Determines if renal failure is due to pre-renal, post-renal, or intrinsic renal pathology.
FENA is only useful in AKI, not CKD.
FENa cannot be used in patients on diuretics. Use FEUrea instead.
Consider obstructive uropathy as a differential for AKI, no matter what the FENa score, as it is easy to correct. You can do so by placing a foley or assessing bladder fullness.
Fractional Excretion of sodium (FENa) = (UNA / PNA) / (Ucr / Pcr)
<1% is prerenal, contrast, HRS or glomerulonephritis;
>2% is ATN.
If the patient is on diuretics, check Fractional excretion of urea.
Fractional Excretion of Sodium (FENa) = (PCr * UNa ) / (PNa x UCr) %
| Prerenal | Intrinsic Renal | Postrenal | |
|---|---|---|---|
| FENa | <1% | >1% | >4% |
| UNa (mmol/L) | <20 | >20 | >40 |
Prerenal: Anything that causes decreased effective renal perfusion: Hypovolemia, CHF, Renal Artery Stenosis, Sepsis, etc. Remember, contrast-induced nephropathy will often look pre-renal.
Intrinsic Renal: ATN, AIN, Glomerulonephritides, etc
Postrenal: Obstruction (BPH, bladder stone, bilateral ureter obstruction)