FENa Calculator (Fractional Excretion of Sodium)

The FENa calculator helps you determine the percentage ratio between the amount of sodium excreted through urine and the quantity filtered and reabsorbed by the kidney.

Fractional sodium excretion (FENa) is not a test. Instead, it’s based on blood and urine salt and creatinine concentrations. Urine and blood chemistry tests are needed for this computation.

Urine and blood samples are taken together and sent to a lab. Salt and creatinine levels are checked there. The chemical byproduct of creatine is creatinine.

Once you have received the results, you can calculate and compare the results.

Please note that FENa is helpful only in acute renal failure. Diuretic patients cannot utilize it.

The results are shown by using these criteria:

  • Prerenal cause: FENa <1%
  • Intrinsic Renal cause: FENa 1-4%
  • Postrenal cause: FENa >4%
FENa Calculator

You might want to calculate the best time for the ETG test or remaining medication.


  • Espinel C. H. (1976). The FENa test. Use in the differential diagnosis of acute renal failure. JAMA, 236(6), 579–581. https://doi.org/10.1001/jama.236.6.579
  • Zhang, W. R., & Parikh, C. R. (2019). Biomarkers of Acute and Chronic Kidney Disease. Annual review of physiology, 81, 309–333. https://doi.org/10.1146/annurev-physiol-020518-114605
  • Brosius, F. C., & Lau, K. (1986). Low fractional excretion of sodium in acute renal failure: role of timing of the test and ischemia. American journal of nephrology, 6(6), 450-457.
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