High urea in dogs: the kidney is not always to blame

What this marker measures

Urea is made by the liver. When the body breaks down protein, it produces ammonia, and the liver turns that into urea, which is less toxic. Urea circulates in the blood, then the kidneys filter it and clear it into the urine. Its value therefore depends on three things at once: how much protein there is to process, how the liver is working, and how well the kidneys can clear it. Part of the filtered urea is reabsorbed when the dog is dehydrated, which pushes it up without any organ being damaged. It is a sensitive but not very specific marker: it moves easily, often for ordinary reasons.

What vets look into when the value sits outside the range

Ordered from the most common to the rarest in practice.

The first two rows explain the majority of slightly high ureas found on a routine panel. The third is the one most often forgotten: digested blood represents a substantial protein load, and it pushes urea up well before the dog looks ill.

What this value does not tell you on its own

Urea alone cannot say whether the kidneys are involved. It is read alongside several other lines.

• Creatinine, first. Two high values together point towards the kidney or towards marked dehydration. A high urea with a creatinine inside the laboratory's range points first to the diet, to dehydration or to the digestive tract.
• Urine specific gravity, measured on a sample collected before any fluids. It is what says whether the kidney still concentrates properly.
• The haematocrit and total protein, which rise with dehydration and fall with bleeding. Those two opposite patterns often settle the question.
• SDMA, phosphorus and electrolytes, which complete the kidney reading.
• Albumin and the liver parameters, because a struggling liver makes less urea: a low urea in a sick dog also carries information.
• The time of the last meal and what was in it, data only the owner can supply.

Two reading habits. Ranges vary from one analyser and one laboratory to another, so a value is only compared with the reference values printed on the same report. And a single sample freezes one moment: it is comparing two panels taken under similar conditions, the dog fasted and hydrated, that shows a trend.

Questions to ask your vet

1. Was my dog fasted, and can the urea be interpreted under those conditions?
2. What does the creatinine alongside it say, and does the relationship between the two point to a kidney or a non-kidney cause?
3. Has urine specific gravity been measured before any fluids, and what does it show?
4. Should we look at the haematocrit and investigate bleeding in the gut given the stools?
5. How soon should the panel be repeated for comparison, and under what fasting and hydration conditions?

My dog was not fasted, does that count?

A great deal. Urea comes from the liver breaking down dietary protein. A rich meal in the hours before the sample is enough to push the value out of the laboratory's reference range, while creatinine barely moves. It is the first thing to tell the vet.

High urea but normal creatinine, what does that mean?

That combination points first to a non-kidney cause: a recent meal, dehydration, or digested blood after bleeding in the stomach or intestine. The vet then looks at the haematocrit, total protein and the appearance of the stools before concluding anything about the kidneys.

Should I change my dog's food because of this value?

That decision belongs to the vet, once the cause has been identified. A particular diet is not equally useful depending on whether the value comes from a sample taken after a meal or from established kidney damage. Changing the food before the recheck muddles the comparison between the two panels.