Loading Calculator...
Please wait a moment
Please wait a moment
Distinguish prerenal from intrinsic acute kidney injury
Diuretics increase FENa, making it unreliable. Use FEUrea instead.
FENa = (UNa × SCr) / (SNa × UCr) × 100
Prerenal azotemia - kidneys retaining Na appropriately
Indeterminate - could be prerenal, early ATN, or mixed
Intrinsic renal - tubular damage impairing Na reabsorption
<35% = prerenal, >35% = intrinsic
| Marker | Prerenal | ATN |
|---|---|---|
| FENa | <1% | >2% |
| FEUrea | <35% | >35% |
| Urine Na | <20 mEq/L | >40 mEq/L |
| BUN:Cr | >20:1 | 10-15:1 |
| Urine Osm | >500 | <350 |
FENa measures the percentage of filtered sodium that is excreted in urine. In prerenal states, kidneys retain sodium (low FENa). In tubular injury, this ability is lost (high FENa).
Use FEUrea when the patient is on diuretics, as diuretics increase sodium excretion and falsely elevate FENa. Urea reabsorption is less affected by diuretics.
Yes. FENa can be calculated from a single spot urine sample with simultaneous serum labs. The creatinine ratio corrects for urine concentration.
Limited utility. CKD patients have impaired sodium reabsorption at baseline, so FENa may be elevated even with superimposed prerenal insult.
FENa can help distinguish oliguric prerenal AKI (low FENa, responsive to fluids) from oliguric ATN (high FENa, not fluid responsive).
No. FENa is one piece of the puzzle. Consider clinical context, history, physical exam, urine microscopy, and imaging. No single test is definitive.
The best time to collect urine for FENa is BEFORE giving fluids or diuretics. Once therapy is started, interpretation becomes more difficult.