MRCP revision battle 6.4: Pleural effusions

Pleural effusions for me are one of the most interesting clinical signs.  A dull day can be made just that tad more interesting by the characteristic stoney dull percussion note, reminding you there is a reason you percuss everyone's chest.  A chest xray can then almost instantly confirm your clinical judgment (unlike with a murmur, which you may never know if you were just imagining or not) but then theres the 'oh bugger' moment as you realise you need to work out why this patient has an effusion... and most of the options aren't entirely positive....


The main thing to establish with an effusion is if it is an exudate or a transudate.

Traditionally a transudate was defined as containing <3g/dl of protein.  However, some doctors prefer to use Light's criteria for determining which label to apply (a criteria which will misclassify 25% of transudates as exudates...)

Lights criteria is that for a fluid to be an exudate it must either have:

• pleural:serum protein > 0.5 or
• pleural:serum LDH > 0.6 or
• pleural LDH >2/3 upper limit of serum LDH

So, obviously to make your diagnosis you need a pleural fluid sample and a blood sample.  You should send your pleural sample off for:

• cell count
• cytology
• glucose
• protein
• LDH
• amylase
• pH
• Ziel-nielson staining

and remember to send bloods for glucose, LDH and amylase at the same time (plus any other bloods, like FBC, you might need)


Armed with the knowledge of transudate or exudate, you can spin forth a list of diffentials:

Causes of transudates:

• heart failure
• renal failure
• liver failure
• peritoneal dialysis
• hypothyroidism
• Meigs
• constrictive pericarditis

Causes of exudates:

• infection: TB, pneumonia, subphrenic abscess
• inflammation: Dresslers, pancreatitis, SLE, RA
• malignancy: mesothelioma, local cancer, lymphoma
• other: PE, uraemia, yellow nail syndrome

As the list of differentials is still quite big, apart from your clinical acumen some of the other pleural fluid results can help narrow it out:


Glucose <3.3 or pH <7.2 or raised LDH suggests:

• TB
• malignancy
• empyema
• SLE
• RA

 Its worth noting that *really* low glucoses are usually due to RA or empyema


Raised amylase suggests:

• pancreatitis
• carcinoma
• bacterial pneumonia
• oesophageal rupture

Cytology results can give more clues:

• neutrophils: pneumonia, TB
• lymphocytes: malignancy, TB, RA, SLE, sarcoid
• mesothial cells ++ : pulmonary infarction
• multinucleated giant cells: RA
• lupus cells: SLE

Hopefully these lists should equip you well for any MRCP pleural effusion questions (or even, shock horror, real life on the wards!)


On to the final battle of today, yellow nail syndrome