Another formula that is commonly asked by nephrologist to medical student. Let’s derive it:

FeNa
= Excreted Na / Filtered Na

Excreted Na = Urine Na X Urine Volume
Filtered Na = Plasma Na X GFR

What is GFR?
Simply it is creatinine clearance (it’s not a very very correct statement as it depends on sex, age, but in this formula it is).

What is clearance?
In simpler terms, clearance is what amount of one substance you have cleared out relative to the its plasma amount.
Clearance = [Excreted substance] / [Plasma substance]
Clearance (C) = (U X V) / P
Ccr = (Ucr X V) / Pcr

Putting it together,
FeNa
= Excreted Na / Filtered Na
= (Una x V) / (Pna X  [(Ucr X V) / Pcr])
= Una X Pcr / Pna X Ucr
= Una/Pna X Pcr/Ucr

What is the significance of this formula?
FeNa < 1 = no sodium is excreted
– prerenal failure eg in shock, hypotension

FeNa > 1 = no reabsorption
– eg tubular necrosis, diuretics

Note that, using FeNa > 1 , we could not distinguish ischemic tubular necrosis vs diuretics use.
– It could have implications because ischemic tubular necrosis can have underlying causes eg nephrocalcinosis.

To overcome this, FeUrea has been proposed as a substitute for FeNa
– It makes sense as urea is less predisposed to reabsorption process in the kidney, whereas sodium is virtually reabsorbed in each segment of the nephron.

FeUrea
= FeUr/PUr X Pcr/Ucr