The fractional excretion of sodium (FeNa) is a test that is often used in the setting of acute renal failure to help distinguish between pre-renal and intra-renal causes that has been mentioned in previous blog posts. In general, a FeNa of <1% suggests pre-renal disease, between 1-2% is indeterminate and >2% suggests ATN. There are some exceptions to this but overall, the specificity of this test is more than 80% and this increases if it is used in combination with the fractional excretion of urea.
By definition, the FeNa is the ratio between the quantity of Na excreted in the urine relative to the amount filtered at the glomerulus. So how can we make this calculation with a spot sample without reference to volume of filtrate or urine? Given that Na is freely filtered at the glomerulus, this means that:
Filtered Na = Plasma Na x GFR
Excreted Na = Urine Na x urine flow rate
FeNa = Excreted Na
= urine Na x urine flow
plasma Na x GFR
Because the creatinine clearance is a surrogate for the GFR:
FeNa = urine Na x urine flow
(plasma Na x urine Cr x urine flow)/plasma Cr
= urine Na x plasma Cr
Plasma Na x urine Cr
Or to make it easier to recall:
= Two smaller numbers
Two larger numbers
It is important to remember to take the serum sample at the same time as the urine sample because if the GFR is changing, this will affect the results. Remember also that the test can be unreliable in the setting of a near-normal GFR and with the use of diuretics, which is why combining it with the fractional excretion of urea can be useful.
This derivation can be used to determine the fractional excretion of any substance that is freely filtered at the glomerulus. However, remember that if the substance that you are trying to calculate is partially bound to albumin, an adjustment will have to be made. For example, Mg is about 30% albumin bound so that the plasma level would need to be multiplied by 0.7 for an accurate result.
Thanks to Dr Seifter for his help with this.