Blood Urea Nitrogen

A raised urea (azotaemia) can point to the kidneys, but several non-kidney factors push it up too, and separating them is the whole skill of reading it:

• Dehydration / reduced kidney blood flow — the most common cause of a mildly high urea; it usually falls with fluids.
• A high-protein meal shortly before sampling, which is why a fasted sample is preferred.
• Bleeding into the gut, where digested blood is a protein load that raises urea — a useful clue in itself.
• Reduced kidney filtration, the finding that matters most, especially when creatinine and SDMA are also raised and the urine is dilute.

Because urea moves for so many reasons, it is most informative in company. Urea and creatinine rising together with dilute urine builds a kidney picture; urea up with a normal creatinine points more towards dehydration, diet, or gut bleeding.

A low urea is usually of little concern and can accompany a low-protein diet, increased drinking and urination, or reduced liver production — the liver makes urea, so advanced liver dysfunction or a portosystemic shunt can lower it (read alongside ALT and bile acids). A normal urea, like a normal creatinine, does not by itself exclude early kidney disease.

In established kidney disease urea tracks broadly with creatinine and rises as the disease advances, and a high urea is part of what produces the clinical signs of uraemia — nausea, poor appetite, mouth ulcers — when the kidneys can no longer clear waste. But because hydration and diet shift it so readily, creatinine and SDMA are the steadier numbers for staging and trend-tracking; urea adds context, particularly about hydration and how the patient is feeling, rather than being the primary yardstick.

Urea is interpreted the same way in dogs and cats, with feline reference ranges running higher. The same cautions apply in both — hydration and diet first, then the kidney picture. In cats, the frequent combination of dilute urine and rising urea and creatinine is a common presentation of the CKD that is so prevalent in the species.

Urea is measured on a routine blood chemistry panel — in the UK often reported directly as "urea" in mmol/L, in the US as BUN in mg/dL (the two describe the same thing on different scales). A fasted, well-hydrated sample is most reliable. As always, read the value against your own report's reference range and follow the same-lab trend.

A mildly raised urea on a well pet is often addressed simply by checking hydration and rechecking a fasted sample. A urea that stays up alongside a raised creatinine, SDMA and dilute urine warrants the fuller kidney work-up with your vet; a high urea with normal creatinine may instead point at hydration, diet, or a gut bleed worth investigating.