Effusion analysis (fluid protein and cell count) in cats and dogs

What it shows

In healthy dogs and cats the chest, abdomen and sac around the heart hold very little fluid — generally less than 10 mL. When fluid builds up (an effusion), the vet takes a sample. Two numbers usually head the report:

• Effusion total protein — how much protein is in the fluid. It is often estimated with a refractometer (on the clear top layer if the fluid is cloudy).
• Nucleated cell count — how many cells with a nucleus (white blood cells and other cells) are in each microlitre.

Together with microscopy of the cells, these numbers sort the fluid into types that point to how it formed. Classifications differ between textbooks. The older one uses pure transudate, modified transudate, exudate, chylous and haemorrhagic effusion. A newer, mechanism-based one speaks of low-protein and high-protein transudates, exudates and separate groups such as FIP, tumour-related, haemorrhagic and chylous effusions, bile or urine in the abdomen and gut rupture.

Higher than reference — what it may be a sign of

• High protein, low cell count — a high-protein transudate: protein above 2.5 g/dL (usually below 5.0) with fewer than about 3,000 cells/µL. In the abdomen of dogs it can come from congestion of the liver in heart failure, and from some liver diseases.
• High cell count — an exudate, meaning inflammation. Older texts used more than 5,000 cells/µL; a newer reference leans towards more than 3,000/µL with 3,000–5,000/µL as a grey zone. Protein is usually above 2.5 g/dL and often above 4.0 g/dL. Bacteria seen inside cells (engulfed by neutrophils) confirm a septic (infected) effusion; the fluid is then also cultured.
• Very high protein with relatively few cells in a cat — the typical picture of FIP: protein usually above 2.5 g/dL, often above 4.0, and cells usually below 5,000/µL, in viscous yellow fluid with fibrin strands.
• Many red cells — a haemorrhagic effusion: more than 1 million red cells per µL and a measurable packed cell volume (the share of red cells by volume), usually above 5%.

None of these types is a diagnosis on its own.

Lower than reference

Low protein (below 2.5 g/dL) with few cells is a low-protein transudate; below 1,500 cells/µL it is called a pure transudate. It usually means fluid leaking because of pressure or low blood protein rather than inflammation — for example portal hypertension (raised pressure in the liver’s portal vein) from liver disease, a common cause in dogs and cats, or severe low albumin (below about 1.0–1.5 g/dL). Protein values also have pitfalls: EDTA in an underfilled tube can make the refractometer read too high, and the fat in chylous fluid makes refractometer protein inaccurate.

Look at together with

• Blood total protein and albumin — low blood albumin is one cause of a transudate.
• Triglycerides in the fluid — when the fluid is milky, to check for a chylous effusion.
• Fluid versus blood — creatinine in the fluid usually more than twice the blood value points to urine in the abdomen, and bilirubin twice or more the blood value to a bile leak.
• Rivalta test and coronavirus tests on the fluid — in a cat with suspected FIP.

Units and conversion

Protein in fluid, as in blood, is written in g/L or g/dL: 1 g/dL = 10 g/L, so the often-quoted 2.5 g/dL is 25 g/L. Cell counts are given per microlitre (cells/µL); some forms and textbooks write thousands per µL (×10³/µL), which is the same as ×10⁹/L, so 5 ×10³/µL means 5,000 cells/µL. In the app each row keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form; for body cavity fluid it is usually not a “normal” interval but cut-offs used to classify the fluid, and these cut-offs differ between textbooks and labs. The description of the cells often matters more than either number.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Effusions
2. eClinPath (Cornell University) — Peritoneal fluid
3. Merck Veterinary Manual — Table: Characteristics of transudates and exudates in cattle, horses, dogs and cats
4. Merck Veterinary Manual — Peritonitis in animals
5. Merck Veterinary Manual — Feline infectious peritonitis
6. Merck Veterinary Manual (Cat Owners) — Disorders of the chest cavity of cats