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Respiratory · Clinical Topics
Community-Acquired Pneumonia
Community-Acquired Pneumonia (CAP) is an acute infection of the lung parenchyma occurring in individuals who have not been in hospital within the prior 14 days. Streptococcus pneumoniae is the most common pathogen. Severity assessment is performed using the CURB-65 score, which guides management location (outpatient vs. inpatient) and antibiotic choice. Clinical features include fever, productive cough, and pleuritic chest pain with evidence of consolidation on chest X-ray.
📌 Learning Objectives
- Describe the epidemiology, aetiology, and pathophysiology of Community-Acquired Pneumonia (CAP).
- Identify the key clinical features and diagnostic criteria for CAP.
- Apply the CURB-65 score to assess CAP severity and guide management decisions.
- Explain the principles of antibiotic selection and supportive care for CAP.
- Discuss potential complications of CAP and indications for follow-up.
📋 Overview
CAP remains a leading cause of death from infection in the UK. The most common causative organisms are Streptococcus pneumoniae (approx. 50%), followed by Haemophilus influenzae and 'atypical' pathogens like Mycoplasma pneumoniae, Legionella pneumophila, and Chlamydia psittaci. CAP is diagnosed clinically based on symptoms of lower respiratory tract infection (LRTI) and new focal signs on examination, confirmed by a new opacity on CXR. The CURB-65 score is vital: Confusion (AMTS <=8), Urea (>7 mmol/L), Respiratory rate (>=30), Blood pressure (SBP <90 or DBP <=60), and Age (>=65). Scores 0-1 (Low risk - treat at home), Score 2 (Intermediate risk - consider hospital assessment), Score 3-5 (High risk - inpatient treatment, consider ITU). Management involves prompt antibiotic administration (within 4 hours), oxygen therapy to target SpO2 94-98%, and fluid resuscitation. Antibiotic choice for low severity is usually oral Amoxicillin. Moderate/High severity requires dual therapy to cover atypicals (e.g., Amoxicillin/Co-amoxiclav plus Clarithromycin). Legionella should be suspected in recent travellers or outbreaks (presents with hyponatraemia/deranged LFTs). Follow-up CXR at 6 weeks is mandatory in all patients over 50 or smokers to ensure resolution and rule out underlying malignancy.
🔬 Basic Science
Pneumonia occurs when infectious agents bypass the respiratory tract's upper defences (mucociliary clearance, alveolar macrophages) and reach the distal bronchioles and alveoli. The resulting inflammatory response leads to the accumulation of exudate (fluid, protein, and white blood cells) within the alveolar spaces. This 'consolidation' impairs gas exchange, leading to V/Q mismatch and hypoxia. The process traditionally follows four stages: 1. Congestion (vascular engorgement), 2. Red hepatisation (alveoli filled with RBCs and fibrin), 3. Grey hepatisation (lysis of RBCs, persistence of fibrin/macrophages), and 4. Resolution (enzymatic digestion of exudate). Lobar pneumonia involves a whole lobe and is typically caused by Strep. pneumoniae. Bronchopneumonia presents as patchy consolidation across multiple segments, often in the basal regions, following the terminal bronchioles.
🏥 Clinical Relevance
Presenting symptoms include cough (usually productive of rust-coloured or purulent sputum), fever, rigors, malaise, and pleuritic chest pain. Clinical signs on examination of the consolidated area include: tachypnoea, reduced expansion, dull percussion note, increased tactile vocal fremitus, bronchial breathing (harsh sound with a gap between inspiration/expiration), and fine end-expiratory crackles. Complications include pleural effusion (sterile), empyema (infected pleural space), lung abscess (especially with Staph. aureus or Klebsiella), respiratory failure, and sepsis. Klebsiella pneumonia is characteristically seen in alcoholics and causes 'red currant jelly' sputum. Specific exposures provide clues: parrots (psittacosis), air conditioning/hotels (Legionella), or farming/lambing (Q fever).
🧪 Investigations
Bedside: SpO2, AMTS (for CURB-65). Bloods: FBC (leucocytosis), CRP (elevated, used for trending), U&Es (for CURB-65), LFTs (deranged in Legionella). Imaging: Chest X-ray (Required for diagnosis - shows lobar or patchy infiltrates). Microbiology: Sputum culture/Gram stain; Blood cultures (if CURB-65 >=2); Urinary antigens (for Legionella and Pneumococcus in moderate/severe CAP); Throat swabs for viral PCR (COVID/Flu).
💊 Management
**Low Severity (CURB-65 0-1):** Home treatment. Oral Amoxicillin 500mg TDS for 5 days (alternatives: Doxycycline 200mg then 100mg OD, or Clarithromycin 500mg BD if penicillin-allergic).
**Moderate Severity (CURB-65 2):** Hospital admission. Oral dual therapy: Amoxicillin 500mg TDS + Clarithromycin 500mg BD (or Doxycycline if Clarithromycin not tolerated). Switch to IV if oral not possible.
**High Severity (CURB-65 3-5):** Urgent hospital admission. IV Benzylpenicillin 1.2g QDS + IV Clarithromycin 500mg BD. If penicillin-allergic: IV Ceftriaxone 2g OD + IV Clarithromycin 500mg BD, or IV Levofloxacin 500mg BD as monotherapy. Note: IV Co-amoxiclav is no longer recommended as first-line for high-severity CAP (NICE NG138 2023 antimicrobial update).
**Supportive:** Oxygen to maintain SpO2 94-98% (88-92% if COPD risk), IV fluids for dehydration/hypotension, analgesia for pleuritic pain. Re-evaluate clinically at 48-72 hours.
**Moderate Severity (CURB-65 2):** Hospital admission. Oral dual therapy: Amoxicillin 500mg TDS + Clarithromycin 500mg BD (or Doxycycline if Clarithromycin not tolerated). Switch to IV if oral not possible.
**High Severity (CURB-65 3-5):** Urgent hospital admission. IV Benzylpenicillin 1.2g QDS + IV Clarithromycin 500mg BD. If penicillin-allergic: IV Ceftriaxone 2g OD + IV Clarithromycin 500mg BD, or IV Levofloxacin 500mg BD as monotherapy. Note: IV Co-amoxiclav is no longer recommended as first-line for high-severity CAP (NICE NG138 2023 antimicrobial update).
**Supportive:** Oxygen to maintain SpO2 94-98% (88-92% if COPD risk), IV fluids for dehydration/hypotension, analgesia for pleuritic pain. Re-evaluate clinically at 48-72 hours.
Revision Resources – expand the sections below for high-yield notes, exam pearls, key facts and further reading.
MLA High-Yield Notes & Quick Revision ⌄
A CURB-65 of 3+ warrants a 'Sepsis Six' approach. If a patient is not improving on standard antibiotics, think of 'atypicals' or complications like empyema (demonstrated by non-resolving fever and pleural fluid on CXR). Always repeat CXR in 6 weeks.
Acute respiratory distress
Fever of unknown origin
Cough
Chest pain
Sepsis
- CAP is lung infection outside hospital.
- S. pneumoniae is primary pathogen.
- Diagnosed by symptoms, signs, and new CXR opacity.
- CURB-65 assesses severity and guides treatment location.
- Low risk: oral Amoxicillin.
- Moderate/High risk: dual therapy (e.g., Amoxicillin + Clarithromycin).
Exam Pearls ⌄
⭐ High Yield
Streptococcus pneumoniae is the most common cause of CAP.
CURB-65 score guides CAP management: 0-1 (home), 2 (hospital assessment), 3-5 (inpatient/ITU).
Antibiotics for low severity CAP is typically oral Amoxicillin.
Dual therapy (e.g., Amoxicillin + Clarithromycin) is used for moderate/high severity CAP to cover atypical pathogens.
Target oxygen saturation for CAP is 94-98%.
A follow-up CXR at 6 weeks is recommended, especially for smokers or those over 50, to rule out underlying pathology.
💡 Clinical Pearl
Sepsis: Severe CAP can rapidly progress to sepsis, requiring prompt recognition and management.
Pleural Effusion: Parapneumonic effusions are a common complication of CAP, sometimes requiring drainage.
Lung Cancer: Persistent or recurrent pneumonia, especially in smokers, warrants investigation for underlying malignancy.
⚠️ Exam Tip — Common Mistakes
Underestimating the severity of CAP, especially in elderly or immunocompromised patients.
Failing to consider atypical pathogens, particularly in patients not responding to standard therapy.
Not performing a CURB-65 score, leading to inappropriate management location.
Delaying antibiotic administration, which can worsen patient outcomes.
Discharging patients without adequate safety-netting advice or follow-up.
Key Facts ⌄
Strep. pneumoniae is the most common cause (gram-positive diplococci).
CURB-65 score (or CRB-65 in GP) is the cornerstone of risk stratification.
Consolidation signs: Increased vocal resonance, dull percussion, bronchial breathing.
Legionella presents with 'extra-pulmonary' features like hyponatraemia.
Mycoplasma is common in younger patients and may cause erythema multiforme.
A follow-up CXR at 6 weeks is essential to exclude underlying lung cancer.
Antibiotics should be started within 4 hours of arrival in hospital.
Related Topics ⌄
References ⌄
- NICE NG138 - Pneumonia
- BNF
- BTS Guidelines for CAP
Further Resources
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