Thoracentesis

Thoracentesis is a minimally invasive procedure in which a needle is inserted through the chest wall into the pleural space to remove accumulated fluid. It serves both diagnostic and therapeutic purposes — fluid samples can be analyzed to identify the cause of a pleural effusion, while removing large volumes of fluid provides immediate relief from breathlessness and chest pressure. In the context of mesothelioma, thoracentesis is often the first procedure performed when a patient presents with an unexplained pleural effusion.¹

Pleural effusion is one of the earliest and most common signs of pleural mesothelioma, occurring in approximately 90% of patients at some point during their disease. The effusion develops because mesothelioma tumors on the pleural surfaces disrupt normal fluid dynamics — the cancerous tissue produces excess fluid while simultaneously obstructing the lymphatic channels that normally drain it. As fluid accumulates, it compresses the underlying lung and progressively reduces breathing capacity.²

During a diagnostic thoracentesis, the withdrawn fluid is sent for cytological examination (microscopic analysis of cells), biochemical analysis, and sometimes flow cytometry. In approximately 60% of mesothelioma cases, malignant mesothelial cells can be identified in the pleural fluid. However, distinguishing malignant mesothelial cells from reactive (benign) mesothelial cells can be extremely challenging, and a negative fluid cytology does not rule out mesothelioma. When cytology is inconclusive, a tissue biopsy is required for definitive diagnosis.³

Modern thoracentesis is performed under real-time ultrasound guidance, which has become the standard of care. Ultrasound allows the clinician to visualize the fluid pocket, identify the optimal insertion point, measure the depth to the fluid, and avoid injury to the lung, diaphragm, and intercostal blood vessels. Ultrasound-guided thoracentesis has a pneumothorax rate of less than 2%, compared to 5–15% with landmark-based (non-guided) techniques.⁴

Thoracentesis is classified by its primary purpose, though both functions are often accomplished in a single procedure:¹

• Diagnostic thoracentesis — A small volume of pleural fluid (50–100 mL) is withdrawn for laboratory analysis. The fluid is examined for cell types (cytology), protein and lactate dehydrogenase levels (to classify the effusion as transudative or exudative using Light's criteria), glucose, pH, bacterial cultures, and tumor markers. In mesothelioma workup, cytology and cell block preparation with immunohistochemistry are the key diagnostic tests³
• Therapeutic thoracentesis — A large volume of fluid (up to 1,000–1,500 mL) is removed to relieve symptoms. Rapid removal of more than 1,500 mL in a single session is generally avoided because of the risk of re-expansion pulmonary edema — a potentially serious complication in which the lung, suddenly relieved of external compression, develops edema as capillary permeability increases²

Thoracentesis is indicated when a patient presents with symptoms of pleural effusion, which include:²

• Progressive dyspnea — Shortness of breath that worsens over days to weeks as fluid accumulates and compresses the lung
• Pleuritic chest pain — Sharp or dull chest pain, often worse with deep breathing or coughing, caused by inflammation of the pleural surfaces
• Reduced exercise tolerance — Inability to perform activities that were previously manageable
• Dry, nonproductive cough — Triggered by fluid pressing on the lung and airways
• Orthopnea — Difficulty breathing when lying flat, as fluid shifts in the supine position
• Decreased breath sounds — On physical examination, breath sounds are diminished or absent over the area of fluid accumulation
• Dullness to percussion — Tapping on the chest wall over the effusion produces a dull sound rather than the normal resonant tone

When pleural fluid obtained via thoracentesis is analyzed in the context of a suspected mesothelioma diagnosis, several laboratory studies are performed:³

• Cytology — The fluid is centrifuged and the cell pellet is examined under microscopy for malignant cells. Mesothelioma cells may appear as large, atypical mesothelial cells with prominent nucleoli, papillary clusters, or cell-in-cell patterns. However, reactive mesothelial cells can mimic these features, making definitive cytologic diagnosis challenging³
• Cell block with immunohistochemistry — The cell pellet is embedded in paraffin and sectioned for immunohistochemical staining. Markers such as calretinin, WT1, D2-40, and CK5/6 support a mesothelial origin, while markers like CEA, TTF-1, and MOC-31 argue against mesothelioma and suggest adenocarcinoma⁵
• Biochemical analysis — Mesothelioma-related effusions are almost always exudative (high protein, high LDH) by Light's criteria. Low glucose (<60 mg/dL) and low pH (<7.30) in the fluid are associated with higher tumor burden and poorer prognosis
• Fluid biomarkers — Soluble mesothelin-related peptide (SMRP) levels in pleural fluid may be elevated in mesothelioma and can support the diagnosis when combined with cytology, though biomarkers alone are not sufficient for a definitive diagnosis⁵

If thoracentesis cytology is negative or inconclusive — which occurs in approximately 40% of mesothelioma cases — a tissue biopsy via thoracoscopy or CT-guided needle biopsy is required for definitive diagnosis.³

As a procedure, thoracentesis follows a standardized technique that has been refined for safety and efficacy:⁴

• Patient positioning — The patient sits upright, leaning slightly forward with arms resting on a bedside table. This position allows gravity to pool the effusion at the lung base and widens the intercostal spaces for needle access
• Ultrasound marking — Real-time ultrasound identifies the fluid pocket, measures its depth, and marks the optimal insertion site — typically in the posterior or lateral chest wall, one to two intercostal spaces below the upper fluid border
• Sterile preparation and local anesthesia — The skin is cleaned with chlorhexidine, a sterile drape is applied, and local anesthetic (lidocaine) is infiltrated through the skin, subcutaneous tissue, and down to the parietal pleura
• Needle insertion — A thoracentesis needle or catheter-over-needle assembly is advanced over the top of the rib (to avoid the intercostal neurovascular bundle running along the rib's lower border) until pleural fluid is aspirated
• Fluid aspiration — Fluid is withdrawn by syringe or vacuum bottle. Samples are sent for cytology, biochemistry, microbiology, and cell block preparation. For therapeutic drainage, larger volumes are removed using a drainage catheter connected to a vacuum system
• Post-procedure assessment — A chest radiograph is typically obtained after the procedure to confirm lung re-expansion and rule out pneumothorax

Thoracentesis itself carries an excellent safety profile, particularly when performed under ultrasound guidance. Outcomes and considerations include:⁴

• Complication rate — Ultrasound-guided thoracentesis has a major complication rate of less than 2%. The most common complication is pneumothorax (air entering the pleural space), which occurs in approximately 1–2% of ultrasound-guided procedures and often resolves without intervention⁴
• Symptom relief — Therapeutic thoracentesis provides immediate improvement in dyspnea in the majority of patients. The degree of relief correlates with the volume of fluid removed and the ability of the lung to re-expand
• Recurrence — In malignant pleural effusion from mesothelioma, fluid almost always reaccumulates within days to weeks after thoracentesis. Recurrent effusion requiring repeated drainage is an indication for definitive management with pleurodesis or an indwelling pleural catheter²
• Diagnostic limitations — Approximately 40% of mesothelioma cases cannot be diagnosed by fluid cytology alone, necessitating tissue biopsy. A negative thoracentesis in the setting of high clinical suspicion should always be followed by further investigation³

Patients who undergo thoracentesis, particularly those with recurrent malignant effusions, should understand what to expect:²

• Immediate recovery — Most patients feel noticeably better within hours of the procedure as the lung re-expands. Mild discomfort at the insertion site is normal and typically resolves within 1–2 days
• Activity after procedure — Normal activities can usually be resumed the same day or the following day. Patients are advised to avoid heavy lifting or strenuous activity for 24–48 hours
• Monitoring for recurrence — Patients should contact their physician if dyspnea returns, as this likely indicates fluid reaccumulation. Tracking the interval between thoracentesis procedures helps the care team decide when to recommend a more durable solution such as pleurodesis or a tunneled pleural catheter
• Repeated procedures — Some patients undergo multiple thoracentesis procedures before a definitive management strategy is implemented. Each procedure carries a small cumulative risk, and repeated drainage can lead to protein depletion and loculation (compartmentalization of fluid that becomes harder to drain)