Pleural Effusion
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A pleural effusion is the accumulation of fluid within the pleural space. In healthy individuals a small amount of fluid exists in this space to help lubricate movement of the lungs during respiration.
In a pleural effusion there is an abnormally large amount of fluid in this space. Pleural effusion can cause cough, shortness of breath and pleuritic chest pain. An effusion has to be quite large (typically >500mls) before they cause any symptoms.
There are many causes of a pleural effusion, the most important of which is malignancy – which accounts for about 10% of cases.
Pleural effusion can be caused by pathology of the lungs and the pleura, as well as extra-pleural sources (such as heart failure).
The most common causes are:
  • Congestive heart failure
  • Malignancy – lung cancer and breast cancer are the most common causes, but many cancers can cause pleural effusion. About 15% of cancer patients will get a pleural effusion – this is a bad prognostic sign
  • Pneumonia
  • Pulmonary embolism
Pleural fluid can be aspirated and examined in the lab and the characteristics of the fluid can be said to be a transudate or exudate – which helps to narrow down the potential causes.
Treatment generally depends on treating the underlying cause. In small to medium effusions then treating the underlying cause will cause resolution of the effusion. Large pleural effusions can be treated by insertion of a chest drain to remove the fluid which relieves the symptoms. However, if the underlying cause is not addressed then the fluid can quickly re-accumulate.
It is important to note that empyema (collection of pus within the pleural cavity) and haemothorax (collection of blood within the pleural cavity) are different conditions but can present similarly to pleural effusion.

Pleural Fluid Samples

Straw coloured / clear
Yellow / white, foul smelling
Transudate / exudate
Empyema / parapneumonic effusion
Trauma, malignancy, pulmonary infarct

Clinical features

  • Reduced chest expansion on the affected side
  • Mediastinal displacement away from the affected side
  • Stony dull to percussion
  • Reduced or absent breath sounds
  • Reduced or absent vocal resonance
  • There will be no additional unusual sounds
  • These will generally only be present when the effusion is greater than 500ml!


  • An effusion of less than 500ml is unlikely to cause anything other than blunting of the costophrenic recess. >500ml will cause a clear fluid level
  • An effusion of less than 300ml may not be seen on x-ray
  • Remember to look for the meniscus – it is likely to be a very long curve, perhaps rising all the way to the axilla.
    •  If the fluid level appear perfectly horizontal, it is likely to due co-existing pneumothorax
  • If the effusion is large enough, the whole of one lung field may appear opaque, and the mediastinum may be shifted to the opposite side
  • Fluid below the lung can simulate a raised hemidiaphragm.
Ultrasound is useful for guiding the aspiration, as well as for diagnosis

Further diagnostics

  • Diagnostic aspiration
    • Percuss the upper border of the effusion, then go 1-2 intercostal spaces below (any lower and you might end up in the abdomen!)
    • Use 5-10ml of lignocaine and inject down to the pleura
    • Insert a 21G needle with syringe just above the ribs upper boarder (to avoid the neurovascular bundle)
    • Take 10-30ml of fluid
    • Send sample for:
      • Clinical biochem – glucose, protein, pH, amylase, LDH
      • Bacteria culture
      • Cytology
      • Immunology (if indicated by clinical signs)
  • If diagnosis is not possible from fluid sampling, it may be possible with a pleural biopsy, which may be CT guided for the best results.
  • You should also take bloods for protein (to check for hypoalbuminaemia), glucose (to compare to the pleural fluid sample – see below)and LFTs, U+E’s (to check for renal failure)

Diagnosis – Transudates and Exudates

  • are fluids that have left the circulatory system and have entered lesions or areas of inflammation.
  • The composition of an exudate varies, but it can include pretty much anything that is in blood.
  • It will nearly always have water and dissolved solutes, and may also have white and red blood cells, as well as platelets. Exudates have a high protiein content
  • Definition – Protein content >35g/L (or more correctly, see Light’s criteria below)
  • are fluids that have left the circulatory system and gone into lesions or areas of inflammation.
  • are caused by disturbances in oncotic pressure (i.e. increases in venous pressure), and not by inflammation.
  • They have a lower protein content, and will contain fewer cells.
  • The fluid typically only contains mononuclear cells (such as macrophages and lymphocytes).
  • Definition – Protein content <25g/L (or more correctly, see Light’s criteria below)
Essentially, a transudate is caused by a high pressure forcing plasma and some blood products out of the blood across a membrane, whilst an exudates is leaking of fluid from one space to another.
Transudates are pretty much always passive, unwanted losses of fluid, whilst exudates can sometimes be deliberate secretions.
In transudates, proteins have moved down their concentration gradient. In exudates, proteins have been moved against their concentration gradient. On inspection, transudates also often appear more ‘clear’ than exudates
Note that it is difficult to tell a transudate from an exudate if the protien is 25-35gd/L – and so in clinical practice we use Light’s Criteria to differentiate transudates from exudates.
According to Light’s criteria, the fluid is an exudate if:
  • [Plerual Protein : serumprotein] ratio >0.5
  • [Pleural LDH : serum LDH] ratio >0.6
  • Pleural LDH > 200

Other Tests

  • Glucose  – a low glucose content (in relation to the patient’s current blood glucose) indicates the presence of cells (possibly native or foreign) in the fluid – thus can indicate the presence of infection.
  • pH – pH of <7.2 suggests empyema (exudate)
  • Amylase – raised amylase may indicate carcinoma, pancreatic disease or an oesophageal rupture.



  • If there is clear evidence of a transudative cause (e.g. LVF, renal failure, hypoalbuminaemia), then BTS guidelines suggest to treat the cause with no nee dto drain the effusion.
  • If unsure of a cause, then perform a Pleural Tap (thoracocentesis). This is not the same as a chest drainwhich often remains in place for several days. Instead, a pleural tap just aspirates a sample of fluid. Send the fluid for cytology (cancer), LDH, protein, pH, Gram stain, AFB testing (Acid Fast Bacilli – TB). Also send bloods (ESR, CRP, albumin, amylase, LDH, glucose
Then, use results to make a diagnosis. BTS guidelines advise to avoid draining a transudate, but to treat the underlying cause.
For an exudate:
  • Treat the underlying disorder!
  • DRAIN THE EFFUSION AS NECESSARY! – even if this means draining many times.
    • Fluid should be removed slowly – 2L every 24hr max. Large, fast fluid cahnges can cause pulmonary oedema.
    • Can be removed by needle aspiration (same method as diagnosis), or chest drain. If aspirating, don’t drain more than 1L at a time. Typically used for emypema and haemothorax.
  • In Malignancy – most cases will reccurr within a month, and so pleurodesis or long-term in-dwelling chest drains may be considered. Pleurectomy may be used in certain instances.
  • Pleurodesis – this is where the pleural space is effectively removed, and the two layers of the pleura are stuck together, either chemically or surgically, to prevent the accumulation of fluid. In cases of recurrent pleural effusion, chemical pluerodesis (e.g. with tetracycline, talc or bleomycin) may be useful.
    • Pleurodesis talc is most useful for effusions caused by malignancy
  • Intrapleural streptokinase was used in the past, but is of no benefit


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Dr Tom Leach

Dr Tom Leach MBChB DCH EMCert(ACEM) FRACGP currently works as a GP and an Emergency Department CMO in Australia. He is also a Clinical Associate Lecturer at the Australian National University, and is studying for a Masters of Sports Medicine at the University of Queensland. After graduating from his medical degree at the University of Manchester in 2011, Tom completed his Foundation Training at Bolton Royal Hospital, before moving to Australia in 2013. He started almostadoctor whilst a third year medical student in 2009. Read full bio

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