ABSTRACT: Part 1 in this series focused on the preparation of fine-needle aspirates \VNJ 27- 187-188). The aim of this article is to introduce veterinary nurses to the cytology of body cavity fluids. It will discuss techniques for preparing fluids in order to maximise the diagnostic benefits for the veterinary surgeon or pathologist.

Body cavities – the pleura and the peritoneum, for example – are lined by membranes which are covered by a single layer of mesothelial cells. Under normal circumstances, a small amount of fluid is produced within the cavities, but this is normally reabsorbed into the lymphatic system.

Larger amounts of fluid can accumulate in these areas caused by:

•  increased permeability with vascular damage or inflammation decreased blood oncotic pressure caused by loss of protein, mostly albumin

•  increased blood pressure as a result of fluid overload and congestion reduced lymphatic drainage owing to inflammation, congestion or obstruction.

Fluid samples are commonly collected by abdominocentesis or thoracocentesis, but they may also be taken from cystic masses found in various areas of the body.

The fluid sample should then be divided and placed in an EDTA blood tube, plus a sterile plain tube in case culture is required. It is important to fill the EDTA tube to the correct level as the anticoagulant may have an effect on cell shape and size, as well as on the dilution of the cell numbers.

Sample preparation

Direct smear

First make a direct smear using either the blood-film technique or the line- concentration technique described in Part 1 (VNJ 27: 187-188). If there are ‘floaters’ (floccules), these should be removed gently and prepared using the squash technique previously described. These smears should then be left to air dry before they are stained or packaged ready to send to an external laboratory.


This technique is very similar to that used for preparing urine sediment slides.

Place the fluid sample into a plain tube, such as an Eppendorf tube, and centrifuge the sample at 3000 rpm for about three to five minutes. Exact speeds can be adjusted to suit your individual centrifuge. The most effective way of preparing hypocellular fluid, such as CSF, is to use a cytospin centrifuge, but these may not be available in many practices.

Once centrifuged, most of the supernatant is removed, the sediment is re-suspended and then smeared onto a microscope slide using either the blood- film or line-concentrated method. These slides should then be left to air dry.

For samples such as CSF, in which cell levels will be very low, it is best to place one drop of the sediment on a microscope slide and allow it to dry. The area in which the cells will be found is already known, making examination much easier. The slides should then be stained using your in-house protocol or sent unstained to your external laboratory for examination.

Quantitative assessment

Quantified assessment normally focuses on red and nucleated cell numbers, as well as protein levels and, if required, triglyceride and cholesterol levels.

Cell counts

Automated analysers can be used to perform cell counts, but they are not always reliable as they do not allow for the presence of clots and are not very accurate for very low-cellular fluids, such as CSF. Manual counts should be performed using a counting chamber, such as an improved Neubauer haemocytometer.

If the sample is grossly cellular or turbid, the fluid should be diluted with a buffer solution or a white-cell diluting fluid, if you are just interested in the nucleated cell count (NCC). The exact dilution does not matter as long as it is known and the cells are individually visible and countable in the chamber used.

At this stage, no distinction is made between inflammatory white cells and tissue cells such as mesothelial cells; instead it is simply termed a nucleated cell count.

Total protein (TP)

Total protein can be measured using an automated chemistry analyser or a refractometer. It is easily measured via a refractometer using the same method as for urine specific gravity but using a separate scale. The value of protein obtained should be multiplied by 10, as levels are typically expressed in g/1 and scales typically read in g/dl.

Results obtained in this way are quick and easy, but are not always reliable, and can be affected by haemolysis and the presence of lipids in chyle. If possible, it is preferable to run a total protein and an albumin on a biochemistry analyser.


At this point, it would be prudent to mention effusions that are white and opalescent in appearance. These are commonly called chylous effusions and the grossly white appearance is caused by their lipid content. The best way to confirm a chylous effusion is to assess the triglyceride and cholesterol levels within the fluid, as well as the patient’s serum levels. A fluid triglyceride level >3 usually confirms a chylous effusion.


Once a cell count and a total protein measurement have been performed, it is possible to identify what category the fluid falls into (Table 1). This is by no means definitive, and cytological examination of the fluid will either confirm your findings or point the veterinary surgeon in a different direction; but for those OOH Sunday afternoon samples, having an understanding of what the fluid might be, could be very useful:

•  transudate – the most common form as a consequence of hypoalbuminaemia caused by conditions such as hepatic dysfunction or protein-losing enteropathy 

•  modified transudate – these effusions can be caused by a number of conditions which commonly include congestive heart failure, liver disease and some tumours 

•  exudate – exudates are most commonly caused by infectious and inflammatory conditions and can be classed as septic or non-septic.

Pancreatitis and peritonitis can be causes of both exudates and effusions. Figures 1- 4 show examples of what can be seen in samples of body cavity fluids.

Figure 1: Bronchoalveolar lavage sample

Figure 2: Bronchoalveolar lavage sample – pneumonia with Curschmann's spiral

Figure 3: Chylous pleural effusion with a grass seed fragment (arrow)

Figure 4: Pyothorax wjth bacteria in the neutrophils


The preparation of cytology samples is of great importance, because it plays a significant role in helping to obtain a correct diagnosis. Veterinary nurses can – and should – be instrumental in preparing laboratory samples. On those busy Sunday afternoons, a VN can provide the veterinary surgeon with vital information as he or she sifts through their differential diagnoses. 


Matthew Garland

CertNatSci (Open) VN MBVNA

After qualifying as a VN in 2004, Matthew worked in small animal practice before moving to Torrance Diamond Diagnostic services in 2006. Now, as laboratory manager of TDDS-Rmgwood, he has developed a strong interest in haematology and biochemistry.

To cite this article use either

DOI: 10.1111/j.2045-0648.2012.00185.x or Veterinary Nursing Journal Vol 27 pp 230-231

Further reading

BSAVA Manual of Clinical Pathology (2nd edition) Eds Villers. E. and Blackwood, L. BSAVA. Gloucester.


Veterinary Nursing Journal • VOL 27 • June 2012