Expert: Karen Craft - 8/8/2009

Question
We have three cats (2 female 1 Male).  We are trying to decipher which of the little rascals is urinating outside of the litter box.  Is there a way to do this? All three are spayed/neutered.

Answer
Hi Suzzanne,

There is a way to do this but it is not really approved. You use Flouroscein dye. Give it to one cat at a time and see if the urine you find glows green under a black light.  Here's an article from Dr. Mike Richards which explains what you need to do.

"The product you are looking for is fluoroscein dye. This is used to make corneal (eye) ulcers show up better. It used to be possible to buy the dye in little bottles of the liquid version but now I can only find it in test strips. This might sound a little odd, but if you cut up several of the test strips into small pieces (four to six of the size that contains 9mg of fluoroscein per strip seems to be about the right number) and then stuff the strips into a capsule and give it orally the same effect can be obtained as with the liquid.  You can buy empty gelatin capsules from your vet or from your pharmacy.

Urine from the cat who has been administered the dye will show up light yellow-green even to the naked eye, if you have really good vision. It is easier to find with a black light, though.   The urine that contains the dye will fluoresce well. Normal cat urine does not fluoresce. This  testing is usually done in multiple cat households when one or more cats are spraying furniture but it is hard to determine who the culprit is. If you use it for this purpose try to remember that there can be more than one culprit, so it may be necessary to continue testing even after identifying one cat who is urine marking.

The urine will fluoresce in as short a time as a half-hour or so and will continue to fluoresce for about 24 hours or so. This means that you have to be prepared to do all your searching within the day after administration of the fluoroscein dye. I am not aware of any serious complications from using this dye in this manner but it is definitely not an officially approved use.

Hope this helps some.

Mike Richards, DVM 5/9/2001"


After you find one cat clean the area and test the next cat. This is just in case there is more than one doing it. Hope this helps. Write back when you find your culprit and I'll help you with taking care of the problem. Good luck.

Ciao, Karen