Investigations in patients presenting with pleural effusion will largely be guided by features elucidated from the patient’s history. Investigations are aimed at determining the size, nature (transudate or exudate) and underlying cause of the condition.

Many of these investigations can be initiated in the ED:


Chest X-ray

The most common first line imaging is a chest radiograph. Blunting of the costo-phrenic angle is visible after 250-500ml of fluid accumulation. If other imaging modalities are not available and there is doubt over the diagnosis, a lateral decubitus radiograph may help in demonstrating a shifting fluid level.

Learning bite

A pleural effusion should have a meniscus. If it is completely flat this may suggest a concurrent pneumothorax.


Point of care ultrasound is being increasingly used in Emergency Departments to help guide diagnosis and treatment. It can provide a rapid diagnosis at the bedside and is particularly helpful in those patients who are acutely unwell. It is also recommended for use in all therapeutic aspirations and chest drain insertions [10]. It requires a suitably trained and competent user to be safe and effective.

CT Scan

CT can aid with the identification and quantification of effusions. It can also help with identifying a causative underlying pathology e.g. malignant tumour, pulmonary embolism, aortic dissection. It may also be used by interventional radiologists for difficult drainage procedures and planning biopsy procedures.


  • Arterial Blood Gas (may be required if there are concerns about oxygenation/ventilation)
  • Full Blood Count and CRP (to assess inflammatory response suggesting infection)
  • U&Es, LFTs (identifying renal/liver disease)
  • Serum Protein, LDH and Glucose (for Light’s criteria)
  • Serum Amylase (if pancreatitis or ruptured oesophagus are suspected)
  • D-Dimer (if PE suspected with a low risk Well’s score)