Analysis of fluids: cells and more (Proceedings)


Analysis of fluids: cells and more (Proceedings)

The fluids most frequently sampled for cytology are peritoneal, pleural, synovial, cerebrospinal and pericardial fluids and washes of the respiratory tract. Some of these fluids are more easily obtained than others. All may potentially yield general, or sometimes more specific information about a disease process.

What to do with the sample?

Fluid samples for cytology should be put into EDTA tubes to best preserve the cell morphology. If you're going to mail samples to an outside laboratory, it's a good idea to make smears of the fresh sample and send these along with the fluid sample to the laboratory. If you think that you may culture the fluid, place at least part of the sample in a red-top tube or on a culture swab, as EDTA is bacteriostatic.

Fluid smears can be made like a blood smear, using a small drop of fluid instead of blood, or a drop of fluid can be placed on a slide and the "squash" technique can be used. If the cell count of the fluid is high, make direct smears. If the fluid has a low cell count, "line" smears can be made or some of the fluid should be spun at a low speed for 3-5 minutes and smears should be made from the sediment. Samples should be allowed to air-dry before staining. No other fixation is needed and can actually damage the sample.

What do we evaluate?

  • Physical properties of the fluid including: volume, color, transparency, and odor
  • The nucleated cell count should be determined (except for washes) usually by hemacytometer, using a dilution system if the cell count looks moderate to high (i.e. if the fluid is cloudier than water).
  • Total protein should be measured (except for washes) using the plasma protein scale of a refractometer.
  • PCV or RBC count may also be determined if the fluid looks very bloody.
  • Cytologic evaluation

Body cavity effusions

A small amount of fluid is normally present in the body cavities to allow the visceral and parietal surfaces of organs and the body wall to interact freely without friction. The fluid is essentially an ultrafiltrate of blood, and is continually being added to and removed from the body cavities.

Normal body cavity fluid usually has:

  • <2.5 g/dl protein (typically around 1 g/dl)
  • <5000 cells/μl
  • <10% neutrophils in small animals

An effusion is an increase in the amount of body cavity fluid. Fluid is difficult to obtain in small animals if an effusion is not present, but can usually be obtained from normal horses. Classifying the type of effusion helps narrow down the list of possible causes and helps direct other useful diagnostic steps.


These are non-inflammatory effusions that result from impeded blood flow, impeded lymph drainage, or low osmotic pressure in blood vessels (i.e. hypoalbuminemia). They are roughly divided into:

Pure Transudates

  • Transudates containing very low protein (<1 g/dl) and cell numbers (<1000 cells/μl)
  • Low osmotic pressure is a common cause of pure transudates, typically due to hypoalbuminemia.
  • Pure transudates are less commonly associated with heart and liver disease than are modified transudates.

Modified Tranudates

  • Higher protein content than pure transudates
  • Cell counts frequently higher than pure transudates – but typically within the normal reference range or only mildly increased.
  • Can occur secondarily to a number of different causes, most associated with some type of impaired lymphatic drainage or venous stasis (increased hydrostatic pressure). The most common causes of a modified transudate are cardiovascular disease and neoplasia. Other possible causes include chronic hepatic disease, tissue inflammation, lung torsion, and diaphragmatic hernia.
  • Chylous effusions may fall into this category and have similar causes.