DOI: 10.1373/clinchem.2011.178111


A 5-day-old girl, born at term after an uncomplicated pregnancy, was admitted to the hospital after a routine midwife check showed that she had lost 15% of her original birth weight [6.2 lb (2.83 kg)]. She was being fed normal-term formula milk, and an initial assessment revealed only mild dehydration. The working diagnosis was a feeding problem, and her management plan was to be fed 150 mL/kg formula milk per day with regular monitoring of weight. The patient's serum concentrations of selected analytes were as follows: sodium, 135 mEq/L (135 mmol/L; reference interval, 135–145 mmol/L); potassium, 5.3 mEq/L (5.3 mmol/L; reference interval, 3.5–5.3 mmol/L); and urea, 11.7 mg/dL (4.2 mmol/L; reference interval, 3.5–6.5 mmol/L).