What is the step‑by‑step technique for performing a thoracentesis on an adult with a symptomatic pleural effusion, including informed consent, coagulation criteria (INR < 1.5, platelet count > 50 × 10⁹/L), patient positioning, and ultrasound guidance?

How to Perform Thoracentesis

Pre-Procedure Requirements

Obtain informed consent and verify coagulation parameters before proceeding with thoracentesis. While specific coagulation thresholds are not universally defined in guidelines, clinical practice typically accepts INR < 1.5 and platelet count > 50 × 10⁹/L as safe parameters for the procedure 1.

Patient Positioning

• Position the patient sitting upright, leaning forward over a bedside table with arms supported 1
• This position allows the diaphragm to descend and maximizes the intercostal spaces posteriorly
• For critically ill or mechanically ventilated patients who cannot sit, use lateral or dorsal decubitus positioning 2

Ultrasound Guidance (Mandatory)

Ultrasound guidance must be used for all thoracentesis procedures to minimize complications and maximize success 3, 4.

Ultrasound Technique

• Measure the maximal distance between parietal and visceral pleura at the thoracic base in the posterior axillary line during end-expiration 3
• This provides the most accurate volume estimation
• Ultrasound reduces pneumothorax rates from 33-50% to 0% and overall complication rates from 6.5% to 1.3% 3
• Success rate for fluid retrieval is 97%, even after failed landmark-based attempts 3
• Evaluate for internal echoes, septations, and loculations to distinguish simple from complex effusions 3

Minimum Safe Effusion Size

• Do not attempt thoracentesis for effusions <1 cm thickness on ultrasound measurement 3
• Small effusions (<1 cm) require ultrasound guidance if drainage is clinically indicated 3

Needle Insertion Technique

For Diagnostic Thoracentesis

• Use a fine bore 21-gauge needle with a 50 mL syringe 1
• Insert the needle just above the superior border of the rib to avoid the neurovascular bundle that runs along the inferior rib margin
• Advance the needle perpendicular to the chest wall while maintaining negative pressure on the syringe
• Stop advancing once pleural fluid is obtained

Site Selection

• Use ultrasound to identify the optimal insertion site with adequate fluid depth (≥15 mm interpleural distance visible over three intercostal spaces) 2
• Mark the site with the patient in the position they will maintain during the procedure
• The posterior axillary line at the thoracic base is typically preferred 3

Fluid Collection and Analysis

Sample Collection

• Place fluid samples in both sterile vials and blood culture bottles to increase diagnostic yield 1, 4
• Collect sufficient volume for comprehensive analysis (typically 50-100 mL for diagnostic purposes)

Required Laboratory Tests

Send pleural fluid for the following analyses 1, 4:

• Protein and LDH (to differentiate transudate from exudate using Light's criteria if protein is 25-35 g/L)
• pH (in all non-purulent effusions if infection is suspected)
• Gram stain and culture (aerobic and anaerobic)
• Acid-fast bacilli (AAFB) stain and culture
• Cytology
• Cell count with differential

Fluid Appearance Documentation

• Note the appearance and odor of the pleural fluid 1
• Classify as serous, blood-tinged, frankly bloody, or purulent
• If turbid or milky, centrifuge to distinguish empyema (clear supernatant) from chylothorax (turbid supernatant) 1
• For bloody fluid, obtain a hematocrit: if >50% of peripheral blood hematocrit, this confirms hemothorax 1

Volume Removal for Therapeutic Thoracentesis

• Remove fluid until symptoms improve, the patient develops chest discomfort, or resistance is met
• Monitor for re-expansion pulmonary edema, particularly when removing large volumes
• The procedure should be immediate (less than 10 seconds from needle insertion to fluid return) in most cases 2

Common Pitfalls to Avoid

• Never perform thoracentesis without ultrasound guidance – this dramatically increases pneumothorax risk 3
• Do not rely on chest radiograph alone – ultrasound is more sensitive than bedside X-ray in detecting pleural effusion, with 17 of 44 effusions in one study visible only on ultrasound 2
• Avoid attempting drainage of effusions <1 cm thickness without compelling clinical indication 3
• Do not insert the needle along the inferior rib border – this risks injury to the intercostal neurovascular bundle 1
• Never skip sending fluid in blood culture bottles – this significantly increases microbiological diagnostic yield 1, 4

Post-Procedure Management

• Complications of thoracentesis include site pain (1-15%), pneumothorax (3-15% without ultrasound, near 0% with ultrasound), vasovagal reaction (1-5%), hemothorax (<2%), and site hematoma (<1%) 1, 3
• Only 1% of pneumothoraces require chest drainage 1
• Immediate specialist referral is indicated for: purulent or frankly bloody fluid, pH <7.2 in suspected infection, positive Gram stain or culture, large effusions causing respiratory compromise, or loculated effusions 4