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What do FENA results mean?

Fractional Excretion Of Sodium (FENA)

FENA(%) = {U Na / Pl Na x 100} / {U Cr * 1000 / Pl Cr}

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

Interpretation:
In pre-renal renal failure FENA is generally <1%.

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. Wikepdia