| 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 | PO4=1 | Mg2+=2.0 | |
| 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 (FENa) is a measure of the percentage of sodium excreted in the urine versus the sodium reabsorbed by the kidney. 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 resorption. 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.
Contents |
Calculation
FENa is calculated in two parts—figuring out how much sodium is excreted, and then how much was filtered by the kidney in the first place. 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 dividing the plasma sodium concentration by the glomerular filtration rate. This translates into the formula:
(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]
Interpretation
FENa can be useful in the evaluation of acute renal failure. Low percentages indicate sodium retention by the kidney, suggesting a prerenal cause. Higher values can suggest sodium wasting due to acute tubular necrosis or other intrinsic renal causes of kidney failure. The FENa 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%[citation needed] 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. |
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, where the urinary sodium concentrations may be higher despite possible prerenal pathology.[4]
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:. PMID 6486145.
- ^ 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.
External links
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