AWHONN's High-Risk and Critical Care Obstetrics by Unknown

Author:Unknown
Language: eng
Format: epub
Publisher: Wolters Kluwer

Box 16-1 Formula for Calculation of Fractional Excretion of Sodium

FeNa+ = UNa × PCr/PNa × UCr × 100

PCr = plasma creatinine, PNa = plasma sodium,

UCr = urine creatinine, and UNa = urine sodium

In adults with normal kidney function, FeNa+ is less than 1%; that is, the percentage of Na+ not reabsorbed by the kidneys and allowed to pass into the urine is a small percentage of the total Na+ that was filtered from the blood. If the kidneys are functioning, the FeNa+ should remain very low. Conversely, if ATN develops, renal cells become damaged and can no longer reabsorb the Na+ filtered from the blood. This produces an increase in the FeNa+, frequently to greater than 3%. It is important to note that the calculated values for FeNa+ are not diagnostic if diuretics or volume replacement have been administered. When diuretics are used as treatment of AKI, alternative renal indices must be used to differentiate between prerenal failure and ATN. One recommendation is the measurement of the fractional excretion of urea, which is calculated by the formula in Box 16-2. The recommendation is based on the principle that urea is not affected by diuretics. However, other laboratory indices should be evaluated to differentiate between prerenal and intrinsic renal AKI.

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