Blood Urea Nitrogen (BUN) or Urea

Blood urea nitrogen is commonly known as BUN in the USA, and, together with creatinine, it is one of the two main measures of kidney function. In the UK, BUN is not normally tested, instead you will see urea listed on your cat's bloodwork. 

Blood urea nitrogen is a measurement of the levels of nitrogen in the blood that come from urea. During the breakdown of dietary protein in the digestive process, ammonia is absorbed in the gut. The liver then makes a substance called urea from the ammonia. The urea is carried in the blood, in the form of urea nitrogen, to the kidneys, which filter it out of the blood and excrete it via urination.

BUN rises in CKD because the kidneys are no longer able to excrete it efficiently; but it can also rise for other reasons, such as dehydration, urethral obstruction (a blockage which prevents a cat from urinating, more common in male cats) or gastro-intestinal bleeding. It is also affected by both diet (since it is a by-product of the breakdown of protein) and stress. 

If BUN and creatinine rise suddenly, your cat may have developed acute kidney disease on top of CKD. This is known as AoCRF and usually has a particular cause, so you should consider the possibility that your cat has a kidney infection or hypertension. Kidney stones which cause blockages may also cause a sudden and high rise in both BUN and creatinine.

Cats with a particular form of diabetes called ketoacidosis may also have elevated BUN or urea and creatinine levels, particularly if potassium and phosphorus levels are normal.

Therefore BUN or urea are not an entirely accurate indicator of kidney function, and you should not assume that your cat has CKD based on the BUN or urea measurement alone. BUN is not toxic in itself, but it is used as a marker i.e. if BUN is high, then certain toxins which cannot be easily measured will also be high. Although BUN is not a toxin, it is important to try to control high levels because they can cause lack of appetite, nausea and vomiting. 

If BUN or urea levels are high yet creatinine is normal or only a little elevated, it usually means that the cat is dehydrated, has gastro-intestinal bleeding, or is eating a high protein diet. 

BUN is not normally low in CKD cats, but may be below normal in cats with liver disease or those who are suffering from starvation. Harpsie once had low BUN levels when he was very sick with an infection (possibly in his liver, though I suspect it was actually a kidney infection) and had not eaten much whilst on IV at the vet's, resulting in rapid weight loss.

Many vets who run tests in-house in the USA cannot measure BUN levels over 130, so you may see a reading of >130. If possible, it is better to know the exact level, but don't worry if this is not available. With luck, it is going to come down below 130 anyway following treatment.

Approximate normal levels (precise ranges vary from laboratory to laboratory) for cats are in this table.

Creatinine

Creatinine is another waste product excreted through the kidneys. It is generally considered to be a more accurate measurement of underlying kidney function than BUN or urea because it is less affected by diet, stress and dehydration.

Creatinine is a by-product of muscle. Therefore small, dainty cats would be expected to have lower levels of creatinine than large, muscular cats.

In CKD cats, both BUN or urea and creatinine will be elevated to some degree depending upon the severity of the disease; but if BUN or urea levels are high yet creatinine is only a little elevated, it usually means that the cat is dehydrated, has gastro-intestinal bleeding, or is eating a high protein diet. 

Creatinine is not a linear measurement. This means that an increase in creatinine from, say, US 2 to 3, indicates more loss of function than an increase from US 5 to 6. Thus, whilst your cat might have a relatively high creatinine of, say US 5, if it increases to US 6, then whilst you are right to be concerned, it does not automatically indicate a massive worsening of your cat's condition.

See below for information about low creatinine levels in end stage CKD.

Other Reasons for Raised Creatinine

Cats with a particular form of diabetes called ketoacidosis may have elevated BUN and creatinine levels, particularly if potassium and phosphorus are normal. 

Cats with pancreatitis also sometimes have elevated creatinine levels. If your cat has relatively low creatinine (in the 2s) yet seems lethargic and far more ill than that mild level of kidney failure would suggest, I would consider pancreatitis.

Because creatinine is a by-product of muscle, large, muscular male cats may naturally have high normal levels of creatinine.

An unexpected high prevalence of azotaemia in Birman cats (2002) Gunn-Moore DA, Dodkin SJ & Sparkes AH Journal of Feline Medicine and Surgery 4 pp165-6 (letter) mentions that up to 80% of Birman kittens under six months of age seem to have high levels of creatinine and BUN. As adults, this incidence reduces, but is still high, at 35%. However, these cats do not appear to develop fullblown CKD at a young age as one might expect. Monitoring them regularly would nevertheless be wise.

The effects of cimetidine on renal function in patients with renal failure (1980) Larsson R, Bodemar G, Kagedal B, Walan A, Acta medica Scandinavica 208 (1-2) pp27-31 explains that cimetidine (Tagemet), which is sometimes used to treat stomach acid in CKD cats though it is not the best drug for that purpose, may cause an increase in creatinine. If your cat's creatinine levels rise while using Tagamet, you may find they improve once you stop the medication.

If creatinine and BUN rise suddenly, you should consider the possibility that your cat has a kidney infection or hypertension. Kidney stones which cause blockages may also result in a sudden and high rise in creatinine and BUN.

Approximate normal levels (precise ranges vary from laboratory to laboratory) for cats are in this table. 

Reasons for Low Creatinine (Especially if BUN and Phosphorus are High)

Since creatinine is a by-product of muscle, cats who lose a lot of weight/muscle may have reduced creatinine levels, because they cannot produce as much creatinine. University of Georgia College of Veterinary Medicine explains more about changes in creatinine (scroll down to Variations in Creatinine Concentration). The Merck Veterinary Manual states "Serum creatinine levels can be falsely lowered in patients with severe muscle wasting"

Cats with hyperthyroidism may have low creatinine levels, because they tend to lose a lot of muscle, plus hyperthyroidism masks true kidney function, making BUN and creatinine look lower than they truly are.

If BUN (urea) levels and phosphorus levels are high yet creatinine is normal or only a little elevated, it can mean that there is a pre-renal factor at work, for example:

• the cat is dehydrated;

• the cat has gastro-intestinal bleeding (see BUN:creatinine ratio above);

• the cat is eating a high protein diet;

• the cat has severe metabolic acidosis; or

• in some cases it may indicate heart failure.

Azotaemia

Azotaemia is another way of saying that there is increased nitrogenous waste in the bloodstream, i.e. BUN/urea and creatinine levels are elevated. Azotaemia is divided into three stages (bear in mind that blood flows to the kidneys where it is filtered):

• pre-renal azotaemia ("before" the kidney):

  this means that the azotaemia does not involve the kidney and is caused by some other problem before the blood reaches the kidneys, such as infection, fever, a high protein diet, heart problems or dehydration.

Uraemia

Uraemia means that a cat has azotaemia (increased BUN and creatinine levels) but also has the associated problems commonly seen in CKD patients such as vomiting, increased urination, anaemia etc.

You may see reference to uraemic toxins: these are the toxins which the cat's damaged kidneys are unable to filter properly, so they cause many of the symptoms of CKD. One such toxin is parathyroid hormone. Contrary to popular opinion, BUN and creatinine are not toxins themselves. However, BUN levels correlate with uraemic toxin levels, i.e. if BUN is elevated, it is highly likely that uraemic toxins are also elevated.

Potassium and sodium are electrolytes (salts), which are essential to the functioning of the body. The increased urination that occurs with CKD may cause imbalances in these electrolytes. Medicine Net explains more about electrolytes.

Potassium

Potassium is used at cellular level, in particular to help muscles function properly. If there is an imbalance, weakness, twitching and seizures may be seen.

Since potassium imbalances are so common in CKD cats, there is a dedicated page all about it. Approximate normal levels (precise ranges vary from laboratory to laboratory) for cats are in this table.

Cornell University College of Veterinary Medicine has an overview of potassium.

Cornell University College of Veterinary Medicine has some information on hypokalaemia. 

High Potassium Levels (Hyperkalaemia)

It is important to understand that not all CKD cats need potassium supplements. Some cats never have low potassium levels, particularly if their CKD is relatively advanced (creatinine over 5). Thomas's creatinine was not that high, yet he never needed potassium, and the same study as mentioned above (Lulich et al., 1992) found that around 13% of CKD cats actually have the opposite problem of hyperkalaemia (high potassium levels). Therefore you should not supplement potassium without a bloodtest and your vet's approval.

If your cat has high potassium levels, (over 6), this is potentially very dangerous, and may cause seizures and even a heart attack.

Cornell University College of Veterinary Medicine has some information on hyperkalaemia. 

Sodium

Sodium is excreted by the kidneys, but levels may rise (hypernatraemia) in CKD cats because the kidneys are no longer working as efficiently and cannot adapt to changes in sodium levels as quickly. Vomiting or diarrhoea may also be factors.

In Preparing cats for radioactive iodine treatment (2005), Presentation to the World Small Animal Veterinary Association World Congress, Dr T Shermerhorn mentions that elevated sodium levels may also be seen in cats with hyperthyroidism.

There is some debate as to whether elevated sodium levels can worsen hypertension (high blood pressure). Approximate normal levels (precise ranges vary from laboratory to laboratory) for cats are in this table.

Cornell University College of Veterinary Medicine has some information on hypernatraemia.

Pet MD also discusses high sodium levels.

Magnesium                                                                                                              Back to Page Index

Magnesium is a mineral but I am including it here because there may be correlations between potassium levels and magnesium levels. Magnesium imbalances are not unknown in CKD cats, but for some reason, it is only rarely measured during routine blood tests.

Newman Veterinary explains more about magnesium imbalances.

Cornell University College of Veterinary Medicine has an overview of magnesium.

High Magnesium

In CKD cats, magnesium levels tend if anything to be high, because the damaged kidneys cannot excrete it properly. Therefore CKD cats do not normally need a magnesium supplement. This is one reason why using phosphorus binders containing magnesium is also not recommended.

Pet MD has some information about high magnesium levels.

Low Magnesium

Occasionally, however, a CKD cat will have low magnesium levels. Usually such a cat will also have low potassium levels. Cats with hyperthyroidism tend to have low magnesium levels, so if your cat has both hyperthyroidism and low potassium levels, you may wish to have magnesium levels checked. 

If your cat has both low potassium and low magnesium levels, initially you should ask your vet about starting a potassium supplement. You may well find that both potassium and magnesium return to normal once the potassium supplement is begun. If, however, your cat has low potassium levels which do not rise despite the use of a potassium supplement, it is possible that the low magnesium also needs to be treated. In such cases, you are unlikely to be able to raise the potassium to an acceptable level until you have also treated the low magnesium. Your vet should also consider the possibility of hyperaldosteronism if your cat has persistently low levels of magnesium and potassium, especially if hypertension is also present. 

The Feline Patient (2007) has an article by HR Sims about low magnesium levels.

Medscape discusses low magnesium levels.

Evaluation of ionized and total serum magnesium concentrations in hyperthyroid cats (2006) Gilroy CV, Horney BS, Burton SA & MacKenzie AL Canadian Journal of Veterinary Research 70(2) pp137–142 focuses on cats with hyperthyroidism but also has some useful information about magnesium levels in cats generally.

• Albumin

• Globulin

• Albumin:globulin ratio

Pet MD has information about ascites.

Health A to Z explains more about ascites.

Other Test Results Which May Be Out of Range                                           Back to Page Index

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