(1409) Discordance of Fractional Excretion of Sodium and Uric Acid in a Child With Severe Hyponatremia

Introduction: Hyponatremia is one of the most common electrolyte abnormalities in hospitalized children and is associated with morbidity and mortality. We describe an infant with renal disease presenting with severe hyponatremia ( < 120 mEq/L) and extra-renal sodium losses with discordant fractional excretion of Na and uric acid (FeNa and FeUA).

Description: An 8-month-old, exclusively breastfed female, with history of grade V vesicoureteral reflux and bladder outlet obstruction (underwent surgical repair at 4 months) presented to the Emergency Department (ED) with 1 week of intermittent vomiting and diarrhea due to C. difficile infection and E. cloacae UTI, which was treated with ciprofloxacin. When she presented to the ED, she was awake, alert, and reactive to stimuli, with signs of mild dehydration (HR 138 bpm, RR 41 rpm, BP 106/61 mmHg), admission weight was 6.94 kg (discharge weight 7.49 kg). Initial serum electrolytes were Na 108 mmol/L (baseline 136 mmol/L), Cl 80 mmol/L, CO2 11.6 mmol/L, anion gap 15.4 mmol/L, calculated serum osmolality 236 mmol/L, BUN 39 mg/dL, creatinine 0.42 mg/dL (baseline 0.3 mg/dL), and glucose 137 mg/dL. FeNa was 0.3% and FeUA was 11.8%. She was managed with hypertonic saline bolus followed by isotonic fluid infusion. Her sodium was corrected over 48 hours at 11-12 mmol/L/day. The patient remained clinically asymptomatic throughout her hospital course. Sodium at the time of discharge was 137 mmol/L without sodium supplements.

Discussion: This patient with renal disease exhibited mild to moderate dehydration (7%) and hypotonic hyponatremia, likely due to extra-renal sodium losses from diarrhea and low sodium intake from breastfeeding. Extra-renal losses of fluid and electrolytes lead to pre-renal azotemia with low FeNa, as seen in our case. However, FeUA of 11.8% suggests either associated renal disease or preserved intravascular volume. As the baseline and follow-up Na levels were normal without Na supplements, normal FeUA in the setting of hyponatremia was unlikely due to renal disease. In addition, clinical signs of mild dehydration in a setting of severe hyponatremia suggest relatively preserved intravascular volume. Discordance with FeNa and FeUA can occur in cases of hyponatremia with preserved intravascular volume.