A common condition is Stomach Acid in Cats with Kidney Disease. 
This is probably the most common cause of vomiting, nausea and loss of appetite in CRF cats. Almost every CRF cat will have problems with stomach acid at some point.

Gastrin is a gastro-intestinal hormone which stimulates the secretion of gastric acid, which helps the stomach digest food. The kidneys are responsible for the excretion of gastrin, but in CRF this function may not work so well, resulting in the gastrin remaining in the stomach and stimulating the production of too much gastric acid.

This excess stomach acid can make a cat feel very unwell. In severe cases stomach ulcers may develop, which may cause gastro-intestinal bleeding. Generally speaking, cats with creatinine over 265 (US: 3) will need help controlling excessive stomach acid. And the higher your cat's BUN or urea level, the more likely s/he will feel sick and vomit. Stomach acid can make some cats feel very ill, but it does in part depend upon the cat.

There is no test as such for stomach acid, but these are some of the symptoms you might see (though some of these may also be due to other causes, as mentioned under each category):

Vomiting and Nausea

Nausea
Nausea can be hard to detect, though it is often manifested in a lack of appetite. The cat may also lie scrunched up, looking uncomfortable.   High phosphorus levels, dehydration (this is why Sub Q comes in handy)  or  anaemia conditions may also cause nausea.

Vomiting
There is a difference between vomiting and regurgitation. Vomiting is usually accompanied by a lot of abdominal movement, whereas regurgitation happens suddenly and with less warning.  Vomiting means the cat is emptying the stomach, whereas regurgitation is where food has not yet reached the stomach but is being ejected from the eosophagus more or less intact, often because of eating too fast or hairballs.

Regurgitation
is often sausage-shaped. Like what you see as fur ball vomiting.

Constipation
Occasionally vomiting is caused by constipation, particularly if your cat vomits before, during or immediately after using the litter tray.  Or not eating.

Vomiting Water
Sometimes CRF cats drink a lot on water in one go, them vomit all or most of it up shortly afterwards. This may be a sign of stomach acid. It is possible that the cat has an urge to drink before vomiting so as to dilute the stomach acid.

Loss of Appetite
Loss of appetite is very common in CRF cats, and is often linked to stomach acid.   Human CRF patients have reported that their sense of smell and sometimes taste are impaired; this is thought to be caused by uraemic toxins, and probably occurs in cats too. 

What to do?
Using homeopathic Serum Ang  (which is in the AN019 Reno-Aid) along with the Sub Q fluids at home helps rid this condition.  With an added pinch of chelated Zinc tablet aids better ability to smell. or homeopathic Teucrium Mar 30C if also suffers from colds, sneezing which can cause loss of smell.  Cats will not eat if they cannot smell their food, even when starving.

There are several other possible causes of lack of appetite ...  including dehydration, high phosphorus levels, anaemia, fluid build-up, crashing, metabolic acidosis, mouth ulcers, the use of antibiotics drugs, constipation,  or the use of the drug medication for a hyperactive thyroid.  We have alternatives for all these conditions and preventions.

Dental problems may also cause appetite loss (usually you can smell a bad breath when teeth decay is there, however when you use the Drawing Out (homeopathic drops) can make all the difference in quickly drawing out and expelling (draining) any pus around the teeth and gums (which is not normally visible to the eye).
Using this remedy will prevent any further intervention putting you pet under any further stress of  things like surgery (anaesthetic toxins) or antibiotics that damage kidneys further still.  Far safer to rid the infection using homeopathics.

Disclaimer
Australian Clients should be aware that national legislation prohibits the marketing or advertising of any veterinary products which have not been registered with the N.R.A. These regulations were designed to control the entry onto the market of new chemical drugs and did not foresee the rising demand for proven herbal and homeopathic formulations. Other laws however, allow such treatments to be available for both human and animal treatment after personal consultation. The Treatment Program Descriptions provided are for Australian clients who should ignore the product names and other claims which are provided for International clients.