Diagnostics & Procedures · Topic 9 of 16

Thoracentesis

Procedures · pleural fluid

Thoracentesis

Tap any new effusion at least 10 mm deep under ultrasound, run Light's criteria, and stop draining when the patient feels chest tightness.

Key numbers

Tap any new effusion ≥ 10 mm≥ 10 mm on ultrasound

Light's criteria: any one means exudatePF/serum protein > 0.5

Pus or pH < 7.20: chest tubepH < 7.20

01 · Defined byTap any new effusion ≥ 10 mm

Perform diagnostic thoracentesis for any new or unexplained effusion at least 10 mm thick, and therapeutic thoracentesis for dyspnea from a large effusion; both can be done at the same tap.

02 · DoAlways use ultrasound guidance

Use thoracic ultrasound for every thoracentesis: it raises success and cuts pneumothorax from about 5.7–15% without guidance to 0.3–1.5%, and helps avoid liver, spleen and diaphragm puncture.

03 · Watch forStop draining for chest tightness, not volume

No fixed volume limit applies (1–1.5 L is the traditional cap): stop for chest tightness or pain, dyspnea or hypotension. Re-expansion pulmonary edema is rare and does not track volume removed.

Causes & risk factors

Why it happens

Ventilated patients: tap supine under ultrasoundThoracentesis can be done supine or semi-recumbent in ventilated patients, but only with ultrasound or CT guidance; positive-pressure ventilation raises the risk of complications.

Diagnosis & monitoring

Work-up

Light's criteria: any one means exudate must knowExudate if any one of Light's criteria is met: pleural/serum protein > 0.5, pleural/serum LDH > 0.6, or pleural LDH > two-thirds the upper limit of normal serum LDH. Draw serum at the tap.

Post-tap film only if symptomsA chest radiograph after thoracentesis is needed only if air was aspirated, several passes were made, the patient is mechanically ventilated, or dyspnea, chest pain or hypoxemia develops — not routinely.

Send protein, LDH, glucose, cells, Gram stainAlways send pleural protein, LDH, glucose, cell count with differential and Gram stain; add pH, cultures, cytology, adenosine deaminase, amylase or triglycerides as the clinical picture dictates.

Management

Orders

Sit up, enter over the ribSeat the patient upright leaning forward, mark the site by ultrasound (classically one interspace below the top of the effusion, mid-scapular line), and enter over the upper rib border to spare the neurovascular bundle.

Pus or pH < 7.20: chest tubeIf the tap returns pus or a parapneumonic effusion has pH below 7.20, place a chest tube rather than repeating thoracentesis; with high suspicion of empyema or hemothorax, start with a tube.

Recurrent malignant effusion: catheter or pleurodesisWhen a symptomatic malignant effusion recurs after therapeutic thoracentesis, offer an indwelling pleural catheter or talc pleurodesis rather than repeated taps.

Red flags & don'ts

Avoid harm

No absolute contraindications; mind anticoagulation avoidThere are no absolute contraindications. Bleeding is infrequent with ultrasound guidance even with abnormal coagulation tests; avoid elective taps on therapeutic anticoagulation (reverse if urgent), while aspirin is safe. Never puncture cellulitis or zoster.

Complications: pneumothorax, hemothorax, organ puncture complicationComplications are pneumothorax, hemothorax from intercostal artery injury, re-expansion pulmonary edema, vasovagal reaction, infection and liver or spleen puncture (about 0.3%).

Non-expandable lung: discomfort and ex vacuo pneumothorax complicationChest discomfort during drainage suggests non-expandable lung; the pneumothorax ex vacuo seen afterward is benign and needs no chest tube, and a trapped lung needs an indwelling catheter rather than pleurodesis.

Diagnostic or therapeutic thoracentesis?

Commonly confused

Purpose	Find the cause of a new or unexplained effusion
Volume	A small sample for fluid analysis
Main added risk	Pneumothorax, bleeding

Commonly confused

Tell apart

Bloody fluid that won't clot is realBloody pleural fluid that does not clot was already in the pleural space (free pleural blood defibrinates), not a traumatic tap; a pleural hematocrit above 50% of the blood hematocrit defines hemothorax.

More detail

Reference

Show 4 reference facts

Coughing during drainage is normal and reflects lung re-expansion; above 500 mL, watch symptoms and blood pressure.

Pleural manometry can guide large-volume drainage (stop if end-expiratory pleural pressure falls below −20 cm H₂O), but a randomized trial found it did not reduce procedure-related chest discomfort.

Re-expansion pulmonary edema appears on imaging in 0.2–14% of reported series but is clinically significant in under 1%; it causes ipsilateral ground-glass opacities and hypoxemia within hours.

Use a 16–19-gauge over-the-needle catheter on a 3-way stopcock, withdraw 30 mL for testing, cover the needle hub during inspiration, and remove the catheter during breath-holding or expiration.

Pearl

Ultrasound on every tap, and let symptoms — not a fixed volume — tell you when to stop.

Sources · Merck Manual Professional — How To Do Thoracentesis (Dezube R, rev. May 2025) · Williams JG, Lerner AD. Managing complications of pleural procedures. J Thorac Dis 2021 (PMC8411187) · Merck Manual Professional — Pleural Effusion (Rahman NM, rev. Jul 2025) · Merck Manual Professional — Thoracentesis (overview; Dezube R, rev. Nov 2025) · Pearce C, Crapnell A, Bedawi EO, Rahman NM, Corcoran JP. Pleural Infection: Diagnosis, Management, and Future Directions. 2025 (PMC11899816; pH ≤ 7.2 warrants chest tube per ACCP and BTS) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).