Sun21November0452PM 5

A previously fit and well 65-year-old male is admitted to the critical care unit following an uneventful laparoscopic right hemicolectomy.

You are called to review the patient because the patient has become oliguric.

Which of the following is most likely to point to a prerenal cause of acute kidney injury?

(Please select 1 option)

Concentrated looking urine

Urinary sodium >20 mmol/L Incorrect answer selected

Serum urea: creatinine ratio 200 This is the correct answer

Creatinine >300 µmol/L

Urine output less <10 mL/hour

Explanation

Serum urea:creatinine ratio is >100 in prerenal failure and <40 in acute kidney injury.

ADH is usually high in prerenal failure which leads to water, urea and sodium resorption. The fractional sodium excretion is less than 1% which in acute tubular necrosis is >2%.

With serum urea nitrogen/serum creatinine higher ratios (>20) are seen in prerenal azotaemia while lower ratios (10-15) are seen in acute tubular necrosis. Normal levels are between 12-20.

Urinary sodium in prerenal failure is less than 20 while in acute tubular necrosis is greater than 40.

Urine osmolality is >500 in prerenal failure and <350 in acute tubular necrosis.

Urine/serum creatinine is >40 in prerenal failure and <20 in acute tubular necrosis.

Serum urea or creatinine concentrations change inversely with glomerular filtration. Changes in serum creatinine concentration more reliably reflect changes in GFR than do changes in serum urea concentrations. Creatinine is formed spontaneously at a constant rate from creatine, and blood concentrations depend almost solely upon GFR.

Urea formation is influenced by a number of factors such as liver function, protein intake and rate of protein catabolism. Urea excretion also depends upon hydration status and the extent of water reabsorption as well as upon GFR.

A raised serum creatinine alone will not point to a particular cause of oliguria and nor will a urine output <10mL/hour and the production of a "concentrated looking urine".

Answer Statistics

1

26%

2

23%

3

41%

4

3%

5

8%

Times answered: 301