Aging Cat 15 years old Lost Weight, Drinks Water Excessively

I have a kitty who is about 15 years old +/- who is very sensitive and has had trouble with Stress. We recently went to the vet about Urinating outside of the box. Normally she is good with the box, The vet noticed she lost 2 pounds in 6 months. Mentioned that she drinks water excessively.

The vet did blood tests, urinalysis, and culture with no result.

After he asked to do Chest X-ray and sonogram of abdomen, also no real issue, clean, just some food in the stomach from before 10pm the prior night

we had brought her to the vet before for unitary issues with no organic cause.

The cat also has a sensitive stomach and vomits a lot.

She also has normally a very good appetite and is not picky

I first noticed the drinking because she would drink my water out of my glass very often, and my partner puts substantial water bowls out.

They normally eat wet food Fancy Feast chicken and fish types.

There is another cat in the house, who also causes stress.

I have had back issues and recently had surgery, I have not been able to be as attentive to her, but my partner has done his best.

It looks like sometimes the issue is she loses control of her bladder while sleeping.

Any clues on how to get her to put back on some weight?

Any clues about the water drinking.

I know you say you've had tests done, but diabetes and kidney disease are the two possibilities that spring to mind. I don't know what else to say. Hope you find the root cause soon.

Do the symptoms in either of these articles ring any bells?
http://www.thecatsite.com/a/introduction-to-feline-diabetes
http://www.thecatsite.com/a/your-cat-s-kidneys-called

The vet thought the Kidneys seemed normal, mild kidney damage consistent with age.

I am expecting to talk to the vet tomorrow, I will ask him if there is anything we need to do to test for diabetes, or if he has already done so.

Thanks for your concern. She seems to be relatively normal in a lot of ways, behaves relatively normally. 

The urinating outside of the box, and the vet thinks its due to the urinary production.

The test or diabetes is called a fructosamine. Basically it's similar to a AC1 done on Human diabetics. It gives the average blood glucose level over the past few weeks. An elevated level would indicate diabetes. Ask the vet if a fructosamine was included with the reecnt bloodwork. If not, ask for it to be done. A quick in office blood glucose test with a blood gluocse meter won't tell you anything other than what the cat's blood glucose is at that very moment (most likely high because of stress).

Does the urine smell sugary or seem extra large and hard to scoop? That indicates sugar in the urine, a common symptom of diabetes.

If she has stomach issues and vomits a lot, that might be related to her losing weight, although shouldn't have much to do with her drinking lots of water.  Both of those things (losing weight and drinking lots) are classic signs of what has been mentioned above...kidney disease or diabetes

Back to the vomiting...sounds like she may need a change in her diet.  However, IF she has either one of the issues discussed above, she will need a change in diet anyway, so this discussion may be a moot point.   Vomiting is not normal for a cat.  An occasion hairball, maybe, although hairballs SHOULD go out the back end (inside their stools), not the front.  So, if a cat is vomiting with any regularity, something is wrong.  Usually it's food related.  I say this even if the issue is IBD, because the right food will help that too.  Cats are not meant to eat things like wheat, corn, soy, and often have allergies to it, which can manifest by them itching a LOT, or vomiting.  Unfortunately, the aforementioned items are in many cat foods out there.  I would suggest you try the Fancy Feast CLASSICS, however, since you already said she has "mild" kidney disease, FF CLASSICS aren't the best, since they are really high in phosphorus, and that's the last thing a kidney cat needs   Maybe try the Sheba pates.  They don't contain the  known "baddies", and the phosphorus levels are better.  They are good for diabetic cats too, IF that turns out to be the issue.  Was your girl overweight before? 

I'm still concerned about the urinating outside the box.  Does or does she not have a UTI?  I don't know what you mean by "urinary production".  I understand how stress can cause UTI's, but get a feeling she doesn't have one.  But urinating outside the box is highly unusual if there is no UTI .  If she is stressed, have you thought about using calming sprays and/or Rescue Remedy for Pets?   Also, playing harp music is supposed to be great for calming cats.  There are CD's specifically for calming cats.   And you could try Cat Attract litter to try to get her to go back to the box.