Calcium oxalate crystals (dihydrate, monohydrate) in urine of cats and dogs
Calcium oxalate crystals are common in urine sediment and are often seen in healthy animals. The form named on the report matters: the usual “envelope” dihydrate on its own often means little, while some monohydrate shapes make the vet think about poisoning or high calcium. Crystals are not stones.
What it shows
On a form these crystals can appear in two lines:
- Calcium oxalate dihydrate — small colourless squares crossed by an “X”, like envelopes. They are often seen in normal urine, can occur at any pH and can also form in the sample while it is stored.
- Calcium oxalate monohydrate — oval, spindle or dumb-bell shapes, and a rare flat, elongated “picket fence” form. They are infrequent in normal dogs and cats; the picket-fence form is not seen in healthy animals.
Both mean the urine is supersaturated with calcium and oxalate. Crystalluria is not the same as stones: stones may be present without crystals, and crystals without stones.
Higher than reference — what it may be a sign of
- Nothing at all — dihydrate crystals are often found in healthy animals.
- Crystals that formed in the sample — dihydrate crystals in particular can form in urine that stood or was chilled after collection, so a fresh sample matters (see struvite for how storage affects crystals).
- Risk of calcium oxalate stones — persistent crystals may point to an increased risk of these stones.
- More calcium in urine — for example with high blood calcium due to overactive parathyroid glands, or with some tumours such as lymphoma.
- Ethylene glycol (antifreeze) poisoning — large numbers of dihydrate crystals in a dog or cat with acute kidney failure, or picket-fence monohydrate crystals, make the vet consider it. The sign is not 100% sensitive: not every poisoned animal shows these crystals. Eating oxalate-rich plants is another possible cause.
Lower than reference
No calcium oxalate crystals is a common result. It does not rule out a calcium oxalate stone, since stones may form without visible crystals. Nor does it rule out ethylene glycol poisoning, because the crystals are not found in every case.
Look at together with
- Calcium in blood — high calcium increases calcium in urine.
- Creatinine and urea — sudden kidney failure together with many oxalate crystals is an important combination.
- Urine specific gravity — concentrated urine favours crystals.
- Urine pH — dihydrate occurs at any pH; monohydrate forms are found in neutral to acidic urine.
- Struvite — the other common crystal type in cats and dogs.
Units and conversion
There is no unit to convert: oxalate crystals are graded by eye, and small ones such as the dihydrate “envelopes” are looked for at high magnification. The form may give a count per high-power field (/hpf) or only words such as “few” or “many”, and the line for the dihydrate is often separate from the line for the monohydrate.
Reference range
The reference range is on your lab’s form; for crystals there is often no number at all, and a few dihydrate crystals in a dog or cat are not abnormal in themselves. Because dihydrate crystals easily form in stored urine, the vet first asks how fresh the sample was. Crystals that persist in fresh samples, monohydrate forms, especially picket fences, and crystals together with kidney problems carry far more weight.
Cats and dogs
Calcium oxalate makes up about half of all stones in cats and most kidney stones. Longhaired cats are predisposed, but any breed can be affected. Calcium oxalate and struvite are the most common crystal types in cats. A cat with many oxalate crystals and sudden kidney problems is assessed for ethylene glycol just like a dog.
Calcium oxalate stones can occur in any breed but are more common in a number of small breeds, including the Miniature Schnauzer, Chihuahua, Bichon Frise and Yorkshire Terrier. Miniature Schnauzers are predisposed even with normal calcium excretion in urine. A single finding in a chilled sample means much less than crystals that keep appearing.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.