FENa Calculator

A result between 1% and 2% is indeterminate. It often represents a transition phase where Prerenal Azotemia is evolving into Acute Tubular Necrosis (ATN), or it may be seen in specific conditions like Obstruction (Post-renal). Doctors typically look at the urine sediment ("muddy brown casts") to confirm the diagnosis in this range.

Creatinine serves as the "reference standard." Since water is constantly being removed from urine, the concentration of everything else changes. By comparing how much Sodium is excreted relative to how much Creatinine is filtered, we cancel out the effect of urine dilution, giving a pure percentage of sodium handling.

No. FENa is strictly validated for Acute Kidney Injury (AKI) only. In chronic kidney disease, the remaining healthy nephrons naturally excrete more sodium to maintain balance, so a high FENa is normal and does not indicate acute tubular necrosis.

"Pre-renal" literally means "Before the Kidney." It means the kidney itself is healthy, but it isn't getting enough blood flow. This is usually caused by severe dehydration, heart failure (pump failure), or sepsis (low blood pressure). The treatment is usually restoring perfusion (IV fluids or heart meds), not dialysis.

Loop diuretics (like Lasix/Bumex) work by blocking sodium reabsorption in the Loop of Henle. This causes a massive amount of sodium to leave in the urine. If you run a FENa test on a patient who took Lasix recently, the result will be falsely high (>2%), leading to a misdiagnosis of ATN.