Why Does My Dog Or Cat’s Urine Contain Oxalate Crystals?

Ron Hines DVM PhD

To see what normal blood and urine values are, go here

For an explanation of causes of most abnormal blood and urine tests, go here

To see how tests are grouped, go here

Oxalate Crystals Seen In Your Pet’s Urine

Read more about oxalate urinary tract stones in cats here and in dogs here

Most of the oxalate in your dog and cat’s body is produced by normal processes occurring in your pet’s liver and red blood cells. Most oxalate (C2O4-) is a normal end product of metabolism occurring there  (ref)  An insignificant portion of the oxalate derives from the foods your dog or cat eats. If oxalate has any functions in your pet’s body is something we veterinarians do not know.

Once produced, oxalate usually combines with free calcium in your dog or cat ’s blood stream to form a soluble calcium salts. In that form it normally leaves your pet’s body in its urine. But in certain situations oxalate can form insoluble calcium crystals in your pet’s urinary tract when its urine is acidic (=pH under 7).

For most of the 20th Century, dog and cat diets were formulated to produce acidic urines in order to slow the formation of struvite crystals. At that time struvite was the major cause of urethral, bladder and kidney stones in dogs and cats. However, the situation has now reversed. Oxalate stones are now the most common ones seen in pets. So your veterinarian is likely to become concerned when small oxalate-shaped crystals are seen in large numbers in a fresh sample of your pet’s urine. That is particularly troubling since veterinarians have no way to dissolve these crystals if they eventually form large obstructive oxalate stones.

Reasons Why Oxalate Crystals Might Be Seen In Your Dog Or Cat’s Urine:

A few oxalate crystals seen in your dog or cat’s urine are not of concern unless your pet is experiencing urinary tract problems as well. Those problems almost always include some pain and urgency. When that is not the case, a few oxalate crystals are usually irrelevant. It is considerably more common to see them when urine was not examined microscopically soon after it was collected.  Old urine samples rarely provide much useful information and the results can cause needless worry. 

It may seem strange to you that something like oxalate crystals could sometimes be important and sometimes be unimportant. If veterinarians and physicians better understood why stones form in one pet and not in another we would certainly be able to prevent and treat them better. This is what we know:

Miniature schnauzers seem particularly susceptible to to both oxalate crystals and urinary tract stones. We do not know why. 

Some diseases of the parathyroid gland (hyperparathyroidism) cause high blood calcium levels  and are suspected but not proven to increase urine oxalate levels and, perhaps, oxalate stone formation.

Oxalate crystals also form in the kidney filters (glomeruli) of pets that have consumed antifreeze. Occasionally they also form in cases of cancer that elevate blood calcium level (eg lymphoma).

Whenever crystals of any kind are seen in your pet’s urine, the first thing that should come to mind and be explored is that the pet might not be consuming sufficient water. The more concentrated urine is, the more likely crystals are to form. 

Complementary Tests:

Urinalysis, CBC/ WBC and blood chemistry panel (including evidence of elevated blood calcium level). If accompanied by urinary tract distress or other abnormal urine parameters (tests), the pet needs a diagnostic work-up (x-rays) for potential oxalate urinary tract stones  (=calculi); if the urine pH is basic, for struvite urinary tract stones as well. A review of the dog or cat’s diet and a close review of the adequacy of the dog or cat’s fluid intake.

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