So, you think you’ve got a pleural effusion?

Posted on November 30, 2011


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Last week my friend Helen (who’s blog on tennis you can find HERE) asked if I wanted to go and see the new Twilight movie with her. I didn’t, but apparently all her other friends find saying “no” much easier than me so I ended up agreeing. And whilst watching Breaking Dawn seemed like a perfectly fine way to spend an evening completely devoid of hope, it turned out Helen had to work and so the whole thing was cancelled, leaving me free to do far more enjoyable things like giving birth to a chair or mulling over my next blog post.

So here it is. Pleural effusions are pretty much my favourite thing in medicine at the moment. It turns out quite a lot of people in hospital have them and astonishingly we haven’t ever really covered them in our teaching despite one registrar telling me I “really need to know about them“.

The British Thoracic Society has recently produced some new guidelines on the investigation of pleural effusions in adults, it’s a really good document that you can get by clicking on THIS LINK.

Pleural Effusions are the result of fluid accumulation in the pleural space- that is in the space between the visceral and parietal pleura that cover the lungs.


Pleural effusions can be categorised into 2 types, depending on the protein level. The pleural fluid can be aspirated in order to measure the protein concentration. Transudates have a protein concentration of <30g/L, and exudates have a protein concentration of >30g/L. This is usually enough to differentiate between the two effusion types, although if the protein level is borderline Light’s Criteria can be used instead:

Under Light’s criteria, the pleural fluid is an exudate if one or more of the following are met:

  • Pleural fluid protein/Serum Protein >0.5
  • Pleural fluid lactate dehydrogenase (LDH)/Serum LDH >0.6
  • Pleural fluid LDH more than 2/3 the upper limit of normal serum LDH

But why does protein concentration matter?!

Transudates and exudates are caused by different processes. Transudates are caused when there is a problem with filtration and reabsorption, so any process that affects hydrostatic forces could cause a transudative effusion. Capillary permeability isn’t affected in disease processes resulting in transudates, so protein level is accordingly low. Exudates are caused by diseases that affect membrane permeability (that is, capillary membranes or the pleural membranes themselves) making them more leaky. Protein levels in exudates are therefore high.


The number one cause of transudative pleural effusions is LEFT VENTRICULAR FAILURE, but other common causes include liver cirrhosis and hypoalbuminaemia. Less common causes include hypothyroidism and nephrotic syndrome, but there are many other causes than these.


If you find yourself presented with an exudative effusion, the diagnosis you should be thinking about is MALIGNANCY. Less common causes include pulmonary effusion with infarction, also rheumatoid arthritis and autoimmune diseases as well as connective tissue disorders like Marfans. Again, there are many other causes of exudates. I’d list them all but this post is already quite long so you can go and look them up yourselves.

English: Pleural effusion Chest x-ray of a ple...

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So the X-ray is back, you suspect an effusion and have successfully aspirated a sample for analysis. WHAT NEXT?

M,C and S: you want to send your sample for microscopy and culture to see if you grow any bugs, and if you do you want to know what antibiotics those bugs are sensitive to.

Cytology: If you suspect malignancy, cytology will confirm or refute this.

Those are the main two investigations you should think about, but there are more you could do:

pH: in an infected non-purulent effusion, a pH of <7.2 suggests you need to put a drain in.

Glucose: a glucose of <3.3mmol/L is found in exudative pleural effusions secondary to empyema, rhematoid disease, lupus, TB, malignancy.

Amylase: This could be requested if acute pancreatitis is possible (though I suggested this on a ward round one morning and was told rather bluntly that if they had pancreatitis, they’d probably have symptoms of that before they present with an effusion).