Pediatric emergency medicine trisk 2888 2888
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An assessment of urine chemistries may also be useful in distinguishing
prerenal AKI from ATN, and initial studies to consider include urine
electrolytes and urine creatinine. The fractional excretion of sodium (FENa)
is calculated as follows:
FENa (%) = [(urine Na × serum creatinine)/(serum Na × urine creatinine)]
× 100
In general, a value below 1% suggests prerenal disease and reflects
reabsorption of almost all of the filtered sodium to maintain intravascular
volume, an appropriate response to decreased renal perfusion. A value
greater than 2% is consistent with ATN or other tubular disorders. Of note,
the FENa may be less than 1% in normal subjects reflecting normal tubular
handling of sodium in the setting of relatively low sodium intake. It may
also be low in some cases of AIN, postischemic ATN, and acute
glomerulonephritis. The fractional excretion of urea is similarly calculated
as the FENa with substitution of urine urea nitrogen and BUN and is
generally less than 35% in prerenal states. Diuretics increase urine sodium
excretion but have less effect on urea excretion. Therefore, the FENa may
be less reliable and the fractional excretion of urea more informative if
diuretics have been provided prior to the collection of urine.
Radiographic assessment should be considered in all patients with AKI
of unclear etiology. This is most certainly true in those who present with
acute anuria, as urinary tract obstruction is a possible etiology and would
require intervention. Given its safety and general availability,
ultrasonography of the kidneys and urinary tract should be considered in all
children with AKI. Ultrasound can provide assessment of the renal
parenchyma and may identify conditions of acquired or congenital
obstruction of the urinary tract. Doppler investigation of the renal vessels
should be performed if there is concern for vascular compromise or
thrombosis, though further imaging may be necessary, given the limitations
of this modality.
Management. The initial management of AKI in the ED is largely
supportive and should focus on addressing fluid or electrolyte abnormalities
while avoiding further renal insult. If prerenal physiology is suspected,
appropriate volume resuscitation with isotonic fluids should be provided
