How do you supplement phosphorus in cases of hypophosphatemia?

Phosphorus Supplementation in Hypophosphatemia

For hypophosphatemia, phosphorus supplementation should be administered orally with sodium-based or potassium-based phosphate salts at a frequency of 4-6 times daily in severe cases, avoiding concurrent administration with calcium-containing foods or supplements to maximize absorption. 1

Assessment of Hypophosphatemia

• Hypophosphatemia is defined as serum phosphate <2.5 mg/dL (0.8 mmol/L), with severe hypophosphatemia considered as levels <1.5 mg/dL 2, 3
• Determine the cause of hypophosphatemia by measuring fractional phosphate excretion; values >15% in the presence of hypophosphatemia confirm renal phosphate wasting 3
• Categorize hypophosphatemia based on severity:
  • Mild: 2.0-2.5 mg/dL
  • Moderate: 1.0-1.9 mg/dL
  • Severe: <1.0 mg/dL 4

Oral Phosphate Supplementation Protocol

• For severe hypophosphatemia, administer 20-60 mg/kg/day of elemental phosphorus divided into 4-6 doses daily 2
• Young patients with high alkaline phosphatase levels require more frequent dosing (4-6 times daily) to maintain stable blood levels 1
• Adolescents may benefit from less frequent dosing (2-3 times daily) to improve adherence 1
• Available formulations include:
  • Oral solutions (caution with glucose-based sweeteners due to dental concerns)
  • Capsules
  • Tablets 1
• Dosage should always be based on elemental phosphorus content, as phosphorus content differs between available phosphate salts 1

Administration Considerations

• Do not administer phosphate supplements with calcium supplements or high-calcium foods (e.g., milk) as this reduces absorption through precipitation in the intestinal tract 1, 2
• For patients with X-linked hypophosphatemia, combine phosphate supplements with active vitamin D (calcitriol or alfacalcidol) to counter calcitriol deficiency and prevent secondary hyperparathyroidism 1, 2
• Calcitriol can be given in one or two doses per day, while alfacalcidol should be given once daily due to its longer half-life 1
• The equivalent dosage of alfacalcidol is 1.5-2.0 times that of calcitriol due to differences in oral bioavailability 1

Intravenous Phosphate Supplementation

• Intravenous phosphate is indicated when oral/enteral replacement is not possible, insufficient, or contraindicated 5
• For severe, life-threatening hypophosphatemia (<1.0 mg/dL), administer intravenous phosphate at 0.16 mmol/kg at a rate of 1-3 mmol/h until a level of 2 mg/dL is reached 3
• The maximum initial or single dose of IV potassium phosphate for hypophosphatemia correction is 45 mmol of phosphorus (66 mEq of potassium) 5
• Recommended infusion rate through a peripheral venous catheter is 8 mmol/hour of phosphorus (10 mEq/hour of potassium) 5
• Continuous ECG monitoring is recommended for higher infusion rates 5

Monitoring Protocol

• Monitor serum phosphorus, calcium, and PTH levels regularly during supplementation 2
• Check serum potassium concentration prior to administering potassium phosphate; if ≥4 mEq/dL, consider using an alternative phosphorus source 5
• For patients with moderate renal impairment (eGFR ≥30 to <60 mL/min/1.73 m²), start at the low end of the dose range and monitor serum potassium, phosphorus, calcium, and magnesium concentrations 5
• Monitor for hypercalciuria, which may lead to nephrocalcinosis, especially with high-dose phosphate supplementation 1, 2

Precautions and Potential Complications

• Avoid phosphate supplementation in patients with hyperphosphatemia and/or hypocalcemia 5
• Be aware that IV phosphate administration may decrease serum magnesium levels, particularly in patients with hypercalcemia and diabetic ketoacidosis 5
• For iron deficiency-induced hypophosphatemia (particularly with ferric carboxymaltose), phosphate repletion should be avoided as it raises parathyroid hormone and worsens phosphaturia 1
• In iron deficiency-induced hypophosphatemia, treatment should focus on mitigating secondary hyperparathyroidism with vitamin D supplementation and cessation of the offending iron formulation 1
• For patients with X-linked hypophosphatemia, be aware that conventional treatment with phosphate and active vitamin D may increase calciuria and promote nephrocalcinosis 1

Weight-Based Dosing Algorithm for IV Phosphorus

• For serum phosphorus 0.73-0.96 mmol/L: 0.32 mmol/kg (low dose)
• For serum phosphorus 0.51-0.72 mmol/L: 0.64 mmol/kg (moderate dose)
• For serum phosphorus ≤0.5 mmol/L: 1 mmol/kg (high dose) 6
• Administer IV phosphorus bolus at a rate of 7.5 mmol/hour 6