Abnormal kidney function indicates intrarenal failure.

Sodium

Is the kidney still able to conserve Sodium? Calculate the fractional excretion of Sodium.

This tells you how much sodium is lost in the urine – a healthy kidney can resorb at least 99% of sodium, so the F E NA % should be less than 1%.
If it is higher, this indicates abnormal renal function (i.e. intrarenal failure).

Renal Concentrating Ability

Can the kidney appropriately concentrate urine? i.e. produce urine of an appropriate concentration compared to the plasma osmolality.
Calculate the ratio of urine:plasma osmolality (U/P Osmolality).
It should be >1.3:1

  • A high Urine to Plasma ratio indicates the kidney still has some function.
  • A low Urine to Plasma ratio indicates the kidney is not working.

Nitrogen Clearance

Are the kidneys able to get rid of urea and creatinine? Test the ratios of urea and creatinine in the urine compared to that of urine in the blood:

  • Urine:Plasma UREA ratio (U/P Urea)
  • Urine:Plasma CREATININE ratio (U/P Creatinine)

Both of these should be >20:1
If it is lower than this, this indicates abnormal function.

urine Sediment

Cell debris, red cells, casts etc. are a sign of intra-renal failure.

Summary

Signs of intra-renal failure.

Urine Na+ mmol/l

>20

Fractional excretion of Na (%)

>1

Urine Cl- mmol/l

>20

Urine/plasma osmolality ratio

<1.3

Urine/plasma urea ratio

<14

Urine/plasma creatinine

<14

Urine sediment

Cell debris, red cells, casts etc