Diagnosis of Empyema
Empyema should be suspected in any patient with pneumonia failing to respond to appropriate antibiotic therapy, and diagnosis requires imaging confirmation of pleural effusion followed by diagnostic thoracentesis with pleural fluid analysis showing frank pus, positive Gram stain/culture, or pH <7.2. 1, 2
Clinical Presentation
Common symptoms include fever, chest pain, and breathlessness. 2 These symptoms in the context of pneumonia that is not improving with antibiotics should immediately raise suspicion for empyema. 1, 2
The clinical onset may be insidious, particularly with anaerobic infections, which can present with less fever, greater weight loss, and are more common following aspiration pneumonia or in patients with poor dental hygiene. 1
Imaging Strategy
Initial Radiographic Assessment
Posteroanterior and lateral chest radiographs should be obtained first, as this combination has significantly greater sensitivity (83.9%) compared to single-view AP radiographs (67.3%) for detecting pleural effusions. 2
The co-existence of pulmonary infiltrates and pleural fluid should alert clinicians to possible parapneumonic collection. 1
Ultrasound (Preferred Modality)
Chest ultrasound is the preferred imaging modality to confirm presence of pleural fluid, differentiate free from loculated fluid, and guide thoracentesis or chest drain placement. 1, 2
All echogenic effusions on ultrasound are exudates, and homogeneous echogenic effusions are due to either empyema or hemorrhage. 1
Ultrasound enables exact location of fluid collections and allows guided diagnostic aspiration. 1
CT Scanning
Contrast-enhanced CT scanning is indicated in complicated cases to delineate loculated pleural fluid, detect airway or parenchymal lung abnormalities, and identify the "split pleura" sign. 1, 2
The "split pleura" sign on contrast-enhanced CT (caused by enhancement of visceral and parietal pleura separated by fluid) helps differentiate empyema from lung abscess. 1
Empyemas are typically lenticular in shape and compress lung parenchyma, while lung abscesses have indistinct boundaries between lung parenchyma and collection. 1
Pleural Fluid Analysis
Mandatory Testing
Diagnostic thoracentesis should be performed in all patients with suspected pleural infection. 2
Gram stain and bacterial culture (both aerobic and anaerobic) of pleural fluid must be performed whenever a specimen is obtained. 1 Culture in blood culture bottles increases yield to 75% positivity. 1
Pleural fluid should be sent for WBC count with differential analysis primarily to help differentiate bacterial from mycobacterial, fungal, or malignancy etiologies. 1
Biochemical Parameters
For pediatric empyema, biochemical analysis of pleural fluid (pH, glucose, protein, LDH) rarely changes patient management and is not recommended. 1 This represents a weak recommendation with very low-quality evidence. 1
For adult empyema, pleural fluid pH is the most diagnostically accurate parameter (area under ROC curve 0.92), with pH <7.2 indicating complicated parapneumonic effusion requiring drainage. 1, 2
When frank pus is present, no additional biochemical tests are necessary as the diagnosis of empyema is established. 1
Enhanced Microbiological Detection
- Antigen testing or nucleic acid amplification through PCR increases pathogen detection in pleural fluid (identifying bacteria in 42-80% of samples versus <25% with culture alone) and may be useful for management, especially in patients pretreated with antibiotics. 1
Classification System
Simple Parapneumonic Effusion
- Clear fluid with pH >7.2, LDH <1000 IU/L, glucose >2.2 mmol/L, and negative Gram stain/culture. 1, 2 These will usually resolve with antibiotics alone. 1
Complicated Parapneumonic Effusion
- Clear or cloudy/turbid fluid with pH <7.2, LDH >1000 IU/L, glucose <2.2 mmol/L, and may have positive Gram stain/culture. 1, 2 These require chest tube drainage. 1
Empyema
- Frank pus on visual inspection, which may have positive Gram stain/culture. 1, 2 No additional biochemical testing is needed when pus is present. 1
Blood Cultures
- Blood cultures should be performed in all patients with suspected empyema before starting antibiotics. 2 While blood culture yield is approximately 11% in community-acquired pneumonia, identification of the organism allows antimicrobial de-escalation. 2
Size-Based Management Algorithm
Small Effusions
- <10 mm on lateral decubitus radiograph or opacifies less than one-fourth of hemithorax: Sampling of pleural fluid is not routinely required; treat with antibiotics alone. 1
Moderate Effusions
- >10 mm rim but opacifies less than half of hemithorax: Thoracentesis may help determine presence of empyema and need for drainage. Drain if patient has respiratory compromise or if pleural fluid is consistent with empyema. 1
Large Effusions
- Opacifies more than half of hemithorax: Drainage indicated in most cases. 1
Special Considerations
Spontaneous Bacterial Pleural Empyema in Cirrhosis
- Diagnosed by pleural fluid analysis showing positive culture and >250 neutrophils/mm³, or negative culture and >500 neutrophils/mm³, in the absence of lung infection. 1, 2 This is associated with spontaneous bacterial peritonitis in 50% of cases. 1
Anaerobic Infections
- Anaerobes may be present in up to 76% of empyema cases and should be suspected with aspiration pneumonia, poor dental hygiene, or insidious clinical onset. 1, 2 Anaerobic cultures must be specifically requested. 1
Critical Pitfalls to Avoid
Do not delay antibiotic therapy while waiting for imaging or culture results in patients with clinical signs of infection. 2
Do not rely solely on chest radiograph when empyema is suspected; ultrasound should be performed to confirm fluid and guide sampling. 1, 2
Do not assume culture-negative effusions are sterile—nucleic acid amplification identifies pathogens in 42-80% of culture-negative cases. 1
Do not perform biochemical analysis on grossly purulent fluid—the diagnosis is already established and pH measurement is unnecessary. 1