FENa stands for fractional excretion of sodium, which is a calculation based on the concentrations of sodium and creatinine in the urine and plasma. It is the percentage of the sodium filtered by the kidney that is excreted in the urine. FENa is used to evaluate acute kidney failure in the context of low urine output. A low FENa indicates sodium retention by the kidney, suggesting pathophysiology extrinsic to the urinary system such as volume depletion or decrease in effective circulating volume (e.g. low output heart failure), while higher values can suggest sodium wasting due to acute tubular necrosis or other causes of intrinsic kidney failure. FENa can be affected or invalidated by diuretic use, since many diuretics act by altering the kidneys handling of sodium. FENa is a simple test to obtain yet complex to interpret.