Community-Acquired Pneumonia

Definition

Pneumonia is inflammation of lung tissue, presenting with cough, fever, purulent sputum, focal chest signs, and consolidation on CXR. Community-acquired pneumonia (CAP) develops in a patient in the community, or one who has been in hospital for less than 48 hours. Patients — particularly the elderly and immunosuppressed, and those with atypical pathogens — may present with non-specific symptoms (tachypnoea, lethargy, headache, functional decline, new incontinence/diarrhoea, loss of appetite, increased confusion) rather than classic respiratory symptoms.

Streptococcus pneumoniae is the most common causative pathogen and the most common cause of high-severity illness and death from CAP — low-dose penicillin-based antibiotics are a recognised cause of poor outcomes. Other causes include atypical bacteria (Mycoplasma, Chlamydia, Legionella) and viruses (influenza and others).

Red flags

Acute confusion, SaO2 ≤93%, respiratory rate ≥30/min, signs of sepsis.

Assessment

1. History: acute lower respiratory illness symptoms (cough plus ≥1 of dyspnoea, sputum, pleuritic chest pain); systemic illness symptoms (temperature >38°C, sweats, fevers, rigors, myalgia); non-specific symptoms in immunosuppressed/older patients; exposures (unwell contacts, travel, birds/animals/potting mix/contaminated water); occupation, smoking/vaping, drug use; respiratory disease history (asthma, COPD, comorbidities); current vaccination status for COVID-19, pneumococcal, RSV and influenza.
2. Risk factors for CAP: age >65, low socioeconomic status, malnutrition, poor housing (damp/mould/overcrowding), tobacco smoke exposure, chronic disease (diabetes, ESKD, respiratory disease), alcohol misuse, immunosuppression, travel from endemic regions, poor dental health, gastric acid-suppressing medication.
3. Examination: general appearance, acute confusion; respiratory rate, temperature, heart rate, blood pressure, SaO2 (measured for ≥1 minute for accuracy); cardiorespiratory exam.
4. Differentials: acute bronchitis (fever, rigors, pleuritic pain, tachypnoea, crackles not clearing with cough, SaO2<95%, HR>100, dullness to percussion all favour pneumonia over bronchitis), heart failure, pulmonary embolism, sepsis, aspiration pneumonitis, interstitial lung disease, pneumothorax, exacerbation of COPD/bronchiectasis/asthma, tuberculosis, underlying lung pathology (e.g. lung cancer), acute abdomen.

Severity assessment

Severity	Definition
High	≥1 red flag for intensive care support (RR≥30; SpO2<90%; systolic BP<90 mmHg; acute confusion; poor peripheral perfusion/mottled skin; acute oliguria). Consider sepsis if red zone observations present (RR>30 or <5, SpO2<90%, systolic BP>200 or <90, HR>140 or <40, neurological status P/U on AVPU) — probable sepsis, high deterioration risk.
Moderate	≥1 feature that may indicate hospital admission (RR≥22; SaO2<92% on room air, or lower than the patient’s own baseline with chronic lung disease; systolic BP<90; HR>100; acute confusion) but no red flags for intensive care.
Low	None of the above hospital-admission features.

Also weigh other factors affecting the admission decision: social circumstances/support at home, age and frailty, comorbidities, and ability to take/absorb oral medication.

Consider pathogens other than S. pneumoniae

• Legionnaires’ disease — from Legionella contaminating cooling towers, spas, showerheads, other water bodies; also gardening/potting-mix exposure. Incubation 2–10 days (typically presents day 5–6). Fever, chills, cough, dyspnoea ± myalgia, headache, fatigue, anorexia, diarrhoea. (Pontiac fever — same organism, milder influenza-like illness without pneumonia, resolves spontaneously in 2–3 days.)
• Mycoplasma pneumoniae — crowded living/working conditions; most common in young adults but any age; symptoms develop over 1–3 weeks (fever, dry cough, headache, sore throat, malaise); patients may appear well.
• Staphylococcal pneumonia — uncommon cause of CAP; consider in severe disease. Risk factors: chronic alcohol misuse, IV drug use, long-term high-dose steroids, T2DM, severe chronic renal disease, other immunocompromise, First Nations peoples. Also consider with significant haemoptysis (necrotising pneumonia), severe illness, or new cavitation/ multifocal pneumonia/secondary pneumothorax on CXR.
• Q fever (Coxiella burnetii) — 50–60% of infections are asymptomatic/ subclinical. Associated with livestock exposure (goats, sheep, cattle) but also dogs, cats, possums, rodents, kangaroos and other marsupials (and their ticks); transmission via inhalation of infected particles. Non-respiratory signs/symptoms may dominate; confirm on serology.
• COVID-19, influenza, other viruses (e.g. RSV).

Investigations

Severity	Investigations
High (likely arranged in hospital)	CXR (PA+lateral), blood culture, NAAT (viral/fastidious bacterial pathogens), sputum gram stain + culture, arterial blood gas; consider pneumococcal and Legionella urinary antigen
Moderate	CXR (PA+lateral), blood culture; consider NAAT, sputum gram stain + culture (before starting antibiotics)
Low	CXR (PA+lateral); consider NAAT. Do not collect blood cultures for patients managed in the community.

Management

Practice point: start appropriate antibiotics early and at sufficient dosage — delay in starting correct-dose antibiotics is associated with increased mortality.

By severity

Low severity CAP — start monotherapy unless significant comorbidities or follow-up is unlikely:

• Amoxicillin 1 g PO TDS for 5 days.
• Penicillin hypersensitivity or atypical pathogen suspected: doxycycline 100 mg PO BD for 5 days.
• Significant comorbidities/follow-up unlikely: combination amoxicillin 1 g PO TDS plus doxycycline 100 mg PO BD, both for 5 days.

Moderate severity CAP — if not suitable for community management, arrange emergency assessment. If suitable for oral therapy: combination amoxicillin 1 g PO TDS plus doxycycline 100 mg PO BD for 5 days. Penicillin hypersensitivity: severe — moxifloxacin 400 mg PO daily for 5 days monotherapy (not PBS-listed); non-severe — replace amoxicillin with cefuroxime 500 mg PO BD for 5 days.

High severity CAP — arrange emergency assessment; give first dose before transfer if available: ceftriaxone 2 g IV/IM plus azithromycin 500 mg IV (do not give ceftriaxone with severe delayed penicillin hypersensitivity — e.g. SJS/TEN, DRESS, significant organ involvement such as acute interstitial nephritis — seek infectious diseases/emergency advice instead).

Directed therapy once a pathogen is identified

• Legionella: low-moderate severity — azithromycin 500 mg PO daily for 3–7 days depending on response; high severity managed in hospital.
• Influenza: if diagnosed in the CAP setting with symptom duration <48 hours, consider oseltamivir.
• Mycoplasma: doxycycline 100 mg PO BD for 7 days.
• Staph aureus: arrange emergency assessment (usually very unwell, needs hospitalisation); if identified in mild/moderate CAP, consider colonisation vs true infection and seek urgent infectious diseases/ respiratory advice.
• Penicillin-resistant pneumococcus: seek infectious diseases/respiratory advice.

Review

• Moderate CAP managed in the community: review daily until improving (can deteriorate quickly).
• Low severity CAP: review within 48 hours.
• Monitor RR, BP, HR, hydration, SaO2; check for persistent fever/systemic symptoms/confusion; reassess severity at each review.
• Arrange emergency assessment for any deterioration.

If not improving on initial therapy

Reassess diagnosis — consider alternative diagnoses, other pathogens (Legionella, mycoplasma, Burkholderia pseudomallei [melioidosis], influenza, COVID-19, CMV, other viral/zoonotic pneumonias e.g. brucellosis, Q fever), complications (complicated parapneumonic effusion/empyema), or endobronchial obstruction. Consider repeat CXR, sputum PCR, urine antigen testing, and seek respiratory advice.

Recurrent pneumonia

Consider an underlying disorder: IgG/IgA deficiency, HIV, cystic fibrosis, bronchiectasis, lung cancer.

Follow-up

• Repeat CXR at 6–8 weeks if pneumonia was documented radiographically, to confirm complete resolution.
• Manage comorbidities; advise smoking/vaping cessation.
• Consider spirometry once well (if current/ex-smoker) to identify previously unrecognised COPD.
• Recommend at-risk patients receive COVID-19, pneumococcal, RSV, and annual influenza vaccination.

Referral

Arrange emergency assessment for: high severity CAP, low/moderate severity CAP unsuitable for community management, deterioration during community management, or features of staphylococcal pneumonia. Seek respiratory phone advice if needed.