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| <div style="float:right;border:solid 10px white;">{{Test sample values}}</div>
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| The '''fractional excretion of sodium''' (FE<sub>Na</sub>) 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.
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| ==Calculation==
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| FE<sub>Na</sub> 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.
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| 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:
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| '''([[Sodium]]<sub>urinary</sub> × Flow rate<sub>urinary</sub>) ÷ ([[Sodium]]<sub>plasma</sub>) '''/''' ([[Creatinine]]<sub>urinary</sub> × Flow rate<sub>urinary</sub>) ÷ ([[Creatinine]]<sub>plasma</sub>) × 100 | |
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| Sodium (mmol/l)
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| Creatinine (mg/dl)
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| The flow rates cancel out in the above equation, simplifying to the standard equation:<ref name="urlFractional Excretion of Sodium">{{cite web |url=http://intsmain.is.mcw.edu/clincalc/fena.html |title=Fractional Excretion of Sodium |work= |accessdate=2009-05-02}}</ref>
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| <math> FE_{Na} {{=}} 100 \times \frac {\rm sodium_{urinary} \times creatinine_{plasma}}{\rm sodium_{plasma} \times creatinine_{urinary}} </math>
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| 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).
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| ==Interpretation==
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| FE<sub>Na</sub> 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 in 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<sub>Na</sub> may be affected or invalidated by [[diuretic]] use, since many diuretics act by altering the kidney's handling of sodium.
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| {| class="wikitable"
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| ! Value
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| ! Category
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| ! Description
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| | below 1%
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| | [[prerenal disease]]
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| | the physiologic response to a decrease in renal perfusion is an increase in sodium reabsorption to control hyponatremia, often caused by volume depletion or decrease in effective circulating volume (e.g. low output heart failure).
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| | above 2%{{Citation needed|date=May 2009}} or 3%<ref name="urlMedlinePlus Medical Encyclopedia: Fractional excretion of sodium">{{cite web |url=http://www.nlm.nih.gov/medlineplus/ency/article/003602.htm |title=MedlinePlus Medical Encyclopedia: Fractional excretion of sodium |work= |accessdate=2009-05-02}}</ref>
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| | [[acute tubular necrosis]] or other kidney damage
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| | either excess sodium is lost due to tubular damage, or the damaged glomeruli result in hypervolemia resulting in the normal response of sodium wasting.
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| |-
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| | intermediate
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| | either disorder
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| | In renal tract obstruction, values may be either higher or lower than 1%.<ref>{{cite journal |author=Steiner R |title=Interpreting the fractional excretion of sodium |journal=Am J Med |volume=77 |issue=4 |pages=699–702 |year=1984 |pmid=6486145 |doi=10.1016/0002-9343(84)90368-1}}</ref> The value is lower in early disease, but with renal damage from the obstruction, the value becomes higher.
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| |}
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| ==Exceptions in children and neonates==
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| While the above values are useful for older children and adults, the FE<sub>Na</sub> 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<sub>Na</sub> of <3% represents volume depletion, and a FE<sub>Na</sub> as high as 4% may represent maximal sodium conservation in critically ill preterm neonates.<ref>{{cite journal|last=Carmody|first=JB|title=Urine electrolytes|journal=Pediatr Rev|date=Feb 2011|volume=32|issue=2|pages=68-68|pmid=21285302|doi=10.1542/pir.32-2-65}}</ref><ref>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.</ref> The FE<sub>Na</sub> may also be spuriously elevated in children with adrenal insufficiency or pre-existing renal disease (such as obstructive uropathy) due to salt wasting.<ref>Andreoli SP. "Acute kidney injury in children." Pediatr Nephrol 2009; 24: 253-263. PMID 19083019.</ref>
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| ==Exceptions in adults==
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| The FE<sub>Na</sub> is generally less than 1% in patients with hepatorenal syndrome and acute glomerulonephropathy. Although often reliable at discriminating between prerenal [[azotemia]] and acute tubular necrosis, the FE<sub>Na</sub> has been reported to be <1% occasionally with oliguric and nonoliguric acute tubular necrosis, urinary tract obstruction, acute glomerulonephritis, renal allograft rejection, sepsis, and drug-related alterations in renal hemodynamics.<ref>Zarich S, Fang LS, Diamond JR. Fractional excretion of sodium. Exceptions to its diagnostic value" ''Archives of Internal Medicine'' 1985;145:108-112 PMID 3970621.</ref> Therefore, the utility of the test is best when used in conjunction with other clinical data.
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| ==Alternatives==
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| 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<sub>Na</sub> may be higher in patients receiving diuretics in spite of prerenal pathology.<ref>{{cite journal |author=Steinhäuslin F, Burnier M, Magnin J, Munafo A, Buclin T, Diezi J, Biollaz J |title=Fractional excretion of trace lithium and uric acid in acute renal failure |journal=J Am Soc Nephrol |volume=4 |issue=7 |pages=1429–37 |year=1994 |pmid=8161725}}</ref>
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| ==See also==
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| *[[Urinary index]]
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| ==References==
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| {{Reflist}}
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| ==External links==
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| *[http://www.mdcalc.com/fractional-excretion-of-sodium-fena/ Fractional Excretion of Sodium Calculator] by [http://www.mdcalc.com MDCalc]
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| *[http://www.miniwebtool.com/fena-calculator/ FE<sub>Na</sub> Calculator]
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| *[http://www-users.med.cornell.edu/~spon/picu/calc/fenacalc.htm Online FENa calculator] by [[Cornell University]]
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| *[http://www.qxmd.com/calculate-online/nephrology/fractional-excretion-of-sodium FENa calculator available online & for iPhone, BlackBerry and Android] by QxMD Medical.
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| {{Renal physiology}}
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| {{DEFAULTSORT:Fractional Sodium Excretion}}
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| [[Category:Urine]]
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| [[Category:Kidney]]
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