Definition/General
Introduction:
Pleural empyema is purulent infection of the pleural space
It contains frank pus with high bacterial load
Characterized by thick, viscous fluid with neutrophilic predominance
Represents complicated parapneumonic effusion
Requires urgent drainage and antibiotic therapy.
Origin:
Results from bacterial invasion of pleural space
Most commonly from pneumonia extension (60% cases)
Post-surgical complications (thoracic surgery)
Chest trauma with secondary infection
Hematogenous spread from distant infection sites.
Classification:
Classified as complicated parapneumonic effusion
Stage 1: Simple parapneumonic (sterile)
Stage 2: Complicated parapneumonic (fibrinopurulent)
Stage 3: Frank empyema (organizing stage)
Based on Light criteria for empyema.
Epidemiology:
Incidence 2-10 per 100,000 population annually
Male predominance (2:1 ratio)
Bimodal age distribution: children <5 years and adults >65 years
Mortality 5-15% with treatment
Higher prevalence in immunocompromised patients.
Clinical Features
Presentation:
High-grade fever with chills and rigors
Severe chest pain (sharp, stabbing)
Marked dyspnea and tachypnea
Productive cough with purulent sputum
Night sweats and malaise
Sepsis syndrome may develop.
Symptoms:
Fever >38.5°C (90% cases)
Severe pleuritic chest pain (85% cases)
Dyspnea and tachypnea (80% cases)
Productive cough (70% cases)
Night sweats (60% cases)
Weight loss and anorexia (50% cases)
Fatigue and weakness.
Risk Factors:
Pneumonia (most common predisposing factor)
Immunocompromised state
Diabetes mellitus
Chronic lung disease
Alcohol abuse
Post-thoracic surgery
Chest trauma
Intravenous drug use.
Screening:
Chest X-ray shows pleural opacity
CT thorax for loculation assessment
Ultrasound-guided thoracentesis
Blood cultures (positive in 20-30%)
Procalcitonin and inflammatory markers elevated.
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Gross Description
Appearance:
Pleural fluid is thick, viscous, and purulent
Yellow-green to gray-brown color
Foul odor may be present (anaerobic bacteria)
Opaque and turbid appearance
May contain fibrin clots and debris.
Characteristics:
pH <7.30 (usually <7.20)
Glucose <60 mg/dL (often <30 mg/dL)
LDH >1000 U/L
Protein >3 g/dL
Low complement levels
Gram stain positive in 60-80% cases.
Size Location:
Usually unilateral (90% cases)
Right-sided slightly more common
Loculated due to fibrin deposition
Multi-loculated in advanced cases
May form pleural peel in chronic cases.
Multifocality:
Typically unilateral involvement
Multiple loculations common
Pleural thickening and adhesions
Trapped lung in chronic cases
Bronchopleural fistula may develop (5-10% cases).
Microscopic Description
Histological Features:
Pleural fluid shows massive neutrophilic infiltration (>75% neutrophils)
Very high cell count (>50,000 cells/μL)
Degenerative neutrophils with nuclear fragmentation
Abundant cellular debris
Bacteria visible on gram stain.
Cellular Characteristics:
Mature neutrophils with multi-lobed nuclei
Degenerative changes: pyknosis, karyorrhexis
Toxic granulation in neutrophils
Reactive mesothelial cells (if viable)
Macrophages with phagocytosed debris.
Architectural Patterns:
Sheets of neutrophils
Inflammatory cell aggregates
Fibrin strands and clots
Necrotic background with debris
Bacterial colonies (when visible)
Reactive mesothelial proliferation (minimal).
Grading Criteria:
Empyema criteria: Gross pus
pH <7.30
Glucose <60 mg/dL
LDH >1000 U/L
Positive gram stain/culture
Neutrophil count >75% of total cells.
Immunohistochemistry
Positive Markers:
Myeloperoxidase (MPO) positive in neutrophils
CD15 positive in neutrophils
Lysozyme positive in macrophages
CD68 positive in macrophages
Calretinin in reactive mesothelial cells (if present).
Negative Markers:
Cytokeratins (except mesothelial cells)
CEA, TTF-1 to exclude adenocarcinoma
CD20, CD3 (lymphoid markers)
S-100 to exclude melanoma
PSA to exclude prostate metastases.
Diagnostic Utility:
IHC rarely needed for empyema diagnosis
Morphology and gram stain usually diagnostic
May help distinguish from necrotizing malignancy
Neutrophil markers confirm inflammatory nature
Bacterial culture most important for diagnosis.
Molecular Subtypes:
No molecular subtypes
Bacterial species identification by culture/PCR
Antibiotic resistance patterns
16S rRNA sequencing for difficult organisms
MALDI-TOF for rapid bacterial identification.
Molecular/Genetic
Genetic Mutations:
No genetic mutations in host cells
Bacterial DNA detection by PCR
Resistance genes in bacteria (MRSA, ESBL)
Virulence factors in pathogenic bacteria
16S rRNA gene for bacterial identification.
Molecular Markers:
Procalcitonin markedly elevated
C-reactive protein >200 mg/L
Interleukin-6, TNF-alpha elevated
Lactate levels increased in pleural fluid
Complement levels decreased.
Prognostic Significance:
Early drainage improves prognosis
Multi-drug resistant organisms worsen outcomes
Delayed treatment leads to organizing empyema
Comorbidities affect prognosis
Complete drainage prevents recurrence.
Therapeutic Targets:
Broad-spectrum antibiotics initially
Culture-guided therapy after sensitivity
Pleural drainage: chest tube/VATS
Fibrinolytic therapy (streptokinase, urokinase)
Decortication in organizing empyema.
Differential Diagnosis
Similar Entities:
Complicated parapneumonic effusion (pre-empyema)
Necrotizing pneumonia with pleural extension
Malignant pleural effusion with secondary infection
Fungal empyema (immunocompromised)
Tuberculous empyema (rare).
Distinguishing Features:
Empyema: Frank pus
Empyema: pH <7.30
Empyema: Very high neutrophil count
Parapneumonic: Turbid but not purulent
Malignant: Cytological atypia
Fungal: Fungal elements on stain.
Diagnostic Challenges:
Distinguishing from complicated parapneumonic effusion
Secondary infection in malignant effusion
Anaerobic empyema may be culture-negative
Antibiotic-treated empyema may show fewer bacteria
Need for rapid diagnosis and treatment.
Rare Variants:
Anaerobic empyema (foul-smelling)
Fungal empyema (immunocompromised)
Tuberculous empyema (chronic cases)
Post-operative empyema
Empyema necessitans (chest wall extension).
Sample Pathology Report
Template Format
Sample Pathology Report
Complete Report: This is an example of how the final pathology report should be structured for this condition.
Specimen Information
Pleural fluid, volume [X] mL, thick, purulent, yellow-green color
Adequacy
Adequate for cytological and microbiological evaluation
Cellularity
Very high cellularity with [X] cells/μL (>50,000 cells/μL)
Cell Differential
Neutrophils: [X]% (>75%), Macrophages: [X]%, Lymphocytes: [X]%, Mesothelial cells: [X]%
Morphological Features
Massive neutrophilic infiltration with degenerative changes. Abundant cellular debris and fibrin. Background shows purulent inflammation.
Gram Stain
Gram stain: [Positive/Negative for bacteria] - [Gram positive/negative cocci/rods]
Biochemical Analysis
pH: [X] (<7.30), Glucose: [X] mg/dL (<60), LDH: [X] U/L (>1000)
Culture Results
Bacterial culture: [Organism identified/Pending/No growth] - Sensitivity: [Pattern]
Final Diagnosis
Pleural empyema - Urgent drainage recommended
Comment
URGENT: Findings consistent with pleural empyema. Immediate chest tube drainage and antibiotic therapy recommended. Clinical team notified.