Synopsis
Recognise pleural fluid on radiography and ultrasound, distinguish uncomplicated effusion from pleural infection and pursue safe sampling, drainage and diagnostic follow-up.
- On an erect radiograph, free pleural fluid blunts the costophrenic angle and forms a meniscus; on a supine film it may layer posteriorly as diffuse hemithorax haze.
- Thoracic ultrasound distinguishes fluid from consolidated lung, identifies septations and a safe intervention site, but sonographic complexity alone does not define every cause.
- Suspected pleural infection needs prompt antibiotics and fluid assessment; pus or non-purulent fluid with pH at or below 7.2 supports drainage when a safe ultrasound-accessible volume exists.
Key red flags
Pus, pleural fluid pH at or below 7.2 in a compatible presentation, positive microbiology or loculated infected fluid supports urgent drainage assessment.
Hypotension, increasing oxygen requirement, severe sepsis or rapid unilateral opacification requires immediate review for empyema, haemothorax or another acute collection.
Unilateral or unexplained recurrent effusion, pleural nodularity, weight loss or asbestos exposure raises pleural malignancy and needs a diagnostic pathway despite negative cytology.
A post-procedure increase in breathlessness, chest pain, hypoxia or subcutaneous emphysema raises pneumothorax, bleeding or re-expansion complication.
Absence of a large meniscus on supine radiography does not exclude substantial dependent pleural fluid; ultrasound or CT may be needed.
Fever, inflammatory illness, adjacent consolidation and complex fluid raise infection; pleural pH, glucose, LDH, Gram stain and culture determine risk more directly.
New pleuritic pain, oxygen decline, haemodynamic change or radiographic air after intervention requires prompt assessment for pneumothorax, bleeding or organ injury.
Investigation priorities
Confirm fluid, assess accessibility and guide aspiration or drain placement.
Management branches
An adult with pneumonia remains febrile and ultrasound shows a drainable pleural collection.
- Assess sepsis and respiratory status, start or optimise antibiotics, review ultrasound and obtain image-guided pleural fluid without delaying necessary treatment.
- Inspect the sample for pus and measure pH promptly in a blood-gas syringe while also sending microbiology, protein, LDH, glucose and other targeted tests.
A stable adult has a unilateral effusion without an established systemic cause.