What is the dose for potassium phosphate (K Phos)?

Potassium Phosphate (K Phos) Dosing Guidelines

For hypophosphatemia treatment, potassium phosphate should be dosed at 0.16-0.64 mmol/kg of phosphate IV (depending on severity), with a maximum initial dose of 45 mmol phosphate (66 mEq potassium) for severe cases, or 750-1,600 mg daily in 2-4 divided doses orally for mild to moderate cases. 1

IV Dosing for Hypophosphatemia

Dosing is based on severity of hypophosphatemia:

• Severe hypophosphatemia (<1.0 mg/dL or <0.32 mmol/L):

  • 0.44-0.64 mmol/kg IV phosphate 1
  • Maximum initial dose: 45 mmol phosphate (66 mEq potassium) 1, 2
  • Infusion rate: 1-3 mmol/hour until level reaches 2 mg/dL 1
  • Alternative regimen: 9 mmol phosphate as KH₂PO₄ every 12 hours for patients with normal renal function 3

• Moderate hypophosphatemia (1.0-2.0 mg/dL or 0.32-0.65 mmol/L):

  • 0.32-0.64 mmol/kg IV phosphate 1, 4
  • Infusion rate: Maximum 7.5-10 mmol/hour 4, 5

• Mild hypophosphatemia (2.0-2.5 mg/dL or 0.65-0.81 mmol/L):

  • 0.16-0.32 mmol/kg IV phosphate 1, 4

Oral Dosing for Hypophosphatemia

• Mild to moderate hypophosphatemia:
  • 750-1,600 mg elemental phosphorus daily in 2-4 divided doses 1
  • For kidney transplant patients: Target serum phosphorus level of 2.5-4.5 mg/dL (0.81-1.45 mmol/L) 6
  • For children with X-linked hypophosphatemia: 20-60 mg/kg body weight daily (0.7-2.0 mmol/kg daily) divided into 4-6 doses 1

Administration Guidelines

• IV administration:

  • Must be diluted in IV fluids; never administer undiluted 2
  • Maximum potassium infusion rate: 10 mEq/hour through peripheral vein 2
  • Continuous ECG monitoring recommended for higher infusion rates 2
  • Check for precipitates in solution and infusion set 2

• Oral administration:

  • Divide into 4-6 doses daily for better absorption and to minimize GI side effects 1
  • Avoid doses >80 mg/kg daily of elemental phosphorus to prevent GI discomfort 1

Monitoring

• Check serum potassium before administration; if ≥4 mEq/L, consider sodium phosphate instead 2
• Monitor serum phosphate within 24 hours of initiating therapy 1
• Continue monitoring every 1-2 days until stable, then weekly until normalized 1
• Monitor serum calcium, potassium, and magnesium concurrently 1, 2
• For kidney transplant patients requiring long-term supplementation, check PTH levels if supplements needed >3 months 6

Precautions and Contraindications

• Contraindicated in:

  • Severe renal impairment or end-stage renal disease (risk of hyperkalemia) 2
  • Hyperphosphatemia or hypercalcemia 2
  • Severe hyperkalemia 2

• Use with caution in:

  • Moderate renal impairment (eGFR 30-60 mL/min/1.73m²): Start at low end of dose range 2
  • Cardiac disease (increased susceptibility to hyperkalemia effects) 2
  • Adrenal insufficiency 2
  • Patients on medications that increase hyperkalemia risk 2

Potential Adverse Effects

• Hyperkalemia: Life-threatening cardiac events, especially with rapid infusion 2
• Hypocalcemia: Due to calcium-phosphate precipitation 2
• Hypomagnesemia: Monitor serum magnesium during treatment 2
• Vein irritation/thrombosis: From hypertonic solutions in peripheral veins 2
• Pulmonary embolism: Due to calcium phosphate precipitates 2
• GI symptoms: With oral administration (constipation, diarrhea, nausea) 6

Clinical Pearls

• For patients with normal renal function, phosphate replacement with dose calculation based on serum phosphate levels and a distribution volume of 0.5 L/kg is effective and safe 7
• High-dose phosphate treatment can lead to hypokalemia in some conditions like hypophosphatemic osteomalacia 8
• Phosphate supplements may decrease serum calcium and increase PTH levels; consider concurrent calcium supplementation if needed 1
• Severe hypophosphatemia (<1.0 mg/dL) is associated with 30% mortality and requires prompt treatment 1