Pleural Effusion

Pleural effusion is the abnormal accumulation of fluid in the pleural space, the potential cavity between the visceral and parietal pleura. It is one of the most heavily tested topics in NEET PG Medicine because it integrates physiology (Starling forces), the diagnostic algorithm (Light's criteria), and the interpretation of pleural fluid values — a classic single-best-answer format. Mastering the transudate–exudate split and the fluid analysis grid will reliably fetch you marks.

Definition & basic physiology

Normally the pleural space contains only 5–15 mL of lubricating fluid, turned over continuously (≈0.01 mL/kg/h). Fluid enters from the parietal pleural capillaries (systemic circulation) and is drained by the parietal pleural lymphatics (stomata). An effusion develops when fluid formation exceeds reabsorption.

Mechanisms (governed by Starling forces):

1. ↑ Hydrostatic pressure → e.g. left ventricular failure (transudate).
2. ↓ Oncotic pressure (hypoalbuminaemia) → nephrotic syndrome, cirrhosis (transudate).
3. ↑ Capillary permeability (inflammation) → infection, malignancy (exudate).
4. ↓ Lymphatic drainage → malignant lymphatic obstruction, post-radiation (exudate).
5. Movement of fluid from peritoneum → ascites tracking via diaphragmatic defects/lymphatics (e.g. Meigs syndrome, hepatic hydrothorax).

High-yield: A transudate results from altered hydrostatic/oncotic pressures with intact capillaries (the membrane is "innocent"); an exudate results from a diseased, leaky pleural surface. The first job at the bedside is always to place the fluid in one of these two boxes.

Classification — transudate vs exudate

The single most examined distinction. Light's criteria define an exudate; the fluid is an exudate if any one of the three is met:

Light's criterion	Exudate if
Pleural fluid protein / serum protein	> 0.5
Pleural fluid LDH / serum LDH	> 0.6
Pleural fluid LDH	> 2/3 (≈0.67×) upper limit of normal serum LDH

High-yield: Light's criteria are highly sensitive for exudates but misclassify ~25% of transudates (especially diuretic-treated cardiac failure) as exudates. If clinically a transudate is expected but Light labels it exudate, check serum–pleural fluid albumin gradient > 1.2 g/dL (favours transudate) or serum–effusion protein gradient > 3.1 g/dL → transudate. This rescue test is a favourite distractor.

Common causes

Transudates (clear, low protein)	Exudates (cloudy/bloody, high protein)
Congestive cardiac failure (commonest transudate)	Parapneumonic effusion / pneumonia
Cirrhosis with ascites (hepatic hydrothorax)	Tuberculosis (commonest exudate in India)
Nephrotic syndrome	Malignancy (lung, breast, lymphoma)
Hypoalbuminaemia / protein-losing enteropathy	Pulmonary embolism (can be either; usually exudate)
Peritoneal dialysis, myxoedema, SVC obstruction	Pancreatitis, connective tissue disease (RA, SLE)
Constrictive pericarditis, urinothorax	Empyema, chylothorax, haemothorax

High-yield: CCF is the commonest cause of transudate worldwide; in the Indian exam context, tuberculosis is the commonest cause of an exudative effusion and the commonest cause of effusion overall in young adults, while malignancy dominates in the elderly.

Clinical features

Symptoms depend on volume and rate of accumulation:

• Dyspnoea (most common) — from compression of lung and impaired diaphragmatic mechanics.
• Pleuritic chest pain — implies parietal pleural inflammation (parapneumonic, PE, TB) rather than the effusion itself.
• Dry cough.
• Constitutional symptoms point to aetiology (fever → infection/TB; weight loss → malignancy/TB).

Signs over the effusion (classic OSCE/MCQ stem):

Finding	Effusion
Mediastinal/tracheal shift	Away from effusion (large)
Chest movement	Reduced on affected side
Percussion	Stony (woody) dullness
Breath sounds	Diminished or absent
Vocal/tactile fremitus & resonance	Reduced
Just above the effusion	Bronchial breathing + aegophony (E→A change)

High-yield: "Stony dull percussion with absent breath sounds and reduced vocal resonance" = pleural effusion. Contrast with consolidation (dull, increased fremitus, bronchial breathing, present breath sounds) and collapse/pneumothorax (trachea pulled towards collapse; pushed away in tension pneumothorax).

Investigations

Stepwise diagnostic flow:

Chest X-ray → confirm & quantify → Ultrasound (best for detection/guidance) → Diagnostic thoracocentesis → Pleural fluid analysis (Light's criteria + targeted tests) → CT thorax/pleural biopsy if undiagnosed exudate.

Imaging cut-offs (frequently asked)

• Lateral decubitus / USG can detect as little as 5–50 mL.
• Blunting of the costophrenic angle on a PA film needs ≈200–300 mL.
• A lateral film detects blunting of the posterior costophrenic angle earlier (~50–75 mL).
• Meniscus sign (Ellis S-shaped curve / Damoiseau line) = the upward-sloping fluid level.
• A massive effusion opacifies a hemithorax and pushes the mediastinum to the opposite side; if the trachea is central or pulled towards a "white-out", suspect associated collapse or mesothelioma.
• Ultrasound is superior for detecting loculations/septations and guiding aspiration — reduces pneumothorax risk and is the investigation of choice for guidance.

Diagnostic thoracocentesis

Indicated for any new effusion > 1 cm on lateral decubitus/USG without an obvious explanation. Aspirate from the upper border of the lower rib to avoid the neurovascular bundle that runs in the subcostal groove.

Routine pleural fluid panel: appearance, protein, LDH, glucose, pH, cell count + differential, cytology, Gram stain & culture, ADA, and (if indicated) amylase, triglycerides, cholesterol, AFB/GeneXpert.

Pleural fluid analysis — the master interpretation grid

Parameter	Significance
Bloody (RBC >100,000/µL)	Malignancy, PE with infarction, trauma (haemothorax)
Milky	Chylothorax (triglycerides) vs pseudochylothorax (cholesterol)
Frank pus / turbid	Empyema
Glucose < 60 mg/dL	Empyema, TB, malignancy, rheumatoid (lowest, often <30)
pH < 7.2	Complicated parapneumonic effusion/empyema → drain; also TB, malignancy, oesophageal rupture
Lymphocyte predominance	TB, malignancy, lymphoma, chronic effusions
Neutrophil predominance	Acute — parapneumonic, PE, early TB
Eosinophils > 10%	Air/blood in pleura, drug reaction, parasitic, asbestos
ADA > 40 U/L (with lymphocytosis)	Strongly suggests tuberculous effusion
Amylase ↑	Pancreatitis, oesophageal rupture, malignancy
Triglycerides > 110 mg/dL	Chylothorax

High-yield: A pleural fluid pH < 7.2 in a parapneumonic effusion is the strongest single trigger for tube thoracostomy (chest drain). Pair it with glucose <60 mg/dL and positive Gram stain/pus.

High-yield: ADA (adenosine deaminase) > 40 U/L plus lymphocytic predominance has high sensitivity and specificity for TB pleuritis — the highest-yield single value in the Indian exam. Pleural fluid AFB smear is usually negative; closed pleural biopsy (Abrams/Cope needle) showing caseating granuloma or GeneXpert confirms it. Pleural fluid culture positivity is low (~25–30%).

High-yield: Markedly low glucose (<30 mg/dL) and very low pH with high LDH but low complement points to rheumatoid pleuritis; SLE pleuritis shows positive ANA and LE cells.

Specific syndromes you must distinguish

Empyema (pus in the pleural space)

Evolution of a parapneumonic effusion through three stages:

1. Exudative (simple) — free-flowing, pH >7.2, glucose normal → antibiotics alone.
2. Fibrinopurulent (complicated) — pH <7.2, glucose <60, loculations, positive culture → chest tube drainage + antibiotics ± intrapleural fibrinolytics (tPA + DNase).
3. Organising — thick peel, trapped lung → decortication (VATS/open).

High-yield: Commonest organisms — community: Streptococcus pneumoniae, Staph aureus, anaerobes; hospital/post-op: MRSA, Gram-negatives. Indications to drain a parapneumonic effusion: frank pus, positive Gram stain/culture, **pH <7.2**, loculation, or large size (>½ hemithorax).

Chylothorax vs pseudochylothorax

Feature	Chylothorax	Pseudochylothorax
Mechanism	Thoracic duct disruption	Chronic effusion (TB, RA)
Onset	Acute	Long-standing
Triglycerides	> 110 mg/dL	Low
Cholesterol crystals	Absent	Present (high cholesterol)
Chylomicrons	Present	Absent
Classic cause	Lymphoma, trauma/surgery	Chronic TB, rheumatoid

High-yield: Triglycerides > 110 mg/dL = chylothorax; < 50 mg/dL excludes it; 50–110 needs lipoprotein analysis for chylomicrons. Commonest non-traumatic cause = lymphoma; commonest traumatic = oesophagectomy/thoracic surgery. Management: low-fat/MCT diet, octreotide, drainage; refractory → thoracic duct ligation or pleurodesis.

Haemothorax

Defined when pleural fluid haematocrit > 50% of blood haematocrit (not merely blood-tinged fluid). Usually traumatic. Management: large-bore intercostal chest tube; thoracotomy if initial output >1500 mL or ongoing >200 mL/h for 2–4 h.

Malignant effusion

Exudative, often bloody and lymphocytic, low glucose/pH if large tumour burden, cytology positive (sensitivity rises with repeated/large-volume samples). Commonest primaries: lung (men), breast (women), lymphoma, ovary. Management is palliative — therapeutic thoracocentesis, indwelling pleural catheter, or pleurodesis (talc).

Meigs syndrome

Triad: benign ovarian fibroma + ascites + right-sided pleural effusion, which all resolve after tumour removal. CA-125 may be mildly raised (classic trap — do not call it malignant).

Management — principles & drug/procedure of choice

• Transudates → treat the underlying cause (diuretics + salt restriction for CCF; albumin/TIPS for cirrhosis). Therapeutic tap if dyspnoeic.
• TB effusion → standard ATT (2HRZE + 4HR); effusion resolves over weeks. Steroids are not routinely recommended.
• Parapneumonic/empyema → antibiotics + drainage as per stage (above); fibrinolytics for loculated; decortication for organised peel.
• Malignant → therapeutic drainage; talc pleurodesis or indwelling catheter for recurrent effusions.
• Therapeutic thoracocentesis caution: remove ≤1–1.5 L at a time to avoid re-expansion pulmonary oedema; stop if chest tightness/cough develops.

High-yield: Re-expansion pulmonary oedema is precipitated by rapid removal of large volumes or by draining a chronically collapsed lung too fast — limit to 1–1.5 L and monitor.

Complications

• Re-expansion pulmonary oedema (over-rapid drainage).
• Pneumothorax / haemothorax from the tap (USG guidance reduces risk).
• Empyema and trapped lung (fibrothorax) needing decortication.
• Pleural fibrosis and restrictive defect.
• Infection of an indwelling catheter, and tumour seeding along the needle track in mesothelioma.

Key differentials (the "white-out" / dull base problem)

• Massive effusion vs complete collapse vs pneumonectomy vs large mass vs diaphragmatic elevation — use the trachea/mediastinum: pushed away = effusion/mass; pulled towards = collapse/pneumonectomy/fibrosis.
• Consolidation — dull but increased fremitus, bronchial breath sounds present.
• Subpulmonic effusion — apparent raised hemidiaphragm; lateral peak of "diaphragm" shifted laterally; confirm with decubitus film/USG.

Recently asked / exam angle

• Pleural fluid panel interpretation MCQs: given protein/LDH ratios → apply Light's criteria → exudate vs transudate. Expect at least one such stem.
• ADA >40 U/L + lymphocytosis → TB (very frequent single-answer item).
• pH <7.2 in parapneumonic effusion → insert chest tube (best next step).
• Lowest pleural fluid glucose → think rheumatoid arthritis (then empyema/TB/malignancy).
• Triglycerides >110 → chylothorax; commonest cause = lymphoma.
• Meigs syndrome triad and its reversibility after surgery.
• Aegophony / "E to A" change localising the upper border of an effusion.
• Site of needle insertion = upper border of lower rib (avoid neurovascular bundle).
• Re-expansion pulmonary oedema from rapid large-volume drainage.
• Trachea direction in white-out hemithorax (effusion vs collapse).

Rapid revision

1. Normal pleural fluid = 5–15 mL; effusion = excess accumulation governed by Starling forces.
2. Light's criteria (any one) = exudate: protein ratio >0.5, LDH ratio >0.6, LDH >2/3 ULN.
3. CCF = commonest transudate; TB = commonest exudate in India, malignancy in the elderly.
4. Stony dullness + absent breath sounds + reduced vocal resonance = effusion; trachea pushed away.
5. CXR needs ~200–300 mL for costophrenic blunting; USG/decubitus detects ~5–50 mL and guides taps.
6. Aspirate at the upper border of the lower rib.
7. ADA >40 U/L + lymphocytes → tuberculous pleuritis; AFB smear usually negative.
8. pH <7.2 / glucose <60 / pus → drain (complicated parapneumonic effusion/empyema).
9. Lowest glucose & pH → rheumatoid pleuritis (also TB, malignancy, empyema).
10. Triglycerides >110 mg/dL = chylothorax; commonest cause lymphoma; <50 excludes it.
11. Haemothorax = fluid Hct >50% of blood Hct; thoracotomy if >1500 mL or >200 mL/h.
12. Limit therapeutic drainage to 1–1.5 L to prevent re-expansion pulmonary oedema; Meigs = fibroma + ascites + effusion, reversible.