A FENa calculator is a clinical tool used to calculate the Fractional Excretion of Sodium in the urine, which helps distinguish between prerenal and intrinsic renal causes of acute kidney injury. The formula is FENa equals urine sodium times plasma creatinine, divided by plasma sodium times urine creatinine, all multiplied by 100 to express as a percentage. A FENa below 1 percent suggests a prerenal cause such as dehydration or reduced kidney perfusion. A FENa above 2 percent suggests intrinsic renal injury such as acute tubular necrosis from ischemia or nephrotoxic medications.
The FENa calculator is an essential tool in nephrology, emergency medicine, and critical care. It is most useful in patients not taking diuretics, as diuretics increase urinary sodium excretion and can falsely elevate the FENa result above 1 percent even in prerenal disease. In these patients, FEUrea, or the fractional excretion of urea, is preferred as a more reliable marker of renal perfusion status. Healthcare providers enter the four required lab values into the calculator and receive an instant result to guide fluid and medication management decisions. Understanding the clinical context behind the FENa calculation remains essential, as a single laboratory result must always be interpreted alongside the full patient presentation.