Equis ISSN 2398-2977

Pleural fluid: cytology

Contributor(s): Annalisa Barrelet, Prof Bernard Feldman


  • The pleura of the horse is a thin layer of connective tissue covered by a single layer of mesothelial cells.  The visceral pleura covers the lungs and the parietal pleura covers the chest wall, mediastinum and diaphragm. 
  • In the normal horse the pleural cavity is moistened by a very small amount of fluid to allow friction free movement of the surfaces during respiration.  When fluid in the pleural space exceeds this very small amount, a pleural effusion is said to exist. 
  • Factors responsible for maintenance of the normal pleural fluid are plasma oncotic pressure, capillary hydrostatic pressure, capillary permeability and lymphatic drainage.  A disturbance in any one of these factors may cause fluid to accumulate, producing a pleural effusion.
  • Pleural fluid   Pleural fluid: overview   is a dialysate of plasma with a low cell count   Pleural fluid: cell count and differential  and protein   Pleural fluid: protein  .  The cytological constituents reflect pathophysiological processes in the pleural surfaces.  
  • Cell characteristics vary in septic conditions, toxic processes, chylous processes and with neoplasia.
  • Cytology enables characterization of a thoracic effusion and positive diagnosis in some cases of neoplasia.


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Most veterinary cytopathologists use Romanowsky stain variants. The important difference in sample handling is that samples are allowed to air dry using these stains, whereas specimens should be wet fixed (sample mixed with equal volume of cytopreservative, or smear spray or ethanol-fixed) if Papanicolaou-type stain is to be used.

Result Data

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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Chaffin (1998) Thoracocentesis and pleural drainage in horses. Equine Vet Educ 11 (2), 106-108.
  • Mair T S & Brown P J (1993) Clinical and pathological features of thoracic neoplasia in the horse. Equine Vet J 25 (3), 220-223 PubMed.
  • Mair T S & Sweeney C R (1992) Investigation of pleural effusions in the horse. Equine Vet Educ (2), 80-74.
  • Wagner & Bennett (1982) Analysis of equine thoracic fluid. Vet Clin Pathol 11 (1), 13-17.

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.