initial test of hypophosphatemia

• serum calcium, vitamin D, and parathyroid hormone levels;
• arterial blood gas;
• urine phosphorus and creatinine levels;
• fractional excretion of phosphorus; fractional excretion of phosphorus < 5% suggests a nonrenal mechanism (intracellular uptake or gastrointestinal);
• x-ray, if osteomalacia is suspected.