Community-Acquired Pneumonia

Definition / Overview

• Community-acquired pneumonia (CAP) is an acute infection of the pulmonary parenchyma occurring in a patient who has not been hospitalised within the preceding 14 days (or who develops symptoms within 48 hours of admission).
• It remains one of the most common causes of infection-related hospitalisation and mortality in adults.
• CAP is distinct from hospital-acquired pneumonia (HAP, onset $\geq 48$ hours after admission) and ventilator-associated pneumonia (VAP); the concept of "healthcare-associated pneumonia" (HCAP) has largely been abandoned because it did not reliably predict resistant pathogen risk and led to antibiotic overuse.
• The principal pathogens differ by severity and host factors, driving both the investigation pathway and the choice of empiric therapy.

Microbiology

Common Causative Organisms

• S. pneumoniae remains the most common bacterial cause at all severity levels.
• Atypical organisms (Mycoplasma, Chlamydophila, Legionella) cannot be reliably distinguished clinically from typical bacterial CAP.
• Community-acquired MRSA is an important cause of severe, rapidly progressive necrotising pneumonia, particularly post-influenza.
• Influenza and other respiratory viruses account for a significant minority of cases; consider empiric antiviral therapy during influenza season.

Clinical Features

Symptoms

• Fever, rigors, malaise, anorexia
• Dyspnoea, cough with purulent sputum (classically "rusty" sputum with pneumococcal pneumonia)
• Haemoptysis
• Pleuritic chest pain

Signs

• Pyrexia; rarely hypothermia (especially elderly)
• Tachypnoea ($\geq 30$/min in severe disease), tachycardia, hypotension, cyanosis
• Confusion - may be the only presenting feature in older patients
• Signs of pulmonary consolidation: reduced chest expansion, dullness to percussion, increased tactile vocal fremitus and vocal resonance, bronchial breathing
• Pleural rub if pleural involvement

Atypical Presentation

• Gradual onset over days-weeks with dry cough, headache, myalgia and minimal auscultatory findings suggests Mycoplasma or Chlamydophila
• Failure to respond to beta-lactam or penicillin therapy should raise suspicion of an atypical organism - these agents have no activity against cell-wall-deficient bacteria
• Gastrointestinal symptoms, hyponatraemia, and high fever in a returning traveller or after exposure to air-conditioning systems raises suspicion for Legionella

Severity Assessment: CURB-65

CURB-65 is the preferred bedside severity scoring tool in Australasian practice. One point is awarded for each feature present:

Management Thresholds by CURB-65 Score

Clinical caveats: - CURB-65 can under-score younger patients with significant physiological derangement - always apply clinical judgement. - A score of 2 attributable entirely or mostly to age alone may not mandate admission if social supports are reliable and oral intake is maintained. - Oxygen saturation $< 92\%$ on room air, inability to maintain oral intake, or significant comorbidity should prompt hospitalisation regardless of score. - Neither CURB-65 nor PSI reliably identifies patients requiring ICU admission; use clinical assessment, arterial blood gases, and trajectory.

Additional High-Risk Features

• $\text{PaO}_2 < 8\,\text{kPa}$ ($< 60\,\text{mmHg}$)
• Bilateral or multilobar involvement on chest X-ray
• Significant comorbidities (cardiac failure, COPD, immunosuppression, renal failure)
• Septic shock or requirement for vasopressors
• Hypercapnia (indicates impending respiratory failure)

Investigations

Bedside / Immediate

• Oxygen saturation (SpO₂): perform on all patients; if $\text{SaO}_2 < 92\%$ or severe CAP, proceed to arterial blood gas
• Vital signs - essential for CURB-65 calculation

Blood Tests (all admitted patients)

• FBC: leucocytosis or leucopenia; neutrophilia in bacterial CAP
• U&E: urea for CURB-65; hyponatraemia suggests Legionella
• LFTs: transaminitis in atypical CAP; cholestatic jaundice is a recognised complication
• CRP / ESR: elevated in bacterial infection; useful for monitoring treatment response - rising CRP at 48-72 hours should prompt reassessment
• Blood cultures: collect before antibiotics if CURB-65 $\geq 2$; yield is low but results guide de-escalation
• Procalcitonin (PCT): insufficiently sensitive or specific to confirm bacterial aetiology or to withhold antibiotics; use in conjunction with the full clinical picture only

Respiratory Specimens

• Sputum microscopy and culture: send if CURB-65 $\geq 3$, or if CURB-65 $= 2$ and patient has not yet received antibiotics
• Urinary antigens:
• Pneumococcal urinary antigen: send if CURB-65 $\geq 2$; sensitivity ≈70%, highly specific
• Legionella urinary antigen: send if CURB-65 $\geq 3$ or clinical suspicion (detects serogroup 1 only)
• Pleural fluid: aspirate and send for MC&S if effusion present and CURB-65 $\geq 2$

Imaging

• Chest X-ray (CXR): obtain in all patients; findings include lobar or segmental consolidation, interstitial infiltrates, cavitation, pleural effusion
• A normal CXR in the correct clinical context does not exclude pneumonia - repeat in 24-48 hours if initial film is unremarkable but suspicion remains
• CT thorax: consider if CXR is equivocal, if there is failure to improve, or if an alternative or complicating diagnosis (malignancy, empyema, abscess) is suspected

Additional / Selected Tests

• Nasopharyngeal swab or respiratory PCR panel: send during influenza season or when viral aetiology is suspected
• Mycoplasma serology / PCR: consider if clinical picture suggests atypical infection
• HIV serology: consider in patients with unexplained PCP, recurrent pneumonia, or relevant risk factors
• Bronchoscopy ± BAL: reserve for immunocompromised patients or those not responding to adequate empiric therapy

Management

Step 1: Immediate Stabilisation

1. Ensure patent airway; assess work of breathing.
2. Oxygen: target $\text{SpO}_2 \geq 94\%$ (or $88$-92% if established COPD); aim $\text{PaO}_2 > 8\,\text{kPa}$.
3. IV access, IV fluids if dehydrated, hypotensive, or unable to maintain oral intake.
4. VTE prophylaxis: subcutaneous low-molecular-weight heparin for all hospitalised patients unless contraindicated.
5. Analgesia: simple analgesics or NSAIDs for pleuritic pain.
6. Consider ICU referral early if shock, worsening hypoxia, or hypercapnia.

Step 2: Antimicrobial Therapy

Key principle: Antibiotics should be commenced as soon as CAP is diagnosed - ideally within 4 hours of presentation. Delays are associated with increased mortality.

Narrow therapy once a microbiological diagnosis is established.

Empiric Antibiotic Selection by Severity

Australian context: Always consult your hospital or jurisdictional antibiotic guidelines (e.g., the Therapeutic Guidelines: Antibiotic or local antibiogram) - these take precedence over generic recommendations and account for local resistance patterns.

Fluoroquinolones and CAP

• Ciprofloxacin has poor activity against S. pneumoniae and should not be used as monotherapy for CAP.
• Respiratory fluoroquinolones (moxifloxacin, levofloxacin) have activity against both typical and atypical organisms and are appropriate for penicillin-allergic patients or when dual therapy is not feasible.

Duration of Therapy

• Mild-moderate CAP: 5-7 days is appropriate for most patients who respond to therapy.
• Severe CAP or bacteraemia: 7-14 days depending on organism and clinical response.
• Legionella: minimum 10-14 days (21 days in severe disease or immunocompromised).
• Staphylococcal / Gram-negative CAP with complications: 14-21 days; discuss with infectious diseases.
• Use clinical resolution (improving fever, CRP trending down, tolerating oral intake) to guide switch from IV to oral therapy - do not wait for CXR clearance.

Step 3: Supportive Care and Monitoring

• Daily clinical review with repeat vital signs; reassess CURB-65 trajectory.
• Repeat CRP at 48-72 hours: failure to fall or rising CRP should prompt review of diagnosis, culture results, and coverage.
• Repeat CXR if not improving by 48-72 hours.

Complications and Special Considerations

Common Complications

• Parapneumonic effusion: occurs in up to 20% of hospitalised patients; aspirate and send fluid for pH, glucose, LDH, protein, MC&S, cytology if clinically significant
• Empyema: pH $< 7.2$, glucose $< 2.2\,\text{mmol/L}$, positive Gram stain or culture - requires intercostal catheter drainage and prolonged antibiotics; discuss with thoracics
• Lung abscess: cavitation on imaging; associated with aspiration, S. aureus, anaerobes - requires prolonged antibiotics (4-6 weeks) and possible bronchoscopy
• Respiratory failure: may require high-flow nasal oxygen, NIV, or invasive ventilation
• Septicaemia and septic shock: management per sepsis bundle - early cultures, antibiotics within 1 hour, fluid resuscitation, vasopressors if required
• Cholestatic jaundice: recognised complication of CAP (mechanism uncertain); monitor LFTs
• Pericarditis / myocarditis: rare; consider if unexplained chest pain or ECG changes persist

Special Populations

• Elderly patients: may present atypically (confusion alone, absence of fever, hypothermia); CURB-65 may over-weight age - apply clinical judgement; ensure adequate hydration and early mobilisation to reduce deconditioning
• Immunocompromised patients: broader differential (including PCP, fungal, CMV, mycobacterial); early bronchoscopy and infectious diseases involvement recommended; consider TMP-SMX prophylaxis if PCP risk is high
• Aspiration pneumonia: cover anaerobes (amoxicillin-clavulanate or metronidazole combination); address underlying aspiration risk (swallowing assessment, head-of-bed elevation, dental hygiene)
• Influenza-associated CAP: treat with oseltamivir (75 mg orally 12-hourly for 5 days; continue regardless of duration of symptoms if hospitalised) alongside antibiotics; monitor for secondary bacterial pneumonia including MRSA

Follow-Up and Long-Term Care

Hospital Discharge Criteria

• Clinically stable: improving or resolved fever, heart rate $< 100$/min, respiratory rate $< 24$/min, systolic BP $\geq 90\,\text{mmHg}$, $\text{SpO}_2 \geq 94\%$ on room air (or baseline), tolerating oral intake, normal or improving mental state

Post-Discharge Follow-Up

• Outpatient review at 6 weeks post-discharge is recommended for all patients admitted with CAP.
• Repeat CXR at 6 weeks: to confirm radiological resolution and exclude an underlying lesion (e.g., bronchial carcinoma) - CXR may take 4-8 weeks to clear, particularly in older patients and those with lobar pneumonia.
• Failure to resolve on CXR at 6 weeks should prompt CT thorax and respiratory/oncology referral.

Vaccination

• Pneumococcal vaccine: recommended for all adults $\geq 65$ years and those with chronic cardiorespiratory, renal, hepatic, or immunosuppressive conditions; both the 13-valent conjugate (PCV13) and 23-valent polysaccharide (PPSV23) vaccines have a role - follow current ATAGI recommendations for scheduling.
• Influenza vaccine: annual vaccination recommended for all adults, particularly those $\geq 65$ years and those with chronic disease or immunosuppression.

Key Exam Points

• CURB-65 score of 0-1: outpatient oral antibiotics; score of 2: hospital admission; score $\geq 3$: consider ICU
• Atypicals cannot be excluded clinically - dual-agent empiric therapy (beta-lactam + macrolide or doxycycline) is standard for hospitalised patients
• Ciprofloxacin must not be used as CAP monotherapy - poor pneumococcal activity
• Urinary antigens (pneumococcal at CURB-65 $\geq 2$; Legionella at CURB-65 $\geq 3$) are rapid, specific, and guide targeted therapy
• Failure to improve at 48-72 hours: reassess diagnosis, check culture results, consider resistant/atypical organism, complications, or alternative diagnosis
• 6-week follow-up CXR is mandatory to exclude underlying malignancy
• Antibiotics within 4 hours of diagnosis - delay increases mortality