Monitoring renal function is an important aspect of care in many of our patients. It goes without saying we should do this in patients with acute kidney injury or chronic kidney disease, but many critical patients can also develop renal injury.

Azotaemia (increased urea/creatinine) can be seen due to a number of causes. These are categorised into pre-renal (the cause is BEFORE the kidney, e.g. hypotension under anaesthesia causing reduced renal blood flow), intrinsic renal (changes take place IN the kidney, e.g. toxicities which exert their effects on the kidney, pyelonephritis and glomerular disease), and post-renal (cause is AFTER the kidney, e.g. urinary tract obstruction).

Renal function should be monitored by regular assessment of:

• Blood urea nitrogen (BUN or urea), creatinine, electrolytes, and phosphate levels,

• Urine concentrating ability & presence of proteinuria,

• Through quantification of urine output.

Ideally, a urine sample should be collected prior to treatment to assess specific gravity prior to fluid therapy.

Urine output is ideally quantified through placement of an indwelling urinary catheter and use of a closed collection system, though if this is not possible, weighing bedding, incontinence sheets and litter trays before/after use can provide some estimation (1 g in weight difference is approximate to 1ml of urine).

A normal urine output is approximately 1-2ml/kg/hour; however volumes retrieved should always be compared to fluid therapy/volumes administered. Oliguria is defined as a urine output of less than 0.5ml/kg/hour, and polyuria is defined as a urine output of above 2ml/kg/hour.

Patients with anuric or oliguric renal failure may require diuretics and in severe cases, continuous renal replacement therapy (haemodialysis) or peritoneal dialysis.

References

1. Monitoring and Intervention for the Critically Ill Small Animal: The Rule of 20. Kirby & Linklater, 2017.