Key Facts

Community-acquired pneumonia (CAP) is defined as an acute lower respiratory tract infection with radiological evidence of consolidation, acquired outside hospital or within 48 hours of admission. It is the most common infectious cause of death in developed countries.

Epidemiology

• Incidence: 5-11 per 1,000 adults per year in the UK
• Peak incidence in winter months
• Higher in extremes of age, immunosuppressed, chronic lung disease
• 22-42% require hospitalisation
• 5-10% of hospitalised CAP patients require ICU admission

Aetiology

• Streptococcus pneumoniae (~30-40%): most common; rusty sputum, lobar consolidation
• Haemophilus influenzae (~5-10%): COPD patients
• Mycoplasma pneumoniae (~5-15%): younger adults, 4-yearly epidemics, extrapulmonary features
• Legionella pneumophila (~2-5%): travel, cooling towers; hyponatraemia, deranged LFTs
• Staphylococcus aureus (~1-5%): post-influenza, IV drug users, cavitation
• Chlamydophila pneumoniae, psittaci: atypical pattern
• Viruses: influenza, SARS-CoV-2, RSV (~15-30% of cases)
• No organism identified in ~40-60% of cases

Pathophysiology

• Microaspiration of oropharyngeal organisms overwhelms local defences
• Alveolar macrophages and neutrophils initiate inflammatory response
• Exudate fills alveoli causing consolidation and impaired gas exchange
• Bacteraemia occurs in ~10-25% of pneumococcal pneumonia
• Parapneumonic effusion from pleural inflammation; empyema if infected

Typical (Bacterial) Presentation

• Acute onset productive cough (purulent or rusty sputum)
• High fever with rigors
• Pleuritic chest pain
• Dyspnoea
• Tachycardia, tachypnoea
• Signs of consolidation: bronchial breathing, dull percussion, increased vocal resonance

Atypical Presentation

• Dry cough, gradual onset
• Headache, myalgia, malaise
• Extrapulmonary features (M. pneumoniae: erythema multiforme, cold agglutinins; Legionella: confusion, hyponatraemia, diarrhoea)
• Patchy rather than lobar infiltrates

Elderly Presentation

• Confusion or functional decline may be sole presentation
• Often afebrile
• Falls

Red Flags

• CURB-65 ≥3
• SpO₂ <92%
• Bilateral or multilobar disease
• Haemodynamic instability
• Failure to improve after 48 hours of appropriate antibiotics
• Cavitation on CXR (abscess, TB, Klebsiella, Staph aureus)

Bedside

• Pulse oximetry: SpO₂ on air
• ABG: if SpO₂ <92% or severe features
• CURB-65 score: calculate on admission
• Urinary antigens: pneumococcal + Legionella (moderate-severe)

Bloods

• FBC: leucocytosis, or leucopenia (poor prognosis)
• CRP: elevated; monitor for treatment response
• U&Es: urea >7 (CURB-65), renal function
• LFTs: deranged in Legionella
• Blood cultures x2: before antibiotics in moderate-severe
• Procalcitonin: may guide antibiotic duration
• Lactate: if sepsis suspected

Imaging

• CXR PA: lobar consolidation, air bronchograms, parapneumonic effusion
• Repeat CXR at 6 weeks: essential in all patients to confirm resolution (NICE CG191)
• CT chest: non-resolving, complicated, or alternative diagnosis
• Pleural USS: guide aspiration of effusion

Special Tests

• Sputum MC&S: before antibiotics where possible
• Pleural fluid analysis: if significant effusion (pH, protein, LDH, glucose, MC&S, cytology)
• Mycoplasma serology: IgM (acute), complement fixation titre
• Legionella PCR: on respiratory samples
• Viral PCR: influenza, SARS-CoV-2, RSV (nasal/throat swab)
• Bronchoscopy with BAL: immunocompromised or non-resolving

Non-pharmacological

• Supplemental oxygen (target SpO₂ 94-98% or 88-92% if COPD risk)
• IV fluid resuscitation if septic or dehydrated
• Adequate analgesia for pleuritic pain (paracetamol, NSAIDs if no contraindication)
• VTE prophylaxis (enoxaparin 40mg SC OD)
• Nutritional support

Pharmacological

NICE CG191 empirical antibiotics:

Low severity (CURB-65 0-1):

• Amoxicillin 500mg TDS PO for 5 days
• Penicillin allergy: doxycycline 200mg then 100mg OD or clarithromycin 500mg BD

Moderate severity (CURB-65 2):

• Amoxicillin 500mg TDS PO + clarithromycin 500mg BD PO for 5 days
• Penicillin allergy: doxycycline 200mg then 100mg OD

High severity (CURB-65 3-5):

• Co-amoxiclav 1.2g IV TDS + clarithromycin 500mg IV BD
• Switch to oral at 48-72 hours when improving
• Total course: 7-10 days for severe (may be shorter for uncomplicated)

Specific pathogens:

• Legionella: fluoroquinolone or clarithromycin ± rifampicin; notifiable disease
• Influenza-associated: oseltamivir 75mg BD + anti-staphylococcal cover
• PVL-producing Staph aureus: linezolid + clindamycin (suppress toxin) + rifampicin

Surgical/Interventional

• Chest drain insertion for empyema (pH <7.2) or large complicated effusion
• Intrapleural fibrinolytics (alteplase 10mg + DNase 5mg BD) for loculated empyema
• CT-guided drainage for lung abscess not responding to antibiotics

Referral Criteria

• ICU if CURB-65 ≥3 or clinical deterioration
• Non-resolving pneumonia at 6 weeks: respiratory/radiology review
• Recurrent pneumonia same lobe: bronchoscopy to exclude obstruction
• Immunocompromised patients: early infectious diseases/respiratory input

• Outpatient CAP mortality: <1%
• Hospitalised CAP mortality: 5-10%
• ICU-admitted CAP: 25-50% mortality
• CURB-65 score 0: ~0.7% mortality; score 5: ~57% mortality
• Pneumococcal bacteraemia mortality: ~20%
• Most improve within 48-72 hours of appropriate antibiotics
• CXR resolution: 50% at 2 weeks, 75% at 6 weeks, 95% at 12 weeks
• Post-pneumonia cardiovascular events increased for 1-2 years
• Pneumococcal vaccination reduces invasive disease by ~50-80% in elderly

CURB-65 Score Interpretation

Common Organisms and Clues