Urea and BUN in cats and dogs

What it shows

The kidneys filter urea freely, but part of it is taken back into the blood on the way: according to eClinPath, about 40% when urine flows fast and about 60% when it flows slowly, as in dehydration. That is why urea climbs with any drop in filtration and can rise a little earlier than creatinine when an animal is dehydrated.

Urea also depends on things outside the kidneys: how much protein was eaten or digested (including blood swallowed or lost into the gut), how fast the body breaks down its own protein, and whether the liver can make urea at all. IDEXX notes that it is not specific for kidney disease, so a vet always reads it next to creatinine.

On a form you may see four rows from this group: urea, BUN, and two ratios — urea : creatinine (U/C, from European labs) and BUN : creatinine (B/C, BUN/Crea).

Higher than reference — what it may be a sign of

• Less blood reaching the kidneys: dehydration, heart disease, shock.
• Kidney disease itself: chronic kidney disease or acute kidney injury.
• Urine cannot leave the body: urethral obstruction, a ruptured bladder.
• Causes outside the kidneys: a recent high-protein meal or diet, bleeding into the upper digestive tract, and faster protein breakdown — fever, severe burns, corticosteroids, starvation. eClinPath stresses that gut bleeding and steroids do not always raise urea.
• Artefacts: severe jaundice of the sample; ammonium heparin as the tube anticoagulant.

Lower than reference

Low urea is seen with a low-protein diet, in young animals that use protein for growth, with heavy drinking and urination (for example diabetes or Cushing’s disease), with overhydration and late pregnancy, and when the liver makes too little urea — a portosystemic shunt or liver failure, not just mild liver injury. A vet reads a low urea together with albumin, glucose and liver tests.

Look at together with

• Creatinine — the main partner. eClinPath describes the patterns: urea high with creatinine normal fits early dehydration, a high-protein diet, gut bleeding or fast protein breakdown; urea normal or low with creatinine high fits reduced filtration combined with liver failure, heavy urination or a low-protein diet, or a large muscle mass (greyhounds).
• Urea : creatinine and BUN : creatinine ratios put those patterns into one number. IDEXX lists gut bleeding, marked muscle loss and pregnancy among causes of a high ratio, and severe muscle injury among causes of a low one — but adds that the ratio was once hoped to separate the causes of azotemia and has too many variables to be used as a diagnostic parameter.
• SDMA and urine specific gravity — help decide whether the kidneys themselves are involved.

Units and conversion

Urea in mmol/L and BUN in mg/dL describe the same molecule. To go from urea in mmol/L to BUN in mg/dL, multiply by 2.8: urea 10 mmol/L is about BUN 28 mg/dL. Some forms give urea itself in mg/dL; multiply by 0.166 to get mmol/L, so 60 mg/dL of urea is about 10 mmol/L. The ratios have no unit, but U/C and B/C are calculated from different units and cannot be compared with each other.

Reference range

The reference range is on your lab’s form. Urea responds to meals, hydration and diet, so results are easiest to compare when taken under similar conditions; the trend together with creatinine says more than one value. If the lab prints a range for the ratio, read the ratio against it; the sources we used give no shared threshold for it.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Urea
2. IDEXX — Complete Chemistry Panel Interpretation Guide
3. Whitbread TJ. Clinical Biochemistry. Merck Veterinary Manual
4. VCA Animal Hospitals — Serum Biochemistry
5. Cornell Feline Health Center — Chronic Kidney Disease
6. Cornell Feline Health Center — Feline Lower Urinary Tract Disease
7. VCA Animal Hospitals — Testing and Monitoring in Addison’s Disease