Urate crystals (ammonium biurate) in urine of cats and dogs

What it shows

Ammonium biurate crystals are brown or yellow-brown spheres with irregular spiky projections, sometimes called “thorn apples”; they are often seen together with amorphous urates. They can form at any pH but favour neutral to acidic urine. Because the finding can matter, it is best confirmed at high magnification. Some medicines produce xanthine crystals that look similar, so the vet takes the list of medicines into account.

Normally much of the uric acid is converted into allantoin; in Dalmatians this conversion is reduced. A variant of the SLC2A9 gene, which encodes a uric acid transporter, raises uric acid in urine. Crystals and stones are linked for urates, but crystals can also be found without stones, and stones without crystals.

Higher than reference — what it may be a sign of

• A breed trait — in Dalmatians, English Bulldogs and Black Russian Terriers urate crystals are found in healthy dogs because of a defect in the SLC2A9 transporter. Dalmatians and English Bulldogs are predisposed to urate stones, though not every dog forms them.
• Portosystemic shunt and other portal vascular anomalies — vessels that let blood bypass the liver. Urate crystals are fairly common in dogs and cats with portal vascular anomalies, with or without stones. In other dog breeds and in cats the finding suggests that the liver is not working as it should.
• Urate stones — in a dog of a breed not known for high uric acid in urine, they are a reason to check for a shunt, including serum bile acids.
• Chemotherapy that increases uric acid production.

Lower than reference

No urate crystals is the expected result for most cats and dogs. It does not exclude a liver shunt or a urate stone: stones can form without crystals in the urine, and a shunt is looked for with blood tests such as bile acids, not with the sediment.

Look at together with

• Bile acids — a key blood test when a shunt is suspected.
• Urine pH — urates favour neutral to acidic urine.
• Urine specific gravity — concentrated urine favours crystals.
• Amorphous salts — amorphous urates often accompany biurate crystals.
• The animal’s breed — the same finding means different things in a Dalmatian and in a mixed-breed dog.

Units and conversion

Urate crystals have no unit to convert: the lab grades them by eye. The form may give a count per high-power field (/hpf) or only words such as “few” or “many”; amorphous urates, if present, usually get a separate line.

Reference range

The reference range is on your lab’s form; for crystals there is often no number at all. Like other crystals, urates can form in the sample itself when urine stands or is chilled after collection, so fresh urine matters (see struvite for how storage affects crystals). Outside the predisposed breeds, even a few urate crystals in a fresh sample are worth discussing with the vet.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Urinalysis: crystals
2. eClinPath (Cornell University) — Crystal quick guide
3. Whitbread TJ. Urinalysis. Merck Veterinary Manual
4. Merck Veterinary Manual — Urolithiasis in dogs
5. Merck Veterinary Manual — Urolithiasis in cats
6. Merck Veterinary Manual — Overview of urolithiasis in small animals
7. Cornell Riney Canine Health Center — Hyperuricosuria and hyperuricemia or urolithiasis (HUU)