FENa Calculator

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Fractional Excretion of Na calculator is to measure of the percentage of sodium excreted in the urine versus the sodium reabsorbed by the kidney.FENa is not a test, but rather a calculation based on the concentrations of sodium and creatinine in your blood and urine.

Calculate the fractional excretion of sodium given the serum and urine sodium, and the serum and urine creatinine.

To accurately interpret FENa, patients should not have recently received diuretics. FENa is greater than 1% and usually greater than 3% with acute tubular necrosis and severe obstruction of the urinary drainage of both kidneys. It is generally less than 1% in patients with acute glomerulonephritis, hepatorenal syndrome, and states of prerenal azotemia such as congestive heart failure or dehydration. FENa may also be less than 1% with acute partial urinary tract obstruction.

– Stein JH, ed. Internal Medicine, 4th edition. Mosby-Year Book. 1994.

The equation is:

Where

UNa = Urine Sodium
UCr = Urine Creatinine
PNa = Plasma Sodium
PCr = Plasma Creatinine

What do FENA results mean?

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Fractional Excretion Of Sodium (FENA)

All concentrations in mmol/L except plasma creatinine in umol/L.

Interpretation:

In pre-renal renal failure FENA is generally

FENa can be useful in the evaluation of acute renal failure and oliguria. Low fractional excretion indicate sodium retention by the kidney, suggesting pathophysiology extrinsic to the urinary system such as volume depletion. Higher values can suggest sodium wasting due to acute tubular necrosis or other causes of intrinsic renal failure. The FENa may be affected or invalidated by diuretic use, since many diuretics act by altering the kidney’s handling of sodium.

- Wikipedia