycycline
• C) Urinary antigen; Levofloxacin
• D) Blood film; Quinine
• E) CT Head; Ceftriaxone
• Answer: C. This is the classic triad for Legionella. Urinary antigen is the fastest test, and fluoroquinolones (Levofloxacin) are first-line.

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10. Viva Scenario: The "Non-Resolving" Pneumonia

Examiner: "Your patient has been on IV Co-amoxiclav for 72 hours for a CURB-3 pneumonia, but they are still spiking fevers of 39°C. What is your differential and next step?"

Candidate:

1. Complications: I would first exclude Empyema or a Lung Abscess.
2. Alternative Pathogens: I would consider organisms not covered by standard therapy, such as MRSA, Pseudomonas, or Mycobacterium tuberculosis.
3. Underlying Pathology: I must consider that the "pneumonia" is a secondary event to an obstructing lung malignancy or bronchiectasis.
4. Next Step: I would order an urgent Ultrasound of the Pleura (to look for septated fluid) or a CT Thorax with contrast.

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11. Patient Explanation

"Pneumonia is an infection that fills the tiny air sacs in your lungs with fluid and pus, making it hard for oxygen to reach your blood. It is much more serious than a simple 'bronchitis.' We use a scoring system to decide if you need to be in hospital. You will be treated with antibiotics to kill the bacteria and potentially a short course of steroids to settle the intense inflammation. You should feel better in 48-72 hours, but the 'post-pneumonia' fatigue can last for several weeks. We will repeat your X-ray in 6 weeks to make sure the lung has fully cleared."

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12. References

1. Metlay JP, et al. Diagnosis and Treatment of Adults with Community-acquired Pneumonia (ATS/IDSA). Am J Respir Crit Care Med. 2019. [PMID: 31573380]
2. Dequin PF, et al. Hydrocortisone in Severe Community-Acquired Pneumonia (CAPE COD). N Engl J Med. 2023. [PMID: 36942789]
3. Lim WS, et al. BTS Guidelines for the Management of Community Acquired Pneumonia in Adults. Thorax. 2009 (Updated 2024). BTS
4. Postma DF, et al. Antibiotic Treatment Strategies for Community-Acquired Pneumonia. N Engl J Med. 2015. [PMID: 25830421]
5. NICE NG138. Pneumonia (community-acquired): antimicrobial prescribing. 2019. NICE

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Last Updated: 2026-01-05 | MedVellum Editorial Team