Synopsis
Distinguish fluid overload from exudative pleural disease, investigate an unexplained effusion safely and recognise infection, malignancy and rapid-drainage hazards that change the pathway.
- Confirm pleural fluid with chest radiography and thoracic ultrasound; ultrasound must guide the site of any pleural aspiration or drain insertion.
- Bilateral, symmetric effusions with a classic heart, renal, liver or hypoalbuminaemic picture can often be treated for the systemic cause first; sample when features are atypical, unilateral/asymmetric, febrile, painful or non-responsive.
- An unexplained unilateral effusion usually needs ultrasound-guided aspiration plus blood tests and contrast CT selected around the malignancy/infection question.
- Send pleural fluid for protein and LDH with paired serum samples, cytology, microbiology, glucose and pH as clinically indicated; inoculate microbiology blood-culture bottles when infection is suspected.
- Light's criteria define an exudate when any one is present: fluid/serum protein ratio above 0.5, fluid/serum LDH ratio above 0.6, or fluid LDH above two-thirds the serum laboratory upper limit.
- In suspected pleural infection, frank pus requires drainage. Non-purulent fluid pH at or below 7.2 is high risk and usually needs an intercostal drain if safely accessible.
- Pleural pH above 7.2 but below 7.4 is intermediate risk: consider drainage when LDH exceeds 900 IU/L and supportive features exist such as low glucose, large volume, CT pleural enhancement, septations or ongoing fever.
- Cytology is the initial test for suspected secondary pleural malignancy, but a negative result must lead to further investigation when suspicion persists; do not use blind pleural biopsy.
- Serum NT-proBNP can support heart failure in a unilateral effusion but must not be used alone; pleural NT-proBNP is not routinely needed because it is not superior to serum testing.