Fractional_sodium_excretion v.1
Calculates the fractional excretion of sodium (FENa) which is the percentage of the sodium filtered by the kidney which is excreted in the urine. FENa helps distinguish between prerenal, intrinsic renal and postrenal causes of renal failure, and is widely used to differentiate prerenal disease (decreased renal perfusion) from acute tubular necrosis (ATN) as the cause of acute kidney injury (AKI). The calculation requires serum (sNa) and urine (uNa) sodium concentrations (mmol/L), and serum (sCr) and urine (uCr) creatinine concentrations (umol/L or mg/dl). FENa < 1% and uNa < 20mmol/L suggests kidney injury of prerenal etiology; FENa > 1% and uNa > 40mmol/L suggests an intrinsic renal etiology; while FENa > 4% and uNa > 40mmol/L suggests a postrenal etiology.
To determine the proportion of sodium excreted in the urine compared with the total amount of sodium filtered by the kidneys, and help differentiate between pre-renal, intrinsic renal and post-renal causes of renal failure and acute kidney injury.
To calculate FENa (%) from given values of sodium (mmol/L) and creatinine (umol/L or mg/dl) detected in serum and urine samples. To determine if renal failure/kidney injury is due to prerenal (FENa < 1%), intrinsic renal (FENa > 1%) or postrenal (FENa > 4%) etiology. FENa < 1% may also represent normal renal function with moderate salt intake, or may be due to specific intrinsic renal, postrenal causes, or even non-renal causes. Etiology of AKI should therefore be based upon the totality of the presentation (history, clinical examination, urine microscopy, and, when appropriate, response to volume resuscitation), not just on the FENa.
FENa is only useful in acute renal failure, not chronic renal failure. FENa is less useful in patients on diuretics, and fractional excretion of urea (FEUrea) should be used instead.
Steiner RW. Interpreting the fractional excretion of sodium. The American journal of medicine. 1984 Oct 31;77(4):699-702.
OBSERVATION.lab_test-fractional_sodium_excretion.v1, EVALUATION.fractional_sodium_excretion.v1
