As discussed below, the ability to produce concentrated urine gradually deteriorates in a CKD cat as the kidneys fail. Although the cat will drink more in an attempt to compensate for the increased urination, eventually it becomes impossible to maintain a balance, and dehydration occurs.

Many people believe that dehydration means the loss of water from the body. However, it actually means the loss of both fluids and electrolytes - salts which the body needs in order to function properly.

It is not possible to diagnose clinical dehydration until the cat is already at least 5% dehydrated, so by the time you see signs, you already have a definite problem. Colorado State University gives information on the likely degree of dehydration based on physical signs (scroll down to section 7).

Recently sunken eyes may indicate dehydration, as may vomiting. Cracked paw pads are also occasionally seen, and litter may stick to the cat's paws. Sometimes a cat with dehydration hangs his/her head over the waterbowl, though more commonly that is a sign of stomach acid. The cat may also grind his/her teeth, or lip his/her lips.

You should regularly check your cat's hydration levels: the most common method is to pinch the skin, usually at the scruff of the neck - the skin should fall back into place immediately. Most CKD cats experience some degree of dehydration so the skin may not fall back as quickly as in a healthy cat, but if it takes a few seconds you should look into improving your cat's hydration. Another way to check is to feel your cat's gums: they should feel slick. If they feel sticky, your cat is probably dehydrated. Pet Education has some information about how to use these two assessment methods. Some people weigh their cats daily, finding this a good guide to their cat's degree of hydration.  

This may be a sign of an upper respiratory tract infection (a cold or cat flu). CKD cats may be prone to these viral infections because they are immune-compromised; plus CKD cats are often visiting the vet more often and therefore being exposed to more viruses.

However, if your cat has no other signs of an upper respiratory tract infection, then the sneezing may be caused by dehydration - the nasal passages of a dehydrated cat will be drier and therefore more susceptible to irritation, such as dust.

Dental problems may sometimes cause sneezing.

Another possible cause is the long-term use of cyproheptadine, which may make the nasal tissues swell as the drug becomes less effective.

Sneezing and snorting - what should I do? (2001) is a presentation by Brendan McKiernan to the World Small Animal Veterinary Association World Congress 2001.

In addition to concentrating urine, a cat's body also tries to conserve water by reabsorbing it from the stool through the intestinal wall. This mechanism is very efficient, and remains so even in CKD cats, and since CKD cats are largely on the edge of dehydration most of the time, the intestine will wring every drop of water out of the stool that it can, leaving it quite dry. The lack of moisture as a lubricant makes it more difficult for the cat to have bowel movements and can lead to constipation. Low potassium levels may also cause constipation, as may high calcium levels.

Symptoms of constipation include loss of appetite, pooping next to the litter tray, vomiting before, during or immediately after using the tray, dry stools or an ungainly walk. Occasionally a cat may urinate outside the litter tray when s/he is constipated - our Karma peed on the sofa so we took her to the vet for a suspected urinary tract infection, but in fact she did not have one, her problem was constipation. Once the constipation was under control, her inappropriate elimination ceased.

The vet can usually feel the backed-up stool when s/he palpates the cat's abdomen, but sometimes an x-ray is necessary to confirm the problem.

Sometimes a cat will appear to have diarrhoea but in fact has constipation, and the runny stool is simply what can squeeze around the solid dry stool.

Harpsie once had an episode of fast breathing and fast heart rate. He had severe constipation, and his problems resolved once he had been given an enema. If a cat is very severely constipated, toxins can back up in the cat's system causing such problems. The opposite problem may also occur i.e. lethargy and fainting (vasovagal syncope - syncope means to faint). Medicine Net discusses this. Obviously you do not want your cat to have such severe constipation that these problems arise!

Feline Constipation is a detailed and helpful website about constipation.

Pets Canada has information on the different colours of cat poop.

Newman Veterinary mentions that constipation may cause vomiting (scroll down a bit).

Fluid Retention/Build-Up

In the worst cases, fluid may collect in the lungs (pulmonary oedema), or around the lungs (pleural effusion) or in the abdomen (ascites). Your cat may appear swollen in the legs, face or abdomen. When Harpsie developed ascites (in his case, because of cancer), it felt like he had a hard little football in his abdomen. However, it is normal for cats to have a soft, squishy "pouch" of fluid after sub-Qs, which may move down into the front legs; this should gradually disappear as the fluids are absorbed.

Swelling may also be a sign of proteinuria. Alternatively it may be a sign of heart problems.

Pet Place has some information about pleural effusion (you don't need to register to read the article, just click on the Close This Window link at the bottom of the registration pop-up).

Macmillan Cancer Support explains more about pleural effusion.

Pet MD discusses pulmonary oedema in cats.

Pet MD has information about ascites.

Health A to Z explains more about ascites. 

Sometimes a CKD cat will suddenly gain a lot of weight in a short period of time, a matter of days or a week. This may indicate fluid retention, and needs to be investigated urgently, particularly if accompanied by fast heart rate, coughing, loss of appetite, breathing from the flank (as if pushing every breath out), limping, and particularly open-mouth breathing. If you see the last three symptoms, your cat's heart or lungs may be affected and you need to go to the vet immediately. Do not give any sub-Q fluids if you see any of the above symptoms or if your cat has gained a lot of weight suddenly or quickly until you have had your cat checked by a vet.

Sometimes a weight gain may seem small but we have to allow for the relatively small starting weight of a cat. So if, for example, a cat who weighs 8 lbs gains 1lb in a week, that is a weight gain of 12%. The human equivalent would be a 140lb person gaining almost 17 lbs in a week, which clearly is not possible in terms of actual weight.

I recommend weighing your cat every day and monitoring trends. What is a reasonable weight gain? Be guided by your vet, but if your cat has just eaten or has been given fluids and has not yet urinated, then these will affect weight. 100ml of fluids weighs about 3.5 ounces (100g), for example, so if you weigh your cat immediately after giving fluids you might panic about a non-existent weight gain of 3.5 ounces. If you are trying to get weight onto your cat, an acceptable rate of actual weight gain should be determined by your vet, based on your cat's current weight and goal weight, but roughly speaking an increase of 2-3 ounces (50-75g) a week should be acceptable.

Healthy cats only have tiny amounts of protein in their urine because their kidneys do not allow the protein to leak through. In CKD cats, this mechanism can be faulty and excess levels of protein in the urine, known as proteinuria, may occur. The main proteins which leak through are albumin and globulin. 

The concentrated urine of a healthy cat has the correct levels of acidity and good bacteria, which inhibit the development of bad bacteria which multiply and cause infections. The dilute urine of a CKD cat does not offer this protection, and so urinary tract infections (UTIs) are relatively common in CKD cats: Antech Diagnostics states that up to 30% of CKD cats have them.

Urinary tract infections may manifest themselves in your cat visiting the litter tray more frequently and producing little or no urine; or there may be slight traces of blood in the urine (see below for other causes of blood in urine); or howling in the litter tray; or licking the affected area a lot. Occasionally the urine will smell. Often there are no obvious signs, or less obvious signs such as urinating in inappropriate places such as on rugs or beds (see below).

UTIs can be very painful so keep a close eye on your cat and seek your vet's advice if you have any doubts; it may also be worth having regular urine cultures performed. Antibiotics can usually treat UTIs effectively, though some types of antibiotic may cause inappetance and vomiting in some cats.

V Vet sells a product called Health Alert which can test for blood in the urine.

The Pet Checkup is a similar test that checks for blood in urine, and which may also be used to check for other issues, such as diabetes.

Health Meter Cat Litter can detect blood in urine, and also checks urine pH. 

This may be a sign of a urinary tract infection, or bladder or kidney stones. It may also indicate high blood pressure. Sometimes this is caused by cancer. If your vet obtains a urine sample from your cat via cystocentesis (a needle into the bladder), this may sometimes cause blood in the urine.

Occasionally it may not be possible to ascertain the cause. There is a condition called "benign renal haematuria" which means there is bleeding from the kidneys but the cause cannot be found. However, this is rare in cats.

Persistent haematuria and proteinuria due to glomerular disease in related Abyssinian cats (2008) White JD, Norris JM, Bosward KL, Fleay R, Lauer C & Malik R Journal of Feline Medicine & Surgery 10(3) pp219-29 discusses how in Abyssinian cats with haematuria, the cause may be glomerular disease.

Severe or ongoing haematuria may cause or worsen anaemia, so you should always take your cat to the vet if you see this symptom.

Pet Place has some information about haematuria in cats.

Pet MD explains more about blood in urine.

KV Vet sells a product called Health Alert which can test for blood in the urine.

The Pet Checkup is a similar test that checks for blood in urine, and which may also be used to check for other issues, such as diabetes.

If your cat is struggling to urinate, i.e. visiting the litter tray more frequently but producing little or no urine, the most likely cause is a urinary tract infection.

Other possible causes of reduced or no urination include acute renal failure or kidney stones.

One of my cats used to get regular kidney infections, and the main sign for him was that he became incontinent when the infection first appeared. However, this is not inevitable with kidney infections - you may simply see more subtle signs, such as lethargy and inappetance. If you see these signs, particularly if your cat's bloodwork is worsening, ask the vet to consider that a kidney infection could be the cause.

Cats may lie in the litterbox if they have a urinary tract infection, or are constipated. This may also be a way to comfort themselves - many cats lie in their litter tray whilst hospitalised because it is the only thing that smells familiar to them.

In some cats, lying in the litter box is simply a sign of dominance - Harpsie loved lying in the litter tray, thus controlling the other cats' access to it.

It is possible to treat all of the above symptoms, in many cases effectively, and details can be found in the Treatments section.