Pneumonia and infected pleural collections

Clinical Evaluation and Pathogen Spectrum

Pneumonia represents acute infection and inflammation of the pulmonary parenchymal tissue. Lower respiratory tract infections contribute substantially to acute hospital admissions and critical care utilisation in Australia. Distinguishing community-acquired pneumonia (CAP) from hospital-acquired pneumonia (HAP) and aspiration syndromes is paramount for directing effective empirical antimicrobial therapy.

Causative Pathogens in Community-Acquired Pneumonia

1. Typical Bacterial Organisms:

   • Pneumococcus: An important cause of bacterial CAP, but not the dominant organism in every age group and setting. Viruses, atypical organisms and hospital-acquired pathogens alter the differential and treatment.
   • Haemophilus influenzae: Frequent in patients with pre-existing structural lung disease, particularly COPD and bronchiectasis.
   • Moraxella catarrhalis: Commonly colonises elderly individuals and patients with chronic respiratory compromise.
   • Staphylococcus aureus: Less common in uncomplicated CAP, but frequently causes severe, necrotising, cavitating pneumonia following influenza infection.

2. Atypical Bacterial Organisms:

   • Mycoplasma pneumoniae: Frequently affects school-aged children, teenagers, and young adults. Manifests with gradual insidious onset, constitutional symptoms (headache, malaise, low-grade fever), persistent non-productive dry cough, and extrapulmonary manifestations including autoimmune cold-agglutinin haemolytic anaemia, erythema multiforme, and bullous myringitis.
   • Legionella pneumophila and Legionella longbeachae: In Australia, L. longbeachae is uniquely common and linked to gardening compost and commercial potting mix inhalation, whereas L. pneumophila is contracted from contaminated aerosolised water sources (cooling towers, air conditioning systems). Clinical hallmarks include high spiking fevers, profound confusion or encephalopathy out of proportion to fever, prominent gastrointestinal symptoms (watery diarrhoea, nausea, vomiting), and laboratory abnormalities including marked hyponatraemia, elevated liver transaminases, and microscopic haematuria.
   • Chlamydia pneumoniae and Chlamydia psittaci: C. psittaci arises from bird exposure (parrots, pigeons, poultry) and causes severe systemic illness with hepatosplenomegaly.

3. Hospital-Acquired and Aspiration Pneumonia:

   • Hospital-Acquired Pneumonia (HAP): Pneumonia occurring 48 hours or more following hospital admission that was not incubating at presentation. Pathogen spectrum shifts toward multidrug-resistant hospital flora, including Gram-negative bacilli (Pseudomonas aeruginosa, Klebsiella pneumoniae, Enterobacter species) and Methicillin-Resistant Staphylococcus aureus (MRSA).
   • Aspiration syndromes: Distinguish chemical pneumonitis from bacterial pneumonia after aspiration. Routine extra anaerobic cover is not required for every aspiration event; abscess, empyema and the clinical setting influence treatment. Assess swallowing and recurrence risk.

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Risk Stratification: SMART-COP vs CURB-65

Accurate risk stratification guides site-of-care decisions (outpatient versus general medical ward versus intensive care unit) and determines the breadth and route of antimicrobial coverage.

The SMART-COP Score

The SMART-COP tool was developed and validated in Australia by the Australian CAP Study Collaboration (Charles et al., 2008). It was designed specifically to predict the need for Intensive Respiratory or Vasopressor Support (IRVS), addressing the known limitation of older scores (such as CURB-65) that heavily weight advanced age and may underestimate severity in younger patients with acute respiratory failure.

• S: Clinical Criterion: Systolic Blood Pressure $< 90\text{ mmHg}$; Points Allocated: 2 points

• M: Clinical Criterion: Multilobar chest radiograph involvement; Points Allocated: 1 point

• A: Clinical Criterion: Albumin $< 35\text{ g/L}$; Points Allocated: 1 point

• R: Clinical Criterion: Respiratory Rate $\ge 25\text{ breaths/min}$ (age $\le 50$) or $\ge 30\text{ breaths/min}$ (age $> 50$); Points Allocated: 1 point

• T: Clinical Criterion: Tachycardia with Heart Rate $\ge 125\text{ bpm}$; Points Allocated: 1 point

• C: Clinical Criterion: Confusion (acute onset of altered mental state); Points Allocated: 1 point

• O: Clinical Criterion: Oxygenation Deficit $\text{PaO}_2 < 70\text{ mmHg}$ or $\text{SpO}_2 \le 93\%$ (age $\le 50$); $\text{PaO}_2 < 60\text{ mmHg}$ or $\text{SpO}_2 \le 90\%$ (age $> 50$); Points Allocated: 2 points

• P: Clinical Criterion: Arterial pH $< 7.35$; Points Allocated: 2 points

• SMART-COP 0 to 2 points: Low risk of needing IRVS (typically suitable for outpatient or short-stay management).

• SMART-COP 3 to 4 points: Moderate risk of needing IRVS (general hospital ward admission indicated).

• SMART-COP 5 to 6 points: High risk of needing IRVS (approximately 1 in 3 require ICU/HDU admission).

• SMART-COP $\ge 7$ points: Very high risk of needing IRVS (approximately 2 in 3 require ICU/HDU admission).

CURB-65 Framework

CURB-65 predicts 30-day all-cause mortality:

• C: Confusion (abbreviated mental test score $\le 8$ or new disorientation)
• U: Urea $> 7\text{ mmol/L}$
• R: Respiratory rate $\ge 30\text{ breaths/min}$
• B: Blood pressure (Systolic $< 90\text{ mmHg}$ or Diastolic $\le 60\text{ mmHg}$)
• 65: Age $\ge 65\text{ years}$

A score of 0 to 1 indicates low mortality risk (outpatient management), a score of 2 indicates intermediate risk (short inpatient admission), and a score $\ge 3$ indicates high mortality risk (inpatient admission with urgent ICU assessment).

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Selecting antimicrobial treatment

Assess illness severity, prior antibiotics, allergies, kidney function, exposures and local resistance. Obtain cultures when indicated without delaying treatment of severe disease. Use the current Australian Therapeutic Guidelines or hospital protocol for the regimen and dose, then narrow treatment as microbiology and clinical response become clear. Hospital-acquired infection and immunocompromise need different pathways from uncomplicated community-acquired pneumonia. Review the diagnosis if the patient fails to improve; obstruction, pulmonary embolism, malignancy and a pleural collection can mimic or complicate infection.

Pleural Space Infections: Parapneumonic Effusion and Empyema

Pneumonic consolidation frequently irritates the adjacent visceral pleura, producing a parapneumonic effusion. Effusions evolve through three continuous stages: exudative (simple parapneumonic), fibropurulent (complicated parapneumonic), and organised (chronic empyema with pleural peel).

Diagnostic Thoracentesis

Any pleural effusion associated with pneumonia that measures $> 10\text{ mm}$ on a lateral decubitus radiograph or is clearly visible on thoracic ultrasound warrants prompt diagnostic thoracentesis to differentiate simple from complicated effusions.

• Macroscopic Appearance: Simple Parapneumonic: Clear, straw-coloured; Complicated Parapneumonic: Cloudy, turbid; Empyema: Frank pus, viscous, foul-smelling
• Pleural Fluid pH: Simple Parapneumonic: $\ge 7.20$; Complicated Parapneumonic: $< 7.20$; Empyema: $< 7.20$
• Pleural Fluid Glucose: Simple Parapneumonic: $\ge 3.3\text{ mmol/L}$; Complicated Parapneumonic: $< 3.3\text{ mmol/L}$; Empyema: Markedly reduced ($< 2.2\text{ mmol/L}$)
• Pleural Fluid LDH: Simple Parapneumonic: $< 1000\text{ IU/L}$; Complicated Parapneumonic: $> 1000\text{ IU/L}$; Empyema: Typically $> 1000\text{ IU/L}$
• Microbiology: Simple Parapneumonic: Negative Gram stain and culture; Complicated Parapneumonic: Often negative (sterile inflammation); Empyema: Positive Gram stain or bacterial culture
• Primary Management: Simple Parapneumonic: Systemic antibiotics alone; Complicated Parapneumonic: Chest tube thoracostomy drainage + IV antibiotics; Empyema: Chest tube thoracostomy drainage + IV antibiotics +/- fibrinolytics/VATS

Management of Complicated Parapneumonic Effusion and Empyema

• Pleural infection: Frank pus or positive pleural microbiology requires drainage assessment. A reliable pH at or below 7.2 supports drainage when a safe fluid target exists; glucose, LDH, imaging and clinical course matter in intermediate cases. Involve respiratory specialists and do not apply one isolated number without checking sampling quality.
• Intravenous Antimicrobials: Broad-spectrum coverage targeting community or hospital pathogens plus oral anaerobes: IV ceftriaxone ($2\text{ g}$ daily) PLUS IV metronidazole ($500\text{ mg}$ 8-hourly).
• Intrapleural Fibrinolytics and Surgery: In multiloculated empyemas with persistent non-draining collections, intrapleural tissue plasminogen activator (tPA, alteplase $10\text{ mg}$) combined with deoxyribonuclease (DNase $5\text{ mg}$) twice daily for 3 days facilitates drainage. If medical and intrapleural therapy fails, video-assisted thoracoscopic surgery (VATS) decortication is required.

Primary references (checked 7 October 2026): NSW adult pneumonia pathway.