on call·labs

Hypophosphatemia

at the bedside

• check vitals · confirm PO₄ < 0.81 mmol/L; symptoms only below ~0.64
• look for weakness, bone pain, confusion, seizures, breathing or heart failure
• refeeding after starvation, alcohol, DKA on insulin℞, sepsis?
• identify + treat cause

escalate now if

• respiratory or cardiac failure → IV phosphate
• confusion or seizures → IV phosphate
• severe muscle weakness → IV phosphate

focused check

• shift into cells → refeeding, insulin, respiratory alkalosis, hungry bone
• less in → poor intake, antacids, diarrhea, vitamin D deficiency
• more out → hyperparathyroidism, Fanconi, tenofovir, IV iron, chemotherapy, acetazolamide, dialysis

send now

• K⁺, Mg⁺⁺, Ca²⁺, glucose with the PO₄; cause unclear → urine phosphate (FEPO₄ < 5% = shift or gut, ≥ 5% = renal loss)

treat

• PO₄ > 0.64 → no replacement; treat cause, trend
• PO₄ 0.32–0.64, no symptoms → oral: Phosphate Sandoz 1–2 tablets TDS (≈ 1 mmol/kg/day, max 80 mmol/day); kidney disease → half
• PO₄ < 0.32, or 0.32–0.48 with symptoms → IV, then oral once > 0.48

IV phosphate

• K⁺ < 4 → potassium phosphate · K⁺ ≥ 4 → sodium glycerophosphate (Glycophos)
• PO₄ < 0.32 → 0.4 mmol/kg IV over 6 h (max 50 mmol)
• PO₄ 0.32–0.42 → 0.3 mmol/kg IV over 4 h (max 30 mmol)
• PO₄ 0.42–0.48 → 0.2 mmol/kg IV over 4 h (max 20 mmol)
• dilute in 100–250 mL; peripheral max 7 mmol/h; fast infusion drops calcium

• stop replacing at PO₄ ≥ 0.64; PO₄ every 6 h on IV, next morning on oral; refeeding → PO₄, K⁺, Mg⁺⁺ daily while feeds go up
• hand over → refeeding risk if present, next check due

refeeding risk

• thiamine 100–300 mg IV/PO before any feed or glucose
• replace K⁺ and Mg⁺⁺ with the phosphate: all three fall together
• feed slowly, dietitian in the morning

available phosphate

product	content	usual adult dose	bedside note
IV potassium phosphate	3 mmol PO₄ + 4.4 mEq K⁺ per mL; 15 mL = 45 mmol PO₄ + 66 mEq K⁺	0.2–0.4 mmol/kg IV over 4–6 h by level (treat)	only if K⁺ < 4.0; dilute in 100–250 mL; peripheral max 6.8 mmol PO₄/h (10 mEq K⁺/h), central max 15 mmol/h; start low in renal impairment
IV sodium glycerophosphate (Glycophos®)	1 mmol PO₄ + 2 mmol Na⁺ per mL; 20 mL = 20 mmol PO₄ + 40 mmol Na⁺	0.2–0.4 mmol/kg IV over 4–6 h by level (treat); maintenance 10–20 mmol/day	use when K⁺ ≥ 4.0; must be diluted; infuse over ≥ 8 h; sodium load 2 mmol per mmol PO₄
oral phosphate (e.g. Phosphate Sandoz® effervescent)	1 tablet ≈ 16.1 mmol PO₄ + 20.4 mmol Na⁺ + 3.1 mmol K⁺ (verify local product)	1–2 tablets TDS (≈ 1 mmol/kg/day, max 80 mmol/day)	diarrhea limits dose; check calcium; local formulation verification needed

hyperphosphatemia

• causes → AKI / CKD, tumor lysis, rhabdomyolysis, phosphate laxatives or enemas; spurious with paraprotein, hemolysis, high bilirubin
• check Ca²⁺: high PO₄ pulls it down
• kidneys working → clears itself; NaCl 0.9% if not overloaded
• AKI / CKD → binder with meals (calcium acetate or sevelamer); severe, or symptomatic low Ca → nephrology, dialysis if renal failure

evidence

source	type
ASPEN refeeding-syndrome consensus recommendations	international guideline

Verify indication, dose, allergies, interactions, renal/hepatic function and local protocols before prescribing.

free the on-call checklist covers the whole shift on one printable page.

reviewed Jun 2026updated Oct 2026file lab/rft-phosphate