Blood Urea Nitrogen (BUN)

What it measures

Breaking down protein produces ammonia, which the liver turns into urea for the kidneys to remove. So urea nitrogen depends on the amount of protein, liver production, and kidney excretion. Like creatinine it reflects kidney filtration, but more outside factors affect it.

What a high value may mean

The most important cause is reduced kidney filtration (including dehydration, kidney disease, and urinary blockage). A high-protein meal, bleeding in the upper digestive tract (the digested blood acts like extra protein), and conditions that increase protein breakdown, such as fever or steroid use, can also raise urea, often without a matching rise in creatinine.

What a low value may mean

Possible causes include a low-protein diet, very young age, poor liver function (less urea is made), a portosystemic shunt (an abnormal liver blood vessel), and conditions that cause a lot of urination, such as diabetes.

Usually read together with

Creatinine, a urine test, total protein, albumin, hematocrit, and liver values.

When to follow up

If urea and creatinine rise together, kidney filtration is usually affected, and your vet will look for the cause. If only urea is high and creatinine is normal, think about whether the cat ate shortly before the blood draw or has shown signs of gut bleeding such as black stools, and tell your vet.

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