Fractional sodium excretion

Pathophysiology sample values
BMP/ELECTROLYTES:
Na⁺=140	Cl⁻=100	BUN=20	/
			Glu=150
K⁺=4	CO₂=22	PCr=1.0	\
ARTERIAL BLOOD GAS:
HCO₃^-=24	p_aCO₂=40	p_aO₂=95	pH=7.40
ALVEOLAR GAS:
	p_ACO₂=36	p_AO₂=105	A-a g=10
OTHER:
Ca=9.5	Mg²⁺=2.0	PO₄=1
CK=55	BE=−0.36	AG=16
SERUM OSMOLARITY/RENAL:
PMO = 300	PCO=295	POG=5	BUN:Cr=20
URINALYSIS:
UNa⁺=80	UCl⁻=100	UAG=5	FENa=0.95
UK⁺=25	USG=1.01	UCr=60	UO=800
PROTEIN/GI/LIVER FUNCTION TESTS:
LDH=100	TP=7.6	AST=25	TBIL=0.7
ALP=71	Alb=4.0	ALT=40	BC=0.5
		AST/ALT=0.6	BU=0.2
AF alb=3.0	SAAG=1.0		SOG=60
CSF:
CSF alb=30	CSF glu=60	CSF/S alb=7.5	CSF/S glu=0.4

The fractional excretion of sodium (FE_Na) is the percentage of the sodium filtered by the kidney which is excreted in the urine. It is measured in terms of plasma and urine sodium, rather than by the interpretation of urinary sodium concentration alone, as urinary sodium concentrations can vary with water reabsorption. Therefore the urinary and plasma concentrations of sodium must be compared to get an accurate picture of renal clearance. In clinical use, the fractional excretion of sodium can be calculated as part of the evaluation of acute renal failure in order to determine if hypovolemia or decreased effective circulating plasma volume is a contributor to the renal failure. "Measurement of the urine sodium concentration provides information on the integrity of tubular reabsorptive function. Low urine sodium concentration thus indicates not only intact reabsorptive function but also the presence of a stimulus to conserve sodium "

1 Calculation
2 Interpretation
3 Alternatives
4 References
5 See also
6 External links

Calculation

FE_Na is calculated in two parts—figuring out how much sodium is excreted in the urine, and then finding its ratio to the total amount of sodium that passed through (aka "filtered by") the kidney.

First, the actual amount of sodium excreted is calculated by multiplying the urine sodium concentration by the urinary flow rate. This is the numerator in the equation. The denominator is the total amount of sodium filtered by the kidneys. This is calculated by multiplying the plasma sodium concentration by the glomerular filtration rate calculated using creatinine filtration. This formula is represented mathematically as:

(Sodium_urinary × Flow rate_urinary) / (Sodium_plasma × Creatinine_urinary × Flow rate_urinary ÷ Creatinine_plasma) × 100

Sodium (mmol/l) Creatinine (mg/dl)

The flow rates cancel out in the above equation, simplifying to the standard equation:^[1]

$FE_{Na} {{=}} 100 \times \frac {\rm sodium_{urinary} \times creatinine_{plasma}}{\rm sodium_{plasma} \times creatinine_{urinary}}$

For ease of recall, one can just remember the fractional excretion of sodium is the clearance of sodium divided by the glomerular filtration rate (i.e. the "fraction" excreted).

Interpretation

FE_Na 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 or decrease effective circulating volume (e.g. low output heart failure). Higher values can suggest sodium wasting due to acute tubular necrosis or other causes of intrinsic renal failure. The FE_Na may be affected or invalidated by diuretic use, since many diuretics act by altering the kidney's handling of sodium.

Value	Category	Description
below 1%	prerenal disease	the physiologic response to a decrease in renal perfusion is an increase in sodium reabsorption to control hypovolemia.
above 2% or 3%^[2]	acute tubular necrosis or other kidney damage	either excess sodium is lost due to tubular damage, or the damaged glomeruli result in hypervolemia resulting in the normal response of sodium wasting.
intermediate	either disorder	In renal tract obstruction, values may be either higher or lower than 1%.^[3] The value is lower in early disease, but with renal damage from the obstruction, the value becomes higher.

While the above values are useful for older children and adults, the FE_Na must be interpreted more cautiously in younger pediatric patients due to the limited ability of immature tubules to reabsorb sodium maximally. Thus, in term neonates, a FE_Na of <3% represents volume depletion, and a FE_Na as high as 4% may represent maximal sodium conservation in critically ill preterm neonates. ^[4]^[5] The FE_Na may also be spuriously elevated in children with adrenal insufficiency or pre-existing renal disease (such as obstructive uropathy) due to salt wasting.^[6]

Alternatives

Fractional excretion of other substances can be measured to determine renal clearance including urea, uric acid, and lithium. These can be used in patients undergoing diuretic therapy, since diuretics induce a natriuresis. Thus, the urinary sodium concentration and FE_Na may be higher in patients receiving diuretics in spite of prerenal pathology.^[7]

References

^ "Fractional Excretion of Sodium". http://intsmain.is.mcw.edu/clincalc/fena.html. Retrieved 2009-05-02.
^ "MedlinePlus Medical Encyclopedia: Fractional excretion of sodium". http://www.nlm.nih.gov/medlineplus/ency/article/003602.htm. Retrieved 2009-05-02.
^ Steiner R (1984). "Interpreting the fractional excretion of sodium". Am J Med 77 (4): 699–702. doi:10.1016/0002-9343(84)90368-1. PMID 6486145.
^ Carmody, JB (Feb 2011). "Urine electrolytes". Pediatr Rev 32 (2): 68-68. PMID 21285302.
^ Tapia-Rombo CA, Velasquez-Jones L, Fernandez-Celis JM, et al. "Usefulness of fractional excretion of sodium in critically ill pre-term newborns." Arch Med Res 1997 Summer; 28(2): 253-257. PMID 9204618.
^ Andreoli SP. "Acute kidney injury in children." Pediatr Nephrol 2009; 24: 253-263. PMID 19083019.
^ Steinhäuslin F, Burnier M, Magnin J, Munafo A, Buclin T, Diezi J, Biollaz J (1994). "Fractional excretion of trace lithium and uric acid in acute renal failure". J Am Soc Nephrol 4 (7): 1429–37. PMID 8161725.