How should I replete phosphorus?

Phosphorus Repletion Strategy

For hypophosphatemia, use oral phosphate supplementation at 20-60 mg/kg/day of elemental phosphorus divided into 4-6 doses for moderate cases, or intravenous phosphate for severe cases (<1.0 mg/dL), with the specific dose and route determined by the severity of hypophosphatemia and clinical context. 1

Severity-Based Treatment Algorithm

Severe Hypophosphatemia (<1.5 mg/dL)

Oral Route (Preferred if Patient Can Tolerate):

• Start with 0.44-0.64 mmol/kg of phosphorus (corresponding to potassium 0.64-0.94 mEq/kg), up to a maximum single dose of 45 mmol 1
• Divide into 6-8 doses daily to minimize gastrointestinal side effects 1
• Use potassium-based phosphate salts preferentially over sodium-based preparations to reduce hypercalciuria risk 1
• Target serum phosphorus level of 2.5-4.5 mg/dL 2, 1

Intravenous Route (For Life-Threatening Cases or Inability to Take Oral):

• Administer 0.16 mmol/kg for serum phosphorus 1.8 mg/dL to lower end of normal range 3
• Administer 0.32-0.43 mmol/kg for serum phosphorus 1.0-1.7 mg/dL 3
• Administer 0.44-0.64 mmol/kg for serum phosphorus <1.0 mg/dL, up to maximum 45 mmol (66 mEq potassium) 3
• Infusion rate: maximum 6.8 mmol/hour via peripheral line or 15 mmol/hour via central line for adults 3
• Check serum potassium and normalize calcium before administering potassium phosphate injection 3

Moderate Hypophosphatemia (1.5-2.5 mg/dL)

• Use oral phosphate supplementation at 20-60 mg/kg/day divided into 4-6 doses 4
• Maximum dose should not exceed 80 mg/kg/day to prevent gastrointestinal discomfort 4
• For kidney transplant patients in this range, supplementation is often required to achieve target of 2.5-4.5 mg/dL 2

Mild Hypophosphatemia (>2.5 mg/dL but below normal)

• In non-CKD patients, supplementation is typically initiated when phosphorus falls below 1.0 mg/dL 2
• Consider observation and dietary optimization before pharmacologic intervention 2

Practical Dosing Regimens

Standard Oral Dosing:

• Adults: One tablet 4-8 times daily with full glass of water, with food, and at bedtime 5
• Pediatric patients >4 years: One tablet 4 times daily 5
• Start with 750-1,600 mg elemental phosphorus daily divided into 2-4 doses 1

Critical Monitoring Protocol

Initial Phase (First Week):

• Monitor serum phosphorus and calcium at least weekly during supplementation 2, 1
• Check serum potassium and magnesium regularly, especially with IV potassium phosphate 1
• If phosphorus exceeds 4.5 mg/dL, decrease supplement dosage 2

Ongoing Monitoring:

• For persistent hypophosphatemia requiring supplementation >3 months post-transplant, check PTH levels to evaluate for hyperparathyroidism 2
• Reassess phosphorus levels within 1 month after initiating therapy 1

Special Clinical Contexts

Kidney Transplant Patients:

• Supplement when phosphorus is <1.5 mg/dL (mandatory) or 1.6-2.5 mg/dL (often required) 2
• Target range: 2.5-4.5 mg/dL 2
• May require co-administration of calcium supplements and vitamin D analogs 2

X-Linked Hypophosphatemia:

• Must combine phosphate supplements with active vitamin D (calcitriol 0.5-0.75 μg daily or alfacalcidol 0.75-1.5 μg daily) 1
• Give vitamin D in the evening to reduce calcium absorption after meals and minimize hypercalciuria 1
• Avoid potassium citrate as alkalinization increases phosphate precipitation risk 1

Diabetic Ketoacidosis:

• Add 20-30 mEq potassium per liter IV fluid (2/3 as KCl, 1/3 as potassium phosphate) once potassium falls below 5.5 mEq/L 4

Moderate Renal Impairment (eGFR 30-60):

• Start at the low end of the dose range 3

Critical Contraindications and Precautions

Before Administration:

• Do not administer potassium phosphate if serum potassium ≥4 mEq/dL; use alternative phosphorus source 3
• Normalize calcium before giving potassium phosphate injection 3
• Never infuse with calcium-containing IV fluids to avoid precipitation 3

During Treatment:

• Do not administer phosphate supplements with calcium supplements or high-calcium foods as this reduces absorption 1
• Watch for hypercalciuria and nephrocalcinosis, especially with high-dose therapy (occurs in 30-70% of X-linked hypophosphatemia patients) 1
• Phosphate supplements may worsen hyperparathyroidism in kidney transplant recipients 2

Infusion Rate Precautions:

• Continuous ECG monitoring required for infusion rates >10 mEq/hour potassium in adults or >0.5 mEq/kg/hour in pediatric patients <20 kg 3
• Use central venous catheter for high infusion rates 3

Common Pitfalls to Avoid

• Failing to check potassium before IV potassium phosphate administration can cause life-threatening hyperkalemia 3
• Co-administering with calcium (food, supplements, or IV fluids) dramatically reduces phosphate absorption and risks precipitation 1, 3
• Using sodium-based phosphate salts increases hypercalciuria risk compared to potassium-based preparations 1
• Inadequate dose frequency (giving once or twice daily instead of 4-6 times) reduces efficacy and increases GI side effects 1
• Overlooking underlying hyperparathyroidism when chronic supplementation is required 2