Interpreting the numbers: Why azotaemia and dilute urine does not always = renal failure

Urea and creatinine are tests frequently requested for veterinary patients - for those that are unwell, as pre-anaesthetic tests, as "wellness" screening tests in older animals or before the prescription of specific medications such as NSAIDs. But if a patient is found to be azotaemic, does that mean it must have renal failure? Urine specific gravity is less frequently measured despite being easy and cheap to do so. Yet it is usually impossible to fully assess an azotaemic patient without knowing the urine SG. Interpretation of urine specific gravity (SG) and serum urea and creatinine requires understanding renal physiology (which we all hated learning about at university!) and the mechanisms that control these parameters. In this lecture, we will review the key aspects of renal function in an accessible way to help us understand why a patient with low urine SG and azotaemia does not always have renal failure and to recognise other disorders that impair both urine concentration and cause azotaemia.

• To understand the factors, both renal and non-renal, that cause azotaemia and impaired urine concentration
• To appreciate that not every patient with azotaemia has renal failure - even if urine concentrating ability is impaired