Lung abscess !

A lung abscess is a pus-filled cavity in the lung, usually after inhaling mouth bacteria. It causes fever and foul-smelling sputum.

Updated 2026-11-01

Structures involved

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In the clinical exam

Why it happens

What it is

A localised area of necrosis of lung tissue caused by infection, forming a cavity larger than 2 cm that contains pus and often an air-fluid level. Smaller multiple cavities are called necrotising pneumonia.

Who gets it

Less frequent since antibiotics became widely available, but still seen in people with alcohol use disorder, seizures, poor dentition or swallowing disorders. In Colombia and Latin America, cavitary tuberculosis is a more common cause of a lung cavity and must always be excluded.

Causes

Aspiration of oral anaerobic flora (Peptostreptococcus, Prevotella, Fusobacterium, Bacteroides) is the most common cause
Staphylococcus aureus and Klebsiella pneumoniae (necrotising pneumonia)
Septic emboli from right-sided endocarditis or septic thrombophlebitis
Bronchial obstruction by a tumour or foreign body
Mycobacteria, fungi and parasites in immunocompromised hosts

Risk factors

Reduced consciousness: alcohol intoxication, seizures, anaesthesia, drug overdose
Dysphagia from stroke, neuromuscular disease or oesophageal disease
Poor dental hygiene and gingivitis
Injection drug use
Immunosuppression, diabetes and lung cancer

Pathophysiology

Aspirated oral bacteria cause pneumonitis that progresses to tissue necrosis in 1–2 weeks. The necrotic area liquefies and, when it drains into a bronchus, air enters and an air-fluid level forms. Gravity determines the site: lying down, aspiration goes to the posterior segments of the upper lobes and the superior segments of the lower lobes, most often on the right because the right main bronchus is wider and more vertical.

Classification

Primary: aspiration or pneumonia in an otherwise normal lung
Secondary: bronchial obstruction, septic emboli or immunosuppression
Acute (under 4–6 weeks) or chronic

Symptoms and signs

Insidious fever, night sweats and malaise over weeks
Cough with abundant, foul-smelling and foul-tasting sputum
Pleuritic chest pain
Weight loss and anaemia in chronic cases
Haemoptysis
Poor dentition or gingivitis on examination; digital clubbing in chronic cases

Diagnosis

Clinical: Suspected in a person with aspiration risk, subacute fever and foul sputum. Assessment of dentition, swallowing and level of consciousness.
Tests: Blood count, blood cultures, sputum Gram stain and culture (anaerobes are hard to grow from sputum); acid-fast smears, molecular test and culture for tuberculosis in every cavity; fungal studies when immunosuppressed. Bronchoscopy when obstruction or cancer is suspected or the response is poor.
Imaging: Chest X-ray: thick-walled cavity with an air-fluid level in a dependent segment. CT defines the cavity, distinguishes abscess from empyema (abscess forms acute angles with the chest wall) and looks for an obstructing lesion.

Differential diagnosis

Cavitary pulmonary tuberculosis
Cavitating lung cancer (squamous cell carcinoma)
Empyema with bronchopleural fistula
Granulomatosis with polyangiitis
Infected bulla or cyst, aspergilloma

Goal of treatment

Eradicate the infection, allow the cavity to drain and close, and correct the cause of aspiration or obstruction.

Treatment

Medical: Prolonged antibiotics with anaerobic activity (e.g. a beta-lactam with beta-lactamase inhibitor, or clindamycin), often for several weeks until the X-ray improves.
Conservative: Postural drainage and chest physiotherapy, nutrition support and dental care.
Procedure: Percutaneous or endoscopic drainage for large abscesses that do not respond; bronchoscopy to remove a foreign body or assess a tumour.
Surgical: Lobectomy for failure of medical treatment, massive haemoptysis or suspected cancer.

Complications

Empyema and bronchopleural fistula
Massive haemoptysis
Spread of infection to other lung areas
Bronchiectasis and residual cavity
Brain abscess and sepsis

Prognosis

Most primary abscesses heal with prolonged antibiotics. Outcomes are worse in older patients, immunosuppressed hosts, large cavities, Klebsiella or Staphylococcus infections and abscesses secondary to bronchial obstruction or cancer.

Prevention

Aspiration precautions in patients with dysphagia or reduced consciousness (head of bed elevated, swallowing assessment)
Dental hygiene and treatment of periodontal disease
Treatment of alcohol use disorder

Test yourself

Think of the answer, then tap the question to see it.

What is the most common cause of a primary lung abscess?

Aspiration of oral anaerobic bacteria in people with reduced consciousness, dysphagia or poor dentition.

Which lung segments are most often affected after aspiration in a supine patient?

The posterior segments of the upper lobes and the superior segments of the lower lobes, more often on the right.

What is the classic radiological sign of a lung abscess?

A thick-walled cavity with an air-fluid level.

Which disease must always be excluded in Colombia when a lung cavity is found?

Pulmonary tuberculosis, with acid-fast smears, molecular testing and mycobacterial culture.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: Abscess in the Lungs (Merck Manual Consumer Version, 2024).

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Representative model, for educational purposes only.

Right lower lobe highlighted on the Durgical 3D model