Cats with chronic kidney disease (CKD) may smell because they have bad breath with an ammonia-like smell. I think this is the reason behind the question. The best answer as to why a cat with CKD has bad breath comes from Tanya on her site dedicated to kidney disease. She says that cats with CKD have high levels of urea (the major waste component of urine) which combines with an enzyme in the mouth bacteria called urease. The interaction of chemicals produces a strong and distinctive odour. It seems somewhat like ammonia (a bleach-like smell). Tanya also says the breath might smell a bit like urine which contains ammonia. The smell will be particularly noticeable if your cat ‘crashes’ meaning collapses.
CKD can also cause mouth ulcers. This is because uraemic waste is not eliminated by the kidneys. Mouth bacteria combines with the uraemic waste to create ammonia which, as I understand it causes the gum ulcers. This makes the mouth sore which leads to an inability to self-groom leaving the coat unkempt. This may make the cat smell less clean than normal. We know that a healthy cat’s coat smells nutty-fresh. It is quite distinctive and therefore it would be noticeable if it smelled less good.
The obvious advice is to see your veterinarian if you suspect that you cat has bad breath as described. It is likely, however, that the observant cat guardian who is emotionally close to their cat would have noticed something wrong because of other signs of kidney disease being apparent already. These signs include: increased drining and urination, vomiting, blood in urine, loss of appetite due to nausea, weight loss, sitting hunched or walking stifly and poor coat as mentioned.
SOME MORE ON A CAT HEALTH DIAGNOSIS: