Synovial (joint) fluid analysis in cats and dogs

What it shows

Healthy joint fluid is colourless to light yellow, clear and thick, with very few cells. Most of them are mononuclear — macrophages and cells of the joint lining with some lymphocytes — and neutrophils make up less than 10%.

The form usually shows two results. The nucleated cell count tells how many cells are in one microlitre. Viscosity shows how stringy the fluid is: a drop drawn out of a pipette normally forms a strand of about 2 cm. An older way to judge it, the mucin clot test, is no longer performed by some laboratories. A smear shows which cells dominate, and that often decides the reading more than the count.

Higher than reference — what it may be a sign of

• Non-inflammatory (degenerative or traumatic) joint disease — osteoarthritis, injury: more fluid, a mild rise of cells, usually under 5,000/µL, mostly mononuclear; viscosity reduced. A lymphocyte-rich picture is seen, for example, with cruciate ligament rupture.
• Inflammatory joint disease: neutrophils above 10% of cells, the count usually above 5,000/µL — though both can stay in range.
• Immune-mediated polyarthritis — the most common inflammatory cause in dogs and cats: several joints affected, many intact (non-degenerate) neutrophils. In 39 dogs the count ranged from 3,700 to 170,000/µL with 20–98% neutrophils.
• Septic (bacterial) arthritis: many neutrophils, the fluid can look like pus; bacteria are rarely seen on the smear, and the culture can be negative in up to half of cases.
• Tick-borne infections in dogs (Ehrlichia, Anaplasma): inclusions are found in fewer than 5% of neutrophils and are easy to miss.

Lower than reference

A count below the reference is not a finding — in healthy joints it starts close to zero. The meaningful «low» result is low viscosity, thin and watery fluid. It occurs with degenerative joint disease, injury, inflammation, excess joint fluid, bleeding into the joint and dilution with blood during collection. Blood from the collection itself also changes the picture, so the vet notes whether the fluid turned bloody at the start or the end of sampling.

Look at together with

• Bacterial culture — confirms infection when bacteria grow; a negative culture does not exclude it.
• ANA and rheumatoid factor — worth testing when immune-mediated polyarthritis is suspected.
• X-rays of the joints — separate erosive forms (bone and cartilage destroyed) from non-erosive ones.
• Blood count, chemistry, urinalysis and infection tests — a search for a cause elsewhere in the body: tick-borne panels in dogs; FeLV, FIV, coronavirus and Toxoplasma in cats. Low platelets often accompany tick-borne disease.

Units and conversion

The nucleated cell count is reported in cells/µL (cells per microlitre). Viscosity is a qualitative result: the form describes it in words, such as normal or decreased, and sometimes gives the strand length; the mucin clot is also described in words. Protein, if listed, is measured with a refractometer — with a very small volume in a large EDTA tube that reading can be falsely high. In the app each row keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form. In most healthy joints the count stays under 1,000 cells/µL. For dogs 3,000/µL is a typical cutoff, though many healthy dogs have far fewer — in 12 healthy large-breed dogs the mean was 667/µL (range 33–1,495). In healthy cats one study found a mean of 161/µL (range 2–1,134), and under 1,000/µL is used as the upper limit. Besides the number, the share of neutrophils matters: under 10% fits a healthy joint.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Synovial fluid — cytology.
2. Immune-Mediated Polyarthritis in Dogs and Cats. Merck Veterinary Manual, professional version.
3. Septic Arthritis in Dogs and Cats. Merck Veterinary Manual, professional version.
4. VCA Animal Hospitals. Joint Fluid Analysis (Know Your Pet).