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Rapid phosphorus replacement can cause severe hypocalcemia, tetany, and cardiac arrhythmias. Monitor calcium levels during replacement. Use caution in renal impairment. Infuse slowly.
Calculate phosphorus replacement dosing for hypophosphatemia
Normal: 3.0-4.5 mg/dL (or 1.0-1.5 mmol/L)
| Serum Phos (mg/dL) | Severity | IV Dose | PO Dose | Monitoring |
|---|---|---|---|---|
| <1.0 | Severe | 0.5 mmol/kg over 6-12h | 2000 mg/day | Check phos q6-12h, calcium, mag, K+ |
| 1.0-1.4 | Moderate-Severe | 0.4 mmol/kg over 6-12h | 1500 mg/day | Check phos q12-24h, calcium |
| 1.5-1.9 | Moderate | 0.25 mmol/kg over 4-6h | 1000 mg/day | Check phos in 24h |
| 2.0-2.4 | Mild | 0.16 mmol/kg | 1000 mg/day (preferred) | Check phos in 24-48h |
| 2.5-2.9 | Low-Normal | Not needed | 500 mg/day if indicated | Routine |
| 3.0-4.5 | Normal | None | None | Routine |
| Formulation | Phosphate Content | Cation | Notes |
|---|---|---|---|
| Sodium Phosphate (IV) | 3 mmol phos/mL | 4 mEq Na+ per mmol phos | Use when K+ elevated |
| Potassium Phosphate (IV) | 3 mmol phos/mL | 4.4 mEq K+ per mmol phos | Use when K+ also low |
| Neutra-Phos (PO) | 250 mg elemental P per packet | Na+ and K+ mix | Dissolve in water |
| K-Phos Neutral (PO) | 250 mg elemental P per tablet | Na+ and K+ mix | Easy dosing |
| Fleet Phospho-soda | Variable | Na+ | Also used as laxative |
Common causes include refeeding syndrome (especially in malnourished/alcoholic patients), DKA treatment (insulin shifts phos intracellularly), respiratory alkalosis, chronic diarrhea, malabsorption, TPN without adequate phosphate, and chronic antacid/PPI use. ICU patients are at high risk.
IV phosphate binds with calcium in the blood to form calcium phosphate, which precipitates and lowers serum calcium. This is why we infuse phosphate slowly and monitor calcium levels. Rapid infusion can cause severe symptomatic hypocalcemia with tetany.
Use potassium phosphate (K-Phos) when both phos and potassium are low (most common in ICU). Use sodium phosphate when potassium is normal/high or in renal failure. Note that K-Phos provides ~4.4 mEq potassium per mmol phosphate.
Severe hypophosphatemia (<1.0 mg/dL) causes muscle weakness, respiratory failure (diaphragm weakness), rhabdomyolysis, hemolytic anemia, decreased cardiac contractility, confusion/seizures, and impaired WBC function. It's life-threatening and requires urgent correction.
Use IV for: Phos <1.5 mg/dL, symptomatic patients, NPO status, severe GI losses, or TPN patients. Use PO for: Phos 1.5-2.5 mg/dL in stable patients tolerating PO. Oral causes diarrhea and is less predictable. IV correction is preferred in ICU.