Severe Hypophosphatemia 1 mgdL in adolescents 2 mgdL in children younger than 12 years

1. Enteral supplements. As listed under moderate hypophosphatemia, earlier; use for asymptomatic hypophosphatemia.

2. Parenteral phosphate. Usually used for symptomatic hypophosphatemia. Avoid with renal failure. Potassium phosphate can be given IV at a dose of 2.5 mg (0.08 mmol)/kg body weight in 1/2 normal saline (NS) over 6 hours or, for symptomatic patients, at 5 mg (0.16 mmol)/kg body weight in 1/2 NS over 6 hours. Monitor calcium, phosphate, and potassium every 6 hours. Monitor BP. Stop parenteral replacement when serum phosphate is > 2 mg/dL.

C. Treatment of Primary Etiology. After emergency treatment, recognition and treatment of primary cause is important. May require vitamin D analogues (see Chapter 46, Hypocalcemia, p. 221).

VI. Problem Case Diagnosis. The 10-year-old patient has Fanconi syndrome, likely due to divalproex sodium administration. In addition to low serum phosphate level, he had low serum potassium level, metabolic acidosis, glucosuria, and a very large renal leak of phosphorus. He required large amounts of IV phosphorus and bicarbonate, but the renal tubular defect improved after stopping divalproex sodium.

VII. Teaching Pearl: Question. Why is the expression of serum phosphate as milliequivalents per liter (mEq/L) uniquely confusing as compared with other ions.

VIII. Teaching Pearl: Answer. Because the average charge of phosphate changes at physiologic pH (charge at pH 7.4 is -1.8), the valency and the value for milliequivalents per liter varies with changes in serum pH. Expression of phosphate in millimoles per liter (mmol/L) and milligrams per deciliter (mg/dL) avoids this problem.

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