Diagnosing Cystinuria

Diagnosing the Defect

Cystinuria is a genetic kidney defect. Since normal urine does not contain cystine, the presence of cystine in the urine tells you the dog has cystinuria. The only exception would be if the dog was on certain antibiotics at the time of the test. Other than that, a urine test that is positive for cystine means your dog has cystinuria.

Where some confusion arises is in asking what this means. Since many dogs, perhaps even most, who test positive for cystinuria do not go on to form stones, this does not mean your dog will ever have cystine stones. Your dog still does have cystinuria, however. Since we do not know at this time what the difference is between cystinuric dogs who form stones and those who do not, and many dogs with cystinuria who do form stones don't do so until they are in their senior years, there is at this time no way to make genetic distinctions between cystinuria positive dogs who do, and do not, form stones.

There are three urine tests that can detect cystine. The first is a basic urinalysis. This is only reliable if cystine is present; a negative result doesn't mean your dog does not have cystinuria, but a postive result means he does. This is the least reliable test but the most readily available. Any vet's office can do a urinalysis.

The next-most-reliable test is the nitroprusside test being performed at the University of Pennsylvania. Owners can submit urine for testing without a vet and the cost is minimal ($20 as of June, 2005, although overnight shipping and special packaging add substanitally to this cost). However, just as with a urinalysis, the nitroprusside test is only reliable if it's positive. A negative test does not mean your dog does not have cystinuria. There is no set number of tests that will "clear" your dog, and no age at which the test can be considered definitive if negative. Since the test is inexpensive and non-invasive, most breeders do at least three for each dog before considering them clear, while still realizing even these three tests might be unreliable. You can get information on how to submit urine for this test here.

There is a third test, the urine amino acid quantitation test, or quantitative amino acid analysis, which is performed only in a human medical laboratory. It is extremely expensive but is the most accurate. However, again, a negative test result is not absolutely definitive. Owners interested in this test should contact the University of Pennsylvania and try to arrange testing; a special rate may or may not be available if you go through them. Dr. Paula Henthorn is conducting research on canine cystinuria at the University of Pennsylvania and her address is:

Paula Henthorn Ph.D.
Associate Professor of Medical Genetics
3900 Delancey Street
School of Veterinary Medicine
University of Pennsylvania
Philadelphia, PA 19104-6010

Dr. Henthorn's office may be contacted by phone at 215-898-9601.

The current state of testing for this condition is best outlined by Dr. Henthorn in this article.

There is no question that the lack of a known mode of inheritance and a reliable screening test is a source of intense frustration for breeders.

Diagnosing Stones

Cystine stones are extremely rare and many vets will not be familiar with their unusual qualities. For example, it's not always possible to see cystine stones on an x-ray or ultrasound, so they may wrongly tell you your dog has no stones based on these tests. (Sometimes they can be seen on x-rays and ultrasounds, though, so it's always worth a try.) A contrast study is needed to be sure.

If your dog is showing signs of urinary blockage and you suspect he may have cystinuria, it's essential that your veterinarian pursue this diagnosis and not rule it out prematurely. On the other hand, cystinuria is a rare condition and the odds are your dog does not have it. However, most of us whose dogs do have it were given incorrect diagnoses, sometimes repeatedly, with many vets, and over several years. This can create a very difficult conflict between the owner and the veterinarian, who may well feel that cystinuria is a very unlikely diagnosis. I can only encourage you to be persistent.

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