Your cat’s bloodwork came back with BUN flagged high, and the word “kidneys” is probably already on your mind. High BUN (blood urea nitrogen) in cats is a finding, not a diagnosis. It often reflects hydration, a recent meal, or bleeding in the gut, and reduced kidney filtration is only one item on that list.
Urea is made in the liver when protein is broken down, and the kidneys clear it into the urine. Because urea production rises and falls with protein intake and digestion, BUN mixes kidney information with a lot of noise from diet. Reference intervals also differ by laboratory and analyzer. One university laboratory lists a feline reference interval of 17 to 35 mg/dL, so the range printed beside your own cat’s result is the one that matters.
Urea, the Liver and the Kidneys
Urea is the end product of protein breakdown. The liver builds it from the ammonia released when amino acids are broken down. The kidneys then filter it out of the blood and into the urine, which is what puts BUN on a kidney panel.
Because BUN depends on kidney filtration, it is a late signal rather than an early one. Azotemia (from the French azote, nitrogen, and the Greek haima, blood) means a rise in urea and creatinine. It generally does not appear until about 75 percent of functioning kidney tissue has been lost. A normal BUN therefore does not prove that the kidneys are healthy. It means only that they are not yet failing badly enough to show up in this number.
Creatinine comes from muscle at a fairly steady rate, so it tracks filtration closely. Urea production swings with protein intake, digestion, and tissue breakdown, which makes BUN the less specific of the two. When BUN and creatinine disagree, a urine sample is usually what settles the question.
In nine healthy cats fed after a 12 hour fast, BUN was measurably higher at 2, 4, 6 and 8 hours after the meal. Creatinine did not shift at any point. Most of those after-meal values still stayed inside the reference interval. A meal is therefore a reasonable explanation for a BUN sitting just over the top of the range, and a weak one for a BUN well above it.
Dinner, Dehydration, Gut Bleeding and Kidney Disease
Dehydration and a recent protein-rich meal are the everyday causes, and both can lift BUN while creatinine barely moves. Bleeding into the upper gastrointestinal tract does the same thing, because digested blood is absorbed as a large protein load. Anything that speeds up protein breakdown will also push urea up, including fever, prolonged fasting, and steroid medications.
When BUN rises together with creatinine and SDMA, the combination points more strongly toward reduced kidney filtration, and chronic kidney disease is common in older cats. The signs of kidney failure in cats are worth knowing here, because they show what that disease looks like day to day, beyond the bloodwork. A urinary blockage is a less common but urgent cause of a rapid rise in both values together.
What Does a Mildly High BUN Usually Mean?
A mild elevation in a well-hydrated cat with normal creatinine and SDMA is more often explained by a recent meal or easily corrected dehydration than by kidney disease. The most useful next step is usually a recheck after a fast, with the companion kidney values and a urine sample drawn at the same visit.
Twelve hours without food is the fast used in the feline study above. A morning appointment helps too, because urea and creatinine both run higher in the afternoon than in the morning in healthy cats.
If BUN settles back into range on a fasted, hydrated recheck, diet or hydration very likely explained the first reading. That does not rule out early kidney disease, which is why the urine sample and the other markers are collected alongside it.
High BUN With Symptoms, Bleeding or a Blocked Bladder
BUN deserves closer attention when it rises together with creatinine or SDMA, or when it comes with symptoms. These are the ones to watch for at home.
- 01
Thirst and urination
Drinking more and filling the litter box more than usual
- 02
Weight or appetite loss
Losing weight or eating less than normal
- 03
Vomiting
Especially vomit that looks like blood or coffee grounds
- 04
Black, tarry stool
Dark, sticky stool, which can mean bleeding in the gut
- 05
Pale gums
Gums that look paler than your cat’s usual colour
- 06
Little or no urine
Repeated trips to the litter box with little or nothing passed
Any of these alongside a high BUN is a reason to contact your veterinarian, and two patterns should not wait. Vomiting blood, passing black tarry stool, or having pale gums can reflect active gastrointestinal bleeding and needs prompt veterinary care. A cat who is straining to urinate or producing little urine may have a urethral blockage, meaning urine cannot leave the bladder. It happens almost exclusively in male cats and can be fatal within two to three days if it is not relieved, so go to the vet immediately, the same day.
Reading BUN Against Creatinine, SDMA and Urine
Vets read BUN as one piece of the full bloodwork panel, alongside creatinine, SDMA, hydration status, and your cat’s diet and recent history. The urine sample carries more weight than most owners expect. Concentrated urine in a cat with a high BUN suggests the kidneys are still working and the problem sits upstream, in blood flow or hydration. Dilute urine points to the kidney tissue itself.
The threshold usually quoted for cats is a urine specific gravity above roughly 1.035 to 1.040. Cats complicate that rule: some cats with genuine kidney disease still concentrate their urine well, so a good specific gravity narrows the question without closing it.
The table below pulls those patterns together. When BUN rises with the other kidney markers, the workup is usually the same one used for a high creatinine.
| Pattern | What it often suggests |
|---|---|
| High BUN alone, normal creatinine and SDMA | Diet, mild dehydration, or GI factors likely |
| High BUN with elevated creatinine and SDMA | Reduced kidney filtration considered |
| High BUN and creatinine with concentrated urine | Dehydration or reduced blood flow considered first |
| High BUN with vomiting blood or black stool | Gastrointestinal bleeding considered |
| High BUN with inability to urinate | Urinary blockage; treated as an emergency |
| Normal on fasted, hydrated recheck | Original result likely diet or hydration related |
Fasted Rechecks and Why BUN Does Not Set the Stage
For a mild, isolated elevation, a fasted recheck alongside creatinine, SDMA and a urinalysis is the sensible first step. Bring a note of your cat’s recent diet, any vomiting or stool changes, and drinking habits to that visit, because that is context the number itself cannot supply.
BUN is not part of how kidney disease is staged. The International Renal Interest Society stages feline chronic kidney disease on fasting creatinine, confirmed on at least two occasions in a stable patient, supported by SDMA. So however many times it is repeated, BUN will not tell you how far anything has progressed.
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