Fractional excretion of sodium
BMP/ELECTROLYTES: | |||
Na+ = 140
|
Cl− = 100 | BUN = 20 | / Glu = 150 \ |
K+ = 4 | CO2 = 22 | PCr = 1.0 | |
ARTERIAL BLOOD GAS :
| |||
HCO3− = 24 | paCO2 = 40 | paO2 = 95 | pH = 7.40 |
ALVEOLAR GAS: | |||
pACO2 = 36 | pAO2 = 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 = 0.6 |
The fractional excretion of sodium (FENa) 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 kidney clearance. In clinical use, the fractional excretion of sodium can be calculated as part of the evaluation of
Calculation
FENa 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.[citation needed]
First, the actual amount of sodium excreted is calculated by multiplying the urine sodium concentration by the
[(Sodiumurinary × Flow rateurinary) ÷ ((Sodiumplasma) × ((Creatinineurinary × Flow rateurinary) ÷ (Creatinineplasma)))] × 100
Sodium (mmol/L) Creatinine (mg/dL)
The flow rates cancel out in the above equation, simplifying to the standard equation:[1]
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
FENa can be useful in the evaluation of
Value | Category | Description |
---|---|---|
below 1% | prerenal disease
|
the physiologic response to a decrease in kidney 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). |
above 2%[citation needed] or 3%[2] | acute tubular necrosis or other kidney damage (postrenal disease) | either excess sodium is lost due to tubular damage, or the damaged glomeruli result in hypovolemia 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 kidney damage from the obstruction, the value becomes higher. |
Exceptions in children and neonates
While the above values are useful for older children and adults, the FENa 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 FENa of <3% represents volume depletion, and a FENa as high as 4% may represent maximal sodium conservation in critically ill preterm neonates.[4][5] The FENa may also be spuriously elevated in children with adrenal insufficiency or pre-existing kidney disease (such as obstructive uropathy) due to salt wasting.[6]
Exceptions in adults
The FENa 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 FENa 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.[7] Therefore, the utility of the test is best when used in conjunction with other clinical data.
Alternatives
Fractional excretion of other substances can be measured to determine kidney 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 FENa may be higher in patients receiving diuretics in spite of prerenal pathology.[8]
See also
- Urinary index
References
- ^ "Fractional Excretion of Sodium". Archived from the original on 2009-05-17. Retrieved 2009-05-02.
- ^ "MedlinePlus Medical Encyclopedia: Fractional excretion of sodium". Retrieved 2009-05-02.
- PMID 6486145.
- PMID 21285302.
- PMID 9204618.
- PMID 19083019.
- PMID 3970621.
- PMID 8161725.