Lucan · Conditions

Community-acquired pneumonia (adult)

Australian guidance: eTG Antibiotic — Pneumonia (CAP)

1. Definition and epidemiology

Community-acquired pneumonia (CAP) is an acute infection of the lung parenchyma in a patient who acquires the illness outside of hospital or within 48 hours of admission. In Australia, CAP is responsible for approximately 77,000 hospitalisations per year, with a 30-day mortality of around 10 to 12% in admitted patients and under 1% in those managed in the community. Incidence is highest at the extremes of age and in individuals with comorbidities such as chronic obstructive pulmonary disease, diabetes mellitus, or immunosuppression.

2. Pathophysiology

CAP results when respiratory pathogens overcome mucociliary clearance, cough reflexes, and alveolar macrophage defences. The most common pathogen overall is Streptococcus pneumoniae, which reaches the alveoli via microaspiration of oropharyngeal secretions. Once in the alveoli, bacteria trigger an acute exudative phase: pneumocytes are damaged, capillary permeability rises, and the alveolar spaces fill with proteinaceous fluid, red cells, and neutrophils (red hepatisation), followed by a grey hepatisation stage in which erythrocytes are degraded and macrophages dominate. Resolution occurs via macrophage phagocytosis of debris and fibrin digestion, restoring normal alveolar architecture. Atypical organisms (Mycoplasma pneumoniae, Legionella pneumophila, Chlamydophila pneumoniae) produce interstitial inflammation rather than lobar consolidation, explaining their radiological and clinical differences. Viral causes (influenza, SARS-CoV-2) damage bronchial epithelium and impair mucociliary clearance, predisposing to secondary bacterial superinfection. In severe CAP, uncontrolled cytokine release extends injury beyond the lung, producing sepsis syndrome and multi-organ dysfunction (Robbins Ch 15).

3. Clinical presentation and diagnostic criteria

CAP classically presents with fever, rigors, productive cough (rusty or purulent sputum), pleuritic chest pain, and breathlessness of acute onset. Examination typically reveals tachycardia, tachypnoea, and localised signs of consolidation: dullness to percussion, bronchial breath sounds, and increased tactile vocal fremitus over the affected lobe. Atypical CAP tends to produce a more insidious onset with dry cough, headache, myalgia, and extrapulmonary features (diarrhoea in Legionella; rash or bullous myringitis in Mycoplasma). There is no single clinical finding that reliably confirms or excludes pneumonia; a chest radiograph showing new pulmonary infiltrate in the context of a consistent clinical picture is the accepted diagnostic standard. In the community setting where chest radiograph is unavailable, clinical diagnosis based on at least two of the following is acceptable: fever, new cough, new focal chest signs, or new breathlessness.

4. Investigations and AU-specific cutoffs

The following assessment and investigation pathway applies to adults with suspected CAP.

Severity scoring. The CORB score (Confusion, Oxygen saturation below 90%, Respiratory rate 25 or above per minute, Blood pressure systolic below 90 or diastolic 60 or below) is used in the Australian setting for prehospital and emergency department triage. A CORB score of 0 to 1 supports community management, a score of 2 indicates hospital admission, and a score of 3 or 4 mandates consideration of intensive care unit (ICU) review. The SMART-COP score (Systolic BP, Multilobar infiltrates, Albumin, Respiratory rate, Tachycardia, Confusion, Oxygen, and arterial pH) is used in the inpatient setting to predict need for intensive respiratory or vasopressor support; a score of 5 or above indicates high risk.

Chest radiograph. Mandatory in all patients requiring hospital admission. Identifies lobar consolidation, multilobar involvement, pleural effusion, and cavitation.

Microbiology. Blood cultures (two sets) and sputum culture (before antibiotic administration where possible) are recommended for all hospitalised patients. Urinary antigen testing for Legionella pneumophila serogroup 1 is indicated in moderate to severe CAP or where Legionella is clinically suspected; pneumococcal urinary antigen has AU sensitivity of 70 to 80% and is specifically useful in patients who have already received antibiotics. Nasopharyngeal swab for influenza polymerase chain reaction (PCR) during influenza season. Mycoplasma and Chlamydophila serology or PCR in atypical presentations.

Blood tests. Full blood count, electrolytes, creatinine, liver function tests, C-reactive protein (CRP), and lactate in moderate to severe disease. CRP above 100 mg/L supports a bacterial aetiology. Procalcitonin can guide antibiotic stewardship but is not routinely recommended as an AU-specific mandatory test.

Oxygenation. SpO2 on air in all patients. Arterial blood gas in moderate to severe disease or when SpO2 is below 94%.

5. Management

Community management (CORB 0 to 1, no comorbidities precluding oral therapy). Empirical oral antibiotic therapy targeting typical and atypical organisms is appropriate for most patients. The recommended first-line regimen in AU is amoxicillin 1 g orally three times daily for 5 days as primary coverage for S. pneumoniae, combined with doxycycline 100 mg orally twice daily for 5 days to cover atypical organisms. Where atypical coverage is not required and the patient is younger with no comorbidities, amoxicillin alone is acceptable. Duration of therapy is 5 days for mild CAP; extension to 7 to 10 days is reserved for Legionella or severe CAP.

Hospital admission (CORB 2 or SMART-COP 3 to 4). Intravenous benzylpenicillin 1.2 g four to six hourly plus doxycycline 100 mg twice daily (or azithromycin 500 mg once daily) is the standard combination for moderate CAP without penicillin allergy. The addition of doxycycline or azithromycin ensures atypical organism coverage, which is associated with reduced mortality in hospitalised CAP.

Severe CAP or ICU admission (CORB 3 to 4 or SMART-COP 5 or above). Empirical therapy is expanded to cover Legionella and resistant S. pneumoniae: benzylpenicillin 1.2 g intravenously four to six hourly plus azithromycin 500 mg intravenously once daily. Where Pseudomonas aeruginosa is a risk (structural lung disease, recent hospital admission, heavy immunosuppression), piperacillin-tazobactam replaces benzylpenicillin.

Penicillin hypersensitivity. For nonsevere hypersensitivity, cefazolin or ceftriaxone may be substituted. For severe immediate hypersensitivity to penicillin, moxifloxacin 400 mg orally or intravenously once daily is the recommended monotherapy alternative.

De-escalation and step-down. Intravenous to oral switch is appropriate within 48 to 72 hours when the patient is afebrile, haemodynamically stable, able to swallow, and SpO2 is adequate on low-flow oxygen. Antibiotic choice should be narrowed once microbiology results are available, consistent with antimicrobial stewardship principles.

6. Pharmacology and AU PBS notes

Amoxicillin 500 mg capsules (generic; Amoxil). PBS listed without restriction for community respiratory infections. Preferred over amoxicillin-clavulanate for uncomplicated CAP (reduced adverse effects; adequate spectrum for the most common pathogens).

Doxycycline 100 mg tablets (generic; Vibramycin-D dispersible tablets available). PBS listed. First-dose loading of 200 mg is recommended in some eTG regimens. Avoid in pregnancy and children under 12 years. Take with food and a full glass of water; remain upright for 30 minutes to reduce oesophageal ulceration.

Azithromycin 500 mg tablets (generic; Zithromax). PBS listed for respiratory infections (streamlined authority not required for CAP). Inhibits CYP3A4 and prolongs QTc; use with caution in patients on other QTc-prolonging drugs.

Benzylpenicillin (penicillin G) 600 mg and 1.2 g vials. Intravenous only. No specific PBS inpatient restriction. Superior pharmacodynamic profile for S. pneumoniae over broader beta-lactams, supporting its use as the preferred agent.

Ceftriaxone 1 g and 2 g vials. PBS listed for inpatient use. Once-daily dosing facilitates hospital-in-the-home (HITH) programmes, which are actively used across Australian metropolitan health services.

Moxifloxacin 400 mg tablets and infusion. PBS authority-listed for CAP in penicillin-allergic patients (Authority Required code varies by state). Should not be used as empirical first-line therapy in patients with risk factors for fluoroquinolone-resistant organisms or in those with a history of tendinopathy or QT prolongation.

7. Complications and monitoring

Complications that require active surveillance include parapneumonic pleural effusion and empyema (suspect if fever persists beyond 72 hours of antibiotic therapy; evaluate with chest radiograph or ultrasound and consider diagnostic pleural tap), lung abscess (suspect if cavitation on chest radiograph and failure to improve; anaerobic or S. aureus cause should be considered), bacteraemia and septic shock, and respiratory failure requiring mechanical ventilation.

Patients treated for CAP should demonstrate clinical improvement within 48 to 72 hours, defined as defervescence, improvement in respiratory rate, and stable or improving oxygenation. Failure to improve by 72 hours should prompt review of the antibiotic regimen, a repeat chest radiograph, microbiological results, and consideration of alternative diagnoses including post-obstructive pneumonia, pulmonary embolism, or non-infectious pulmonary infiltrate.

Follow-up chest radiograph is recommended at 6 weeks in patients over 50 years or those who are current or ex-smokers to confirm radiological resolution and exclude an underlying lung malignancy as a predisposing cause. Pneumococcal vaccination (13-valent conjugate vaccine followed by 23-valent polysaccharide vaccine where eligible) should be offered to all patients recovering from CAP who have not previously been vaccinated; influenza vaccination is indicated annually.

Written for Australian practice and grounded in the guidance named above. Study material only — not medical advice. Always check against current guidelines.