Pleural Effusion & Lung Abscess

Harrison's Principles of Internal Medicine, 22e

PART I: PLEURAL EFFUSION

Definition & Pathophysiology

Transudate vs. Exudate: Light's Criteria

Pseudoexudates: Light's criteria misidentify ~25% of transudates as exudates (often "diuresed transudates"). If clinical suspicion favors a transudate, use: Serum-PF protein gradient >3.1 g/dL OR serum-PF albumin gradient >1.2 g/dL - these identify pseudoexudates with ~99-100% sensitivity.

Diagnostic Algorithm

Fig. 305-1: Diagnostic approach to pleural effusions. Transudate → treat underlying cause. Exudate → further workup including glucose, cytology, cultures, TB markers.

Causes

• Congestive heart failure (most common)
• Cirrhosis
• Nephrotic syndrome
• Peritoneal dialysis
• Superior vena cava obstruction
• Myxedema, Urinothorax

• Bacterial pneumonia / parapneumonic (most common exudative cause in USA)
• Malignancy (lung, breast, lymphoma = 75% of malignant effusions)
• Tuberculosis (most common cause worldwide)
• Pulmonary embolism (most commonly overlooked)
• Viral infections
• Collagen vascular disease (RA, SLE)
• GI disease (esophageal perforation, pancreatitis, intra-abdominal abscess)
• Chylothorax, hemothorax

Key Specific Effusions

• Occurs in up to 50% of community-acquired pneumonia
• Usually reactive (sterile cultures)
• 10% progress to complicated PPE or empyema (fibrinopurulent or grossly purulent fluid)
• Indicators requiring drainage (chest tube or intervention):
  • Loculated fluid
  • pH <7.20
  • Glucose <3.3 mmol/L (<60 mg/dL)
  • Positive Gram stain or culture
  • Gross pus (empyema)
  • PF LDH >900 IU/L

• Second most common exudate; poor prognosis (<6-month survival)
• Lung, breast, lymphoma = ~75%
• Usually exudate; reduced glucose if high tumor burden
• Diagnosis: cytology. If negative x2, consider thoracoscopic biopsy
• Treatment: therapeutic thoracentesis, indwelling pleural catheter (IPC), or pleurodesis with talc

• Predominantly lymphocytic exudate
• Markers: adenosine deaminase >40 IU/L or IFN-γ >140 pg/mL
• Diagnosis: pleural biopsy culture (higher yield than fluid); thoracoscopy

• Thoracic duct disruption (trauma/surgery or mediastinal tumor)
• Milky fluid; triglycerides >1.2 mmol/L (110 mg/dL)
• Treatment: chest tube + octreotide + eliminate enteral fat; if refractory - percutaneous thoracic duct blockage or ligation

• PF hematocrit >50% of peripheral blood hematocrit
• Most from trauma; treat with tube thoracostomy
• If bleeding >200 mL/h: consider angiographic embolization or surgery

Imaging of Pleural Effusion

Fig. 305-2: A - Simple (free-flowing) effusion on CT. B - Loculated effusion on CT (two pockets). C - Simple effusion on ultrasound. D - Complex effusion with loculations and septations on ultrasound.

Empyema CT

Fig. 305-3: Empyema on CT - note thick, contrast-enhancing pleural rind (red arrows).

Management of Parapneumonic Effusion/Empyema

1. Simple PPE - antibiotic treatment of pneumonia; no drainage needed
2. Complicated PPE (criteria above) - chest tube drainage
3. Fibrinopurulent stage (cannot fully drain) - intrapleural tPA (10 mg) + DNase (5 mg), or VATS/thoracoscopy to break adhesions
4. Organized stage (pleural peel) - surgical decortication
5. Trapped lung - pleural manometry; decortication if needed

PART II: LUNG ABSCESS

Definition

Classification

Epidemiology & Risk Factors

• More common in middle-aged men
• Major risk factor: aspiration
• At-risk groups: altered mental status, alcoholism, drug overdose, seizures, bulbar dysfunction, stroke, neuromuscular disease, esophageal dysmotility/obstruction, GERD
• Periodontal disease / gingivitis - anaerobic colonization of gingival crevices is critical; lung abscess is extremely rare in edentulous patients

Microbiology (Table 132-1)

Pathogenesis

CT of Lung Abscess

Fig. 132-1: CT chest showing lung abscess - early consolidation (left, black arrow) progressing to cavitation with an air-fluid level (right, white arrow).

Clinical Features

• Subacute presentation is typical for primary (anaerobic): weight loss, low-grade fever, mild anemia, leukocytosis, history of aspiration risk
• Putrid-smelling sputum = virtually diagnostic of anaerobic infection
• Pleuritic chest pain and productive cough
• Large effusion with minimal infiltrate may occur (especially anaerobic)

Differential Diagnosis of Cavitary Lung Lesion

• Lung infarction
• Malignancy (cavitating tumor)
• Pulmonary sequestration
• Cryptogenic organizing pneumonia
• Sarcoidosis, vasculitis (GPA/Wegener)
• Lung cysts/bullae with fluid
• Septic emboli (tricuspid endocarditis)

Diagnosis

• Chest X-ray: thick-walled cavity with air-fluid level
• CT chest: better definition; earlier cavitation; identifies underlying cause (malignancy); distinguishes peripheral lung abscess from empyema (critical distinction for treatment)
• Sputum: Gram stain + culture (may miss anaerobes; likely polymicrobial)
• Molecular methods (16S rRNA gene amplification) - increasingly used
• Secondary abscesses / failed empirical therapy: sputum + blood cultures, serology for fungi/viruses, bronchoscopy with BAL or protected brush, CT-guided needle aspiration (risks: contralateral spillage, pneumothorax, bronchopleural fistula)
• Bronchoscopy early if bronchial obstruction suspected

Treatment

Why clindamycin over penicillin? - Oral anaerobes can produce β-lactamases; clindamycin proved superior to penicillin in clinical trials.

• Typically 3-4 weeks to 14 weeks
• At least 6 weeks may be associated with better outcomes
• Chronic abscesses may need longer courses

• Spontaneous drainage via bronchial tree is most common
• Percutaneous CT-guided catheter drainage - systematic reviews support use for selected cases; consider when medical therapy fails or if patient is a poor surgical candidate
• Surgery (lobectomy/segmentectomy) - reserved for: failed medical therapy, massive hemoptysis, suspected malignancy, bronchopleural fistula, or inability to rule out cancer

Source: Harrison's Principles of Internal Medicine, 22e (2025) - Chapter 132: Lung Abscess; Chapter 305: Pleural Effusion