What tests are recommended for the basic evaluation of pleural effusion?

Basic Evaluation of Pleural Effusion

The basic evaluation of pleural effusion requires thoracentesis with pleural fluid analysis for protein, lactate dehydrogenase (LDH), pH, cell count with differential, Gram stain, culture (including blood culture bottles), acid-fast bacilli (AAFB) stain, and cytology, combined with imaging using chest radiography and thoracic ultrasound. 1

Initial Imaging Studies

Chest Radiography

• Posteroanterior (PA) chest radiograph is the first-line imaging modality and can detect approximately 200 ml of pleural fluid 1
• Lateral chest radiographs can detect as little as 50 ml of fluid by showing posterior costophrenic angle blunting 1
• Look for obliteration of the costophrenic angle, meniscus sign ascending the lateral chest wall, or homogeneous opacity in supine patients 1
• Assess for absence of contralateral mediastinal shift (suggests mediastinal fixation, bronchial obstruction, or extensive pleural involvement) 1

Thoracic Ultrasound (TUS)

• Ultrasound must be used to confirm the presence of pleural fluid and should guide all thoracentesis procedures 1
• TUS is more sensitive than chest radiography for detecting small effusions and can identify effusions as small as 3-5 ml 1
• Evaluate for an anechoic region above the diaphragm as the primary finding of pleural effusion 1
• Assess for internal echoes within the effusion to identify complicated effusions (exudates, empyema, hemorrhage) 1
• Estimate effusion volume and determine optimal drainage site 1
• Ultrasound-guided aspiration yields fluid in 97% of cases when initial thoracentesis fails 1

Diagnostic Thoracentesis Technique

Procedure Details

• Use a fine bore (21-gauge) needle with a 50 ml syringe for diagnostic sampling 1
• Ultrasound guidance should be used to reduce complications and improve success rates 2
• Perform thoracentesis for any new, unexplained unilateral effusion or bilateral effusion with normal heart size 1

Pleural Fluid Analysis - Essential Tests

Send samples in both sterile vials AND blood culture bottles to increase diagnostic yield 1

Biochemical Analysis

• Protein and LDH levels (to apply Light's criteria for transudate vs. exudate differentiation) 1, 3
• pH measurement (perform in all non-purulent effusions when infection is suspected; pH <7.2 indicates complicated parapneumonic effusion requiring drainage) 1, 2
• Glucose level (values <60 mg/dl associated with pH <7.30 suggest increased tumor burden or complicated infection) 1

Cellular Analysis

• Cell count with differential (lymphocyte predominance suggests malignancy or tuberculosis; >25% lymphocytes is unusual) 1
• Note that pleural fluid eosinophilia does not exclude malignancy 1

Microbiological Studies

• Gram stain and culture (send in sterile tube for staining and in blood culture bottles for enhanced organism recovery) 1
• AAFB stain and tuberculosis culture (essential in all unexplained exudates) 1

Cytological Examination

• Cytology for malignant cells (diagnostic in approximately 60% of malignant effusions on first sample, 87% with repeat sampling) 1
• Cell blocks and smears should both be prepared to increase yield 1

Fluid Appearance Assessment

• Document the appearance and odor of pleural fluid 1
• Serous, blood-tinged, frankly bloody, or purulent appearance guides differential diagnosis 1
• Unpleasant odor suggests anaerobic infection 1
• Measure pleural fluid hematocrit if hemothorax is suspected 1

Distinguishing Transudates from Exudates

Light's Criteria Application

• Use Light's criteria when pleural fluid protein is between 25-35 g/l to accurately differentiate exudates from transudates 1
• An exudate meets at least one of the following: pleural fluid protein/serum protein ratio >0.5, pleural fluid LDH/serum LDH ratio >0.6, or pleural fluid LDH >2/3 upper limit of normal serum LDH 3

Clinical Context

• Small bilateral effusions with heart failure, cirrhosis, or hypoalbuminemia are likely transudative and may not require thoracentesis 2
• Approximately one-third of malignant effusions have pH <7.30 at presentation 1

Advanced Imaging When Initial Evaluation is Non-Diagnostic

CT Chest with Contrast

• Request contrast-enhanced CT thorax (venous phase) when diagnosis remains unclear after initial evaluation 1
• CT should ideally be performed with fluid present to better visualize pleural abnormalities and identify optimal biopsy sites 1
• For suspected malignancy, extend CT to include abdomen and pelvis 1
• CT can identify pleural nodularity, circumferential thickening, and mediastinal involvement suggesting malignancy 1

When CT is Not Routinely Indicated

• CT scans should not be performed routinely in pediatric empyema due to high radiation exposure (up to 400 chest radiograph equivalents) 1
• Ultrasound adequately visualizes fibrinous septations better than CT 1

Common Pitfalls to Avoid

• Do not skip thoracentesis in unexplained effusions - approximately 20% remain undiagnosed without fluid sampling 4
• Always send pleural fluid in blood culture bottles in addition to sterile containers to maximize microbiological yield 1
• Do not rely on chest radiography alone - ultrasound is superior for small effusions and procedural guidance 1
• Transudative effusions can occasionally be malignant (paramalignant effusions from mediastinal nodes or concomitant heart failure) 1
• Only 60% of malignant effusions are diagnosed on first cytology; consider repeat sampling or pleural biopsy if suspicion remains high 1