Fractional Excretion of Sodium (FENa)
Determines if renal failure is due to pre-renal, post-renal, or intrinsic renal pathology.
|Prerenal||Intrinsic Renal||Postrenal||UNa (mmol/L)||<20||>40||>40||FENa||<1%||>1%||>4%|
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)