21st February 2025

Investigation of pleural effusion

Imaging

A chest X-ray is the first-line investigation in suspected pleural effusion and can detect effusions from around 200 ml. Ultrasound is more sensitive and specific and should be used to guide any pleural procedure. CT chest is also usually performed, to delineate the effusion and detect loculations, and to look for signs of the underlying pathology such as lung masses or pleural thickening.

Image Source: Wikimedia Commons (2011). Effusion, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Effusion.png#/media/File:Effusion.png (Accessed: 19 January 2024).

Pleural fluid analysis

Often, bilateral pleural effusions can be reliably diagnosed as transudates by history, examination and blood tests. Effusions should be aspirated and analysed if unilateral or if there is not a clear transudative cause.

Recommended for all pleural fluid samples:

• Cell count and differential, cytology
• LDH
• Protein
• Microscopy, culture and sensitivity

Other tests which should be considered include pH (low in infection), glucose, triglycerides and cholesterol (raised in chylothorax), amylase (raised in pancreatitis or oesophageal rupture), NT-proBNP (N-terminal pro b-type natriuretic peptide) (raised in heart failure), tuberculosis culture.

Routine chest X-ray is not recommended by BTS following a pleural aspiration, however a chest X-ray is recommended to check for complications if the procedure was difficult and required multiple attempts, or if air is aspirated during the procedure.

Other investigations

Blood tests should be done to look for evidence of inflammation, bleeding and liver or renal failure. Serum protein and LDH are needed for Light’s criteria. NT-proBNP and transthoracic echo are useful in suspected heart failure.