What is the protocol for intravenous (IV) phosphate repletion in patients with severe hypophosphatemia?

IV Phosphate Repletion Protocol

For severe hypophosphatemia (serum phosphate <1.0 mg/dL), administer intravenous potassium phosphate at 0.16 mmol/kg infused at a rate of 1-3 mmol/hour until serum phosphate reaches 2.0 mg/dL. 1

Pre-Administration Requirements

Before initiating IV phosphate repletion, you must:

• Check serum potassium and calcium concentrations - normalize calcium before administering potassium phosphates injection 2
• Verify serum potassium is <4 mEq/dL - if potassium is ≥4 mEq/dL, use an alternative phosphorus source (sodium phosphate) 2
• Do not infuse with calcium-containing IV fluids - this will cause precipitation 2

Preparation and Dilution

Standard Potassium Phosphate Solution Composition

Each 1 mL contains 4.4 mEq potassium and 3 mmol (93 mg) phosphate 3

Dilution Requirements by Age and Access

Adults and pediatric patients ≥12 years: 2

• Peripheral line: Maximum concentration 6.8 mmol phosphorus/100 mL (10 mEq potassium/100 mL)
• Central line: Maximum concentration 18 mmol phosphorus/100 mL (26.4 mEq potassium/100 mL)
• Recommended total volume: 100-250 mL in normal saline or D5W

Pediatric patients <12 years: 2

• Peripheral line: Maximum concentration 0.27 mmol phosphorus/10 mL (0.4 mEq potassium/10 mL)
• Central line: Maximum concentration 0.55 mmol phosphorus/10 mL (0.8 mEq potassium/10 mL)
• Use smallest volume considering daily fluid requirements

Dosing by Severity

Severe Hypophosphatemia (<1.5 mg/dL)

• Dose: 0.16 mmol/kg body weight 1
• Rate: 1-3 mmol/hour until serum phosphate reaches 2.0 mg/dL 1
• Alternative simplified approach: 1 mL/hour of standard K2PO4 solution is safe and appropriate for most patients 3

Moderate Hypophosphatemia (1.5-2.2 mg/dL)

Weight-based protocol shows superior efficacy: 4

• Single IV dose based on weight and serum phosphorus level
• Success rate of 78% in achieving normal levels with protocol-based approach versus 53% without protocol 4
• Most patients require <25 mmol total replacement therapy 5

Critical Monitoring

• Serum phosphate: Check levels during and after repletion to guide therapy 1
• Serum potassium: Monitor closely during infusion to prevent hyperkalemia 2
• Serum calcium: Verify normal before starting and monitor during therapy 2

Common Pitfalls and Caveats

Do not use phosphate repletion for ferric carboxymaltose-induced hypophosphatemia - this paradoxically worsens the condition by raising parathyroid hormone and increasing phosphaturia 6, 7. For FCM-induced hypophosphatemia, treatment should focus on vitamin D supplementation to mitigate secondary hyperparathyroidism 6

Avoid over-replacement - severe hypophosphatemia typically resolves with small doses and has a short course without sequelae when appropriately managed 5. Patients generally respond quickly, with 50% requiring <25 mmol total replacement 5

Risk of hyperkalemia - each mmol of phosphate delivered as potassium phosphate contains 1.47 mEq of potassium, making hyperkalemia a significant risk in patients with impaired renal function or baseline elevated potassium 2

Precipitation risk - never mix phosphate with calcium-containing solutions as this causes immediate precipitation and potential embolism 2

Storage and Handling

• Single-dose vials (5 mL, 15 mL): Discard after single use 2
• Pharmacy bulk package (50 mL): Use within 4 hours of penetration at room temperature (20-25°C) 2
• Visually inspect for particulate matter and discoloration before and after dilution 2