Pleural Effusion
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Direct answer
Stony dullness with reduced breath sounds, and tracheal shift away when massive — pathological fluid has collected in the pleural space, and the diagnostic thoracentesis with Light's criteria now decides the road: pleural-to-serum protein ratio above 0.5, pleural-to-serum LDH ratio above 0.6, or pleural LDH above two-thirds of the upper limit of normal for serum — any one of the three makes it an exudate. In India tuberculosis is the commonest cause of an exudative effusion; empyema requires intercostal tube drainage in addition to antibiotics.
What you must remember
- Transudates follow systemic states: congestive heart failure (commonest overall), hepatic hydrothorax, nephrotic syndrome, hypothyroidism and Meigs syndrome.
- Exudates follow pleural disease: tuberculosis, malignancy, parapneumonic effusion, pulmonary embolism, pancreatitis, rheumatoid arthritis and SLE.
- Tuberculous effusion: lymphocyte-predominant exudate with high protein; ADA above 40 units per litre is strongly supportive; confirmation by pleural fluid studies, biopsy or CBNAAT, treated with standard antitubercular therapy.
- Malignant effusion: often haemorrhagic, lymphocytic, cytology-positive, massive and rapidly recurring; pleurodesis or an indwelling catheter for symptom control.
- Empyema and complicated parapneumonic effusion: frank pus or pH below 7.2 with glucose below 40 mg per dL and LDH above 1000 units per litre — intercostal drainage with antibiotics; loculated collections need fibrinolytics or surgery.
- Signature fluids: rheumatoid effusion has very low glucose with high rheumatoid factor; chylothorax is milky with triglycerides above 110 mg per dL (thoracic duct injury, lymphoma); haemothorax has a haematocrit above half the blood level.
- Diuretic-treated heart failure can masquerade as an exudate ("pseudoexudate"); a serum-to-pleural albumin gradient above 1.2 g per dL unmasks the transudate.
One tap, read three ways
A 28-year-old has a moderate left effusion; the diagnostic tap returns straw-coloured fluid, sent for protein, LDH, glucose, pH, cell count, cytology, Gram stain, culture, ADA and CBNAAT. Read it three ways. First, the class: pleural protein 4.6 g per dL against serum 6.8 (ratio above 0.5) — an exudate, so the systemic transudate causes drop out and the pleural causes line up: tuberculosis, malignancy, parapneumonic disease, pulmonary embolism, pancreatitis, rheumatoid, SLE. Had the patient been in diuretic-treated heart failure, the fluid might falsely have passed Light's criteria as a pseudoexudate — rescued by a serum-to-pleural albumin gradient above 1.2 g per dL. Second, the cytology: lymphocyte predominance with high protein and ADA above 40 units per litre in a febrile young Indian patient makes tuberculosis the working diagnosis, confirmed by CBNAAT or biopsy; haemorrhagic, cytology-positive, rapidly recurring fluid points to malignancy, managed with pleurodesis or an indwelling catheter. Third, the drain-or-not numbers: pH below 7.2, glucose below 40 and LDH above 1000 (or frank pus) mean complicated parapneumonic effusion or empyema — drainage with antibiotics, not tablets. The signature fluids complete the bank: rheumatoid effusion with very low glucose, chylothorax milky with triglycerides above 110, haemothorax with a haematocrit above half the blood level.
Where students slip
Exudate versus transudate is the topic, and the marks are lost by applying one criterion instead of any one of the three — the numerical question deliberately supplies values that satisfy only one. The pseudoexudate of diuretic-treated heart failure is the second trap, with the albumin gradient as the rescue line worth writing in the margin. The third is the management fork the classification implies: transudates are treated systemically and never serially tapped, whereas exudates demand an aetiological diagnosis — and an exudate that is infected, acidic and glucose-starved needs a chest tube, not a longer course of tablets. In the imaging stems, the massive effusion differential in India runs tuberculosis against malignancy, and the young febrile lymphocytic profile against the old haemorrhagic recurring one decides it.
Frequently asked questions
What are Light's criteria for an exudative effusion?
Pleural-to-serum protein ratio above 0.5, pleural-to-serum LDH ratio above 0.6, or pleural LDH above two-thirds of the upper normal serum limit; any one makes it an exudate.
Which features suggest a tuberculous effusion?
A lymphocyte-rich exudate with high protein and ADA above about 40 units per litre in a febrile young patient.
When does a parapneumonic effusion need drainage?
Frank pus, positive Gram stain or culture, or pH below 7.2 with glucose below 40 mg per dL and LDH above 1000 units per litre.
What is a pseudoexudate?
A transudate, usually from diuretic-treated heart failure, that falsely meets Light's criteria; a serum-to-pleural albumin gradient above 1.2 g per dL unmasks it.
How does a rheumatoid pleural effusion behave?
An exudate with very low glucose, high rheumatoid factor, high LDH and low complement, typically in older men with nodular disease.
What defines a chylothorax?
Milky fluid with triglycerides above 110 mg per dL and lymphocyte predominance, from thoracic duct disruption by trauma, surgery or lymphoma.