Pulmonary · Topic 17 of 28

Pleural Effusion

Pulmonary · pleural disease

Pleural Effusion

Tap any new effusion 10 mm or deeper under ultrasound, then sort transudate from exudate — one Light's criterion makes it an exudate.

Key numbers

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

Tap new effusions 10 mm or deeper≥ 10 mm

Typical heart failure effusion: diurese firsttap if ≥ 3 days

After diuresis, use the serum–fluid protein gradientgradient > 3.1 g/dL

Pleural ADA above 40 U/L suggests TBADA > 40 U/L

01 · Defined byLight's criteria: any one means exudate

The effusion is an exudate if any one of Light's criteria is met: pleural/serum protein ratio > 0.5, pleural/serum LDH ratio > 0.6, or pleural LDH > two-thirds the upper limit of normal serum LDH.

02 · DoTap new effusions 10 mm or deeper

Perform diagnostic thoracentesis, always under ultrasound guidance, for any new or unexplained effusion at least 10 mm thick on ultrasound, CT or lateral decubitus film.

03 · CauseTransudate: heart failure, cirrhosis, nephrotic syndrome

Transudates come from raised hydrostatic or lowered oncotic pressure — heart failure (most common, usually bilateral), cirrhosis with ascites (hepatic hydrothorax, usually right-sided), nephrotic syndrome — and need little further work-up.

What it is

Core idea

Hemothorax: fluid hematocrit over half of blood'sA bloody effusion is a hemothorax when the pleural fluid hematocrit exceeds 50% of the peripheral hematocrit — usually trauma, rarely coagulopathy or a ruptured aorta or pulmonary artery.

Milky fluid: chylothorax from thoracic duct damageMilky, triglyceride-rich fluid is a chylothorax from thoracic duct damage (surgery, trauma, lymphoma); a cholesterol-rich chyliform effusion from long-standing rheumatoid or TB pleurisy looks similar but has low triglycerides.

Causes & risk factors

Why it happens

Exudate: pneumonia, cancer, pulmonary embolism, TB must knowExudates come from local inflammation and leaky capillaries; the most common causes are pneumonia (parapneumonic effusion), cancer, pulmonary embolism and tuberculosis. Every exudate needs its cause found.

Diagnosis & monitoring

Work-up

Pleural ADA above 40 U/L suggests TBPleural adenosine deaminase > 40 U/L is about 95% sensitive and specific for tuberculous pleuritis (cancer can also raise it); pleural fluid TB cultures are positive in fewer than half.

Management

Orders

Typical heart failure effusion: diurese firstSymmetric bilateral effusions in heart failure without fever or chest pain can be diuresed instead of tapped; perform thoracentesis if they persist 3 days or more.

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

Recurrent malignant effusion: catheter or pleurodesisFor a symptomatic malignant effusion that recurs after thoracentesis, an indwelling pleural catheter and talc pleurodesis work equally well; choose by patient preference. A lung that will not re-expand needs a catheter, not pleurodesis.

Bedside ultrasound is the standard detection testThoracic ultrasound is standard of care: more sensitive than radiography for small effusions and it shows septations and pleural thickening. On an upright film about 75 mL blunts the posterior costophrenic angle.

Undiagnosed exudate: contrast CT, then pleural biopsyIf fluid analysis is unrevealing, get a contrast-enhanced chest CT (CT angiography if PE is possible); pleural nodularity or thickening calls for biopsy. Effusions with no obvious cause are often occult PE, TB or cancer.

The rules

Know these

No routine chest film after thoracentesisA chest radiograph after thoracentesis is needed only if the patient develops dyspnea or chest pain or air entry is suspected — not routinely.

Transudate or exudate?

Commonly confused

Mechanism	High hydrostatic or low oncotic pressure
Light's criteria	None met
Common causes	Heart failure, cirrhosis, nephrotic syndrome
Work-up	Treat the cause; little further testing

Commonly confused

Tell apart

After diuresis, use the serum–fluid protein gradientLight's criteria mislabel about 20% of transudates as exudates, especially after diuretics; a serum minus pleural fluid protein gradient > 3.1 g/dL means the effusion is probably a transudate.

Low glucose and pH: infection or rheumatoid pleuritisLow pleural glucose with low pH and high LDH points to a complicated parapneumonic effusion or rheumatoid pleuritis, which must be told apart; tuberculous effusions also tend to have low glucose.

More detail

Reference

Show 7 reference facts

Therapeutic thoracentesis usually removes about 1–1.5 L; there is no fixed volume limit — stop for chest tightness, chest pain or severe cough. Re-expansion pulmonary edema is rare and unpredictable.

About 40% of patients with pulmonary embolism have an effusion — almost always exudative, often hemorrhagic; suspect PE when dyspnea is out of proportion to the effusion size.

After CABG a small left effusion is common: early (< 30 days) effusions are bloody with eosinophils, late ones clear and lymphocytic.

Esophageal rupture with effusion is a medical emergency (mediastinal and pleural infection); pancreatitis causes an exudative effusion in about a third of acute cases. Send pleural amylase when either is suspected.

Drug-induced exudates: amiodarone, nitrofurantoin, dantrolene, bromocriptine, methysergide, interleukin-2 and tyrosine kinase inhibitors such as dasatinib.

Trapped lung (encased by a fibrous peel from empyema or tumor) makes pleural pressure more negative and produces borderline fluid, within 15% of the Light's cutoffs.

Normally 10–20 mL of low-protein pleural fluid lines the pleura; it drains through parietal lymphatics to the right heart, so clearance depends partly on right-sided pressures.

Pearl

One is enough: meet any single Light's criterion and the effusion is an exudate that needs a cause.

Sources · Merck Manual Professional — Pleural Effusion (Rahman NM, rev. Jul 2025) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).