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Pleural effusion
The first step is to separate a transudate (a pressure problem — heart, liver, kidney) from an exudate (a problem of the pleura itself — infection, tumour, inflammation) using Light's criteria, and then to look for the cause in the fluid.
Why it matters in the exam
Exam questions almost always present the results of a pleural tap and ask for their interpretation. The three Light's criteria must be remembered — one positive criterion is enough to call an exudate — together with the trap of the heart-failure patient on diuretics: the fluid "looks" exudative, and the serum-to-fluid albumin gradient (above 1.2 g/dL) then proves it is a transudate after all. In bilateral effusions in a patient with known heart failure and a typical picture, diuretics come first rather than an immediate tap.
The second theme is the parapneumonic effusion: pH below 7.2, low glucose or pus on aspiration call for a chest drain, not antibiotics alone. Other fluid findings point to the diagnosis: lymphocytes with a high ADA — tuberculosis; very low glucose — rheumatoid arthritis, empyema, tuberculosis or tumour; high amylase — pancreatitis or oesophageal rupture; triglycerides above 110 mg/dL — chylothorax; a fluid haematocrit above half of the blood value — haemothorax. In a recurrent malignant effusion, the choice is between talc pleurodesis and an indwelling pleural catheter.
Key points
- Light's criteria: protein above 0.5, LDH above 0.6, fluid LDH above two thirds of the upper limit.
- One positive criterion is enough for an exudate; on diuretics check the albumin gradient (above 1.2 g/dL = transudate).
- pH below 7.2 or pus in a parapneumonic effusion → chest drain.
- ADA above 40 U/L with lymphocytes points to tuberculosis; triglycerides above 110 mg/dL to chylothorax.
- Recurrent malignant effusion: talc pleurodesis or an indwelling pleural catheter.
Summary table
| Fluid test | Value | Meaning | Action |
|---|---|---|---|
| Fluid/serum protein | Above 0.5 | Exudate (Light's criterion) | Look for a local cause |
| Fluid/serum LDH | Above 0.6, or fluid LDH above two thirds of the serum upper limit | Exudate (Light's criterion) | Culture, cytology, ADA |
| Serum-fluid albumin gradient | Above 1.2 g/dL | Transudate despite Light's (diuretics) | Treat the underlying disease |
| pH in a parapneumonic effusion | Below 7.2, or pus | Complicated effusion or empyema | Chest drain + antibiotics |
| ADA and lymphocytes | ADA above 40 U/L with lymphocytosis | Tuberculous pleurisy | Pleural biopsy for confirmation and culture |
| Triglycerides | Above 110 mg/dL | Chylothorax | Look for thoracic duct injury or lymphoma |
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Sources
- British Thoracic Society Guideline for Pleural Disease, 2023 — Investigation of a unilateral pleural effusion; pleural infection; malignant pleural effusion
- Harrison's Principles of Internal Medicine, 21st edition (2022) — Disorders of the Pleura
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.