Equis ISSN 2398-2977

Pleural fluid: protein

Contributor(s): Annalisa Barrelet, Prof Bernard Feldman


  • Pleural fluid is a dialysate of plasma, with low cellularity and protein concentrations   Pleural fluid: overview  .
  • The biochemical composition often reflects the pathophysiological conditions of the pleura and the vascular permeability of the vessels therein.  
  • Protein is elevated in some forms of thoracic effusions.
  • Assists differentiation of transudates, modified transudates and exudates. With a transudate, the pleural surfaces are not involved by the primary pathologic processes.


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  • Standard Biuret method as for serum protein:
    • Widely available.
  • Refractometer:
    • Not accurate at lower protein levels (transudates).  
    • 25 g/l is usually the lowest reading on the protein scale of the refractometer.


  • Widely available.


Predictive value

  • Pleural fluid analysis essential in work up of horses with pleural disease.
  • Very high protein figures (>30 g/l) indicate significant inflammation, often associated with severe inflammation (septic pleuritis). Causes vary with geographic location, breed, age and occupation of the horse.
  • The majority of pleural effusions are secondary to bacterial pleuropneumonia   Lung: pleuropneumonia - bacterial (pleuritis)  , especially in the United States. See Pleural fluid cytology for a list of differential diagnoses of pleural effusions   Pleural fluid: cytology  .
  • Pleural fluid protein is not an accurate prognostic indicator - in one study it was found that of 49 horses with pleuritis, 58.3% of survivors and 52% of non-survivors had pleural fluid protein >50 g/l.

Technique (intrinsic) limitations

  • Presence of high concentration of other solutes, eg lipids can give false high reading. 
  • Significant blood contamination gives false high protein readings.

Result Data

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Further Reading


Refereed papers

Other sources of information

  • DeHeer H L, Parry B W & Grindem C B (2002) Pleural Fluid. In: Diagnostic Cytology and Haematology of the Horse. 2nd edn. Eds: Cowell R L & Tyler R D. Mosby, USA. ISBN: 0323013171. pp 107-126.
  • Raphel C & Beech J (1981) Pleuritis and Pleural Effusion in the Horse. In:  Proc 27th Annual AAEP Convention. pp 17-25.