Rivalta test on effusion fluid in cats

What it shows

The Rivalta test is a simple, inexpensive test done in the clinic on fluid from the abdomen or chest. A test tube is filled with about 7–8 mL of distilled water at room temperature and one drop of concentrated acetic acid (or white vinegar) is mixed in. Then one drop of the cat’s fluid is placed gently on the surface:

• Positive — the drop keeps its shape and stays attached to the surface, or slowly sinks to the bottom.
• Negative — the drop dissolves and the liquid stays clear.
• Questionable — faint cloudy swirls; such results are hard to read.

A positive reaction marks a protein-rich, inflammatory fluid. What exactly makes the drop hold together is still unknown, and it is not simply the amount of protein. Reading the result is subjective, and there is no reference method to compare it with.

Higher than reference — what it may be a sign of

A positive result makes FIP more likely, but it is not specific. Other protein-rich inflammatory effusions — for example bacterial (septic) peritonitis or lymphoma — can also turn the drop positive, and the study authors suggest that long-standing effusions may do so too. That is why cytology of the fluid usually follows.

The largest study of the test included 497 cats with an effusion and a confirmed diagnosis, 172 of them with FIP. The test detected 91.3% of the FIP cases (sensitivity), but only 65.5% of the cats without FIP tested negative (specificity). Of 268 positive results, 111 — about 41% — were in cats with other diseases. When cats with lymphoma and bacterial infections were excluded, specificity rose to 73.0%.

Lower than reference

A negative result makes FIP very unlikely, and this is the most useful side of the test: it helps rule FIP out better than it helps confirm it. In the same study, 93.4% of negative results were in cats that did not have FIP. The picture changes in young cats, in whom FIP is far more common among effusions: in cats up to 2 years old, only 66.7% of negative results were truly free of FIP. So a negative test in a young cat with typical signs does not close the question.

Look at together with

• Effusion analysis — protein and cell count of the same fluid; a typical FIP effusion is very rich in protein with relatively few cells.
• Cytology of the fluid — many neutrophils with bacteria, tumour cells or a large lymphocyte population point away from FIP.
• Coronavirus tests on the fluid — RT-PCR or staining for coronavirus antigen in cells of the fluid; a positive antigen stain in macrophages makes FIP very likely, while a negative one does not exclude it.
• Globulin and the albumin-to-globulin ratio in blood and protein electrophoresis — a low serum ratio supports FIP, a high one makes it very unlikely.

Units and conversion

The test has no unit. It is reported as “positive” or “negative”, sometimes as “questionable” or “weakly positive”. In the app the result is stored as text.

Reference range

The reference range is on your lab’s form or in the clinic notes; the expected result for fluid that is not inflammatory is “negative”. Because the reading is subjective and the test is crude, the vet sees it as one piece of the puzzle and decides on the next steps together with the fluid analysis, blood tests and the cat’s signs.

Cats and dogs

Sources

1. ABCD (European Advisory Board on Cat Diseases) — Guideline for feline infectious peritonitis
2. Fischer Y. et al. — Diagnostic accuracy of the Rivalta test for feline infectious peritonitis (Vet Clin Pathol, 2012; PMC)
3. Merck Veterinary Manual — Feline infectious peritonitis
4. eClinPath (Cornell University) — Effusions