Community Acquired Pneumonia

Acute infection of the lung parenchyma acquired outside hospital, or within the first 48 hours of admission, with a new infiltrate on chest imaging and compatible symptoms. Incidence in adults is about 5 to 11 per 1000 per year and rises steeply after age 65. About 20 to 40% of patients are admitted, and of those about 5 to 10% need intensive care.

• Mortality is less than 1% in outpatients, about 5 to 15% in admitted patients and about 30 to 50% in those needing intensive care.
• Organisms: Streptococcus pneumoniae (about 30 to 50% of identified causes), Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus aureus, Klebsiella pneumoniae, atypical organisms (Mycoplasma, Chlamydophila, Legionella; about 10 to 30%) and respiratory viruses (influenza, respiratory syncytial virus, SARS-CoV-2; about 20 to 30%, often with bacterial co-infection). No cause is found in up to half of patients. In sub-Saharan Africa and in people with HIV, pulmonary tuberculosis, Pneumocystis pneumonia and non-typhoidal Salmonella must also be considered.
• Risk factors: age over 65, smoking, alcohol misuse, COPD, asthma, diabetes, heart failure, chronic kidney or liver disease, HIV, malnutrition, indoor smoke exposure, aspiration risk (stroke, seizures, dysphagia), immunosuppression, absent vaccination and recent viral infection.

• Fever (often absent in the elderly), cough with purulent or rust coloured sputum, pleuritic chest pain, dyspnoea, rigors, malaise, headache, and new confusion in older patients. Diarrhoea and hyponatraemia point to Legionella.
• Signs: respiratory rate 20 or more, tachycardia, focal bronchial breathing, dullness to percussion, coarse crackles, pleural rub, hypoxaemia, hypotension in sepsis.
• Differential diagnosis: tuberculosis, Pneumocystis pneumonia, pulmonary embolism with infarction, acute heart failure, lung cancer with obstructive pneumonia, organising pneumonia, eosinophilic pneumonia, vasculitis, aspiration pneumonitis, drug induced pneumonitis.
• Complications: parapneumonic effusion and empyema (about 10 to 20% of admissions), lung abscess, necrotising pneumonia, respiratory failure, ARDS, septic shock, metastatic infection (meningitis, endocarditis, septic arthritis), and cardiac events such as atrial fibrillation and myocardial infarction (about 10 to 25% of admitted patients).

• Chest X-ray: new focal or lobar consolidation, air bronchograms, effusion, cavitation. An early film can be normal; repeat it, or use ultrasound or CT, when suspicion remains.
• Severity scoring (one point each): confusion, urea greater than 7 mmol/L, respiratory rate 30 or more, systolic pressure less than 90 mmHg or diastolic 60 mmHg or less, age 65 or more. A score of 0 or 1 is low risk (home treatment), 2 means admit, and 3 or more is severe. Severe pneumonia is also defined by need for ventilation or vasopressors (major criteria), or three of: respiratory rate 30 or more, PaO2 to FiO2 ratio 250 or less, multilobar infiltrates, confusion, urea 7 mmol/L or more, white cell count less than 4 x 10^9/L, platelets less than 100 x 10^9/L, temperature less than 36 degrees Celsius, hypotension needing fluids.
• Pulse oximetry in everyone; arterial blood gas when SpO2 is less than 92% or the patient is unwell.
• Blood tests for admitted patients: full blood count, urea and electrolytes, creatinine, liver function, CRP, procalcitonin (helps to stop antibiotics early), glucose, lactate if septic, HIV test, two sets of blood cultures before antibiotics.
• Microbiology in moderate to severe disease: sputum Gram stain and culture, pneumococcal and Legionella urinary antigens, respiratory viral PCR including influenza and SARS-CoV-2, Xpert MTB/RIF when cough lasts more than 2 weeks or in HIV, weight loss, night sweats or endemic settings.
• Pleural fluid if the effusion is more than 10 mm: pH, glucose, LDH, protein, Gram stain, culture. Empyema is pus, pH less than 7.2, glucose less than 2.2 mmol/L, or a positive culture.
• CT chest and bronchoscopy for failure to respond by 72 hours, suspected abscess, obstruction or immunosuppression.

• Give the first antibiotic dose within 1 hour in severe disease or sepsis, and within 4 hours in other admitted patients.
• Low severity, home treatment:
  • Amoxicillin 500 mg to 1 g orally three times daily for 5 days.
  • Penicillin allergy: doxycycline 200 mg on day 1 then 100 mg twice daily, or clarithromycin 500 mg twice daily, for 5 days.
  • Review at 48 to 72 hours; admit if there is no improvement.
• Moderate severity, admitted:
  • Amoxicillin 1 g orally or IV three times daily plus clarithromycin 500 mg twice daily (or doxycycline 100 mg twice daily) for 7 days. Switch to oral when stable.
  • Comorbidity or antibiotics in the previous 3 months: co-amoxiclav 625 mg orally three times daily (or 1.2 g IV three times daily) plus a macrolide or doxycycline, or levofloxacin 750 mg daily, or moxifloxacin 400 mg daily.
• Severe:
  • Ceftriaxone 2 g IV daily (or co-amoxiclav 1.2 g IV three times daily) plus clarithromycin 500 mg IV twice daily or azithromycin 500 mg daily.
  • Penicillin allergy: levofloxacin 750 mg IV daily plus a macrolide when Legionella is possible.
  • Add vancomycin 15 to 20 mg/kg IV every 8 to 12 hours (trough 15 to 20 mg/L) or linezolid 600 mg twice daily with flucloxacillin 2 g IV every 6 hours when Staphylococcus aureus is likely (post-influenza, cavities, injection drug use). For Pseudomonas risk (bronchiectasis, prior isolation), use piperacillin tazobactam 4.5 g IV every 6 to 8 hours plus ciprofloxacin 400 mg IV every 8 to 12 hours.
  • Hydrocortisone 200 mg/day IV (50 mg every 6 hours) for up to 7 days in severe pneumonia with respiratory failure or shock. Avoid in influenza and suspected fungal infection. Evidence is moderate.
• Duration: 5 days is enough if afebrile for 48 hours and clinically stable; 7 days for moderate to severe; 10 to 14 days for Staphylococcus aureus, bacteraemia with metastatic focus or Gram negative disease; Legionella 7 to 14 days (14 to 21 if immunosuppressed).
• Supportive care:
  • Oxygen to SpO2 94 to 98% (88 to 92% in COPD with carbon dioxide retention); high flow nasal oxygen or non-invasive ventilation for respiratory failure; intubation for refractory hypoxaemia, exhaustion or shock.
  • IV fluids for volume depletion; noradrenaline 0.05 to 0.5 micrograms/kg/min for septic shock.
  • Paracetamol 1 g four times daily; enoxaparin 40 mg SC daily for thromboprophylaxis; early mobilisation; smoking cessation with nicotine patch 21 mg/24 h.
• Failure to improve at 72 hours: confirm adherence and diagnosis, review cultures, exclude empyema, abscess, tuberculosis, Pneumocystis pneumonia (co-trimoxazole 120 mg/kg/day in 3 to 4 divided doses for 21 days, with prednisolone 40 mg twice daily for 5 days when PaO2 is less than 9.3 kPa), obstruction and non-infective mimics, and broaden cover.
• Complicated effusion or empyema: chest drain with intrapleural alteplase 10 mg plus dornase alfa 5 mg twice daily for 3 days, or video assisted thoracoscopic debridement if the patient is failing.
• Special groups:
  • Children: amoxicillin 40 to 90 mg/kg/day in 2 to 3 divided doses for 5 days; macrolide for atypical infection; IV ampicillin or ceftriaxone 50 to 100 mg/kg/day (maximum 4 g) if severe.
  • Pregnancy: amoxicillin 500 mg to 1 g three times daily plus azithromycin 500 mg daily; avoid doxycycline and fluoroquinolones; keep maternal SpO2 greater than 95%.
  • Elderly: lower threshold for admission; watch for delirium and aspiration; review swallowing.
  • Renal impairment: adjust levofloxacin, vancomycin and piperacillin tazobactam doses.
  • Fluoroquinolones: avoid with aortic aneurysm risk, tendon disease, QT prolongation, and when tuberculosis is possible (partial treatment masks it).
  • HIV: test for tuberculosis and Pneumocystis pneumonia; co-trimoxazole 960 mg daily prophylaxis when CD4 count is less than 200; start antiretroviral therapy.
• Prevention: pneumococcal conjugate vaccine (20-valent single dose, or 15-valent followed by 23-valent polysaccharide vaccine 8 weeks or more later) for adults 65 or older and those with chronic disease or HIV; yearly influenza vaccine; smoking cessation.
• Red flags: respiratory rate 30 or more, SpO2 less than 90%, confusion, systolic pressure less than 90 mmHg, lactate greater than 2 mmol/L, multilobar disease, large effusion, no response at 72 hours.
• Follow up: clinical review at 48 to 72 hours for outpatients and at 6 weeks; repeat chest X-ray at 6 weeks in smokers over 50 or those with persisting symptoms, to exclude malignancy; HIV test, vaccination and review of underlying lung disease.