The Fractional Excretion of Sodium Calculation (FENa) requires the following patient parameters : Plasma Sodium and Urine Sodium in mEq/L. Plasma Creatinine and Urine Creatinine in mg/dL.
Formula for FENa in  % =  [(PCr × UNa ) / (PNa × UCr)] x 100
PCr plasma creatinine, UNa urine sodium, PNa plasma sodium, and UCr urine creatinine
Interpretation of FENa
Renal handling of sodium (filtered sodium) provides supportive information about the possible etiology of acute kidney injury.
| FENa value | UNa (mmol/L) | Cause of renal injury | Etiology |
| <1% | <20 | Pre-renal AKI | 1. Congestive heart failure 2. Renal Artery Stenosis 3. Septic Shock 4. Hypovolemia 5. Hepatorenal syndrome |
| 1-4% | >40 | Intrinsic renal injury | 1. Acute Tubular Necrosis 2. Acute interstitial nephritis 3. Glomerulonephritis |
| >4% | >40 | Post-renal AKI | Obstructive uropathy (ureteric urolithiasis, benign prostatic hyperplasia) |
AKI Acute Kidney Injury, UNa Spot urine Sodium
Application
This calculator is used for distinguishing between the various causes of AKI (pre-renal, post-renal, or intrinsic/renal causes).
Limitations
FENa estimation is likely to be inaccurate in the setting of uncontrolled glucosuria (diabetes mellitus), concomitant diuretic exposure, known chronic kidney disease, or known obstructive uropathy. For patients taking diuretics, fractional excretion of urea is recommended. This is due to the natriuresis induced by diuretics.
References
- Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976 Aug 9;236(6):579-81. doi: 10.1001/jama.236.6.579. PMID: 947239.
- Kaplan AA, Kohn OF. Fractional excretion of urea as a guide to renal dysfunction. Am J Nephrol. 1992;12(1-2):49-54. doi: 10.1159/000168417. PMID: 1415365.
