Fluids and electrolytes

Calcium, magnesium, phosphate and refeeding syndrome

Reviewed Nov 2026·1 references
New· views rounded down, updated weekly

Use ionised calcium in the ICU. Treat symptomatic hypocalcaemia with IV calcium; replace magnesium early. Identify refeeding risk before feeding: start low, give thiamine, and check phosphate, potassium and magnesium every 12 h for 3 days.

At the bedside

Timed steps
  1. 1

    Measure ionised

    Calcium

    Normal iCa 1.1-1.3 mmol/L. Total calcium correction for albumin is unreliable in ICU.

  2. 2

    Symptomatic

    Hypocalcaemia

    Tetany, seizures, prolonged QT, hypotension: 10-20 mL 10% calcium gluconate over 10 min, then infusion. Correct Mg first if low. Common in massive transfusion, pancreatitis, rhabdomyolysis, CRRT citrate.

  3. 3

    Severe (> 14 mg/dL or symptomatic)

    Hypercalcaemia

    0.9% saline to euvolaemia, then zoledronic acid 4 mg IV (adjust in renal impairment) or calcitonin 4 IU/kg SC for rapid effect; dialysis if renal failure. Look for malignancy, hyperparathyroidism, vitamin D excess.

  4. 4

    Replace

    Magnesium

    Mg < 0.7 mmol/L (1.7 mg/dL) or arrhythmia: 2 g MgSO4 IV over 15-60 min; repeat to keep ≥ 0.8-1.0 in cardiac patients.

  5. 5

    Identify risk

    Refeeding

    ASPEN: BMI < 16 (severe), weight loss > 7.5% in 3 months or > 10% in 6, negligible intake > 7 days, low baseline K/P/Mg, alcohol use, malignancy, prolonged NPO.

  6. 6

    Feed carefully

    Refeeding

    High risk: start 100-150 g dextrose or 10-20 kcal/kg in first 24 h, advance by 33% of goal every 1-2 days. Thiamine 100 mg before feeding and daily for 5-7 days.

  7. 7

    Monitor

    Refeeding

    K+, PO4, Mg every 12 h for first 3 days in high-risk patients; if levels fall, replace and do not advance calories.

Hypomagnesaemia

Magnesium sulphate

IV
Dose
2 g (8 mmol) over 15-60 min

Severe hypophosphataemia (< 0.3 mmol/L / < 1 mg/dL)

Potassium / sodium phosphate

IV
Dose
0.5 mmol/kg over 8-12 h (max ~50 mmol), then recheck
Worked example
50 kg: 25 mmol phosphate over 8 h

ASPEN refeeding syndrome severity (fall in P, K or Mg within 5 days of feeding)

SeverityFall in any of P, K, Mg
Mild10-20%
Moderate20-30%
Severe> 30% and/or organ dysfunction from these deficits or thiamine deficiency

Organ dysfunction includes arrhythmia, respiratory muscle weakness, heart failure, delirium.

Red flag · call your senior now
Phosphate < 0.3 mmol/L with weakness, failed weaning or heart failure: replace IV now.
Pitfall
Starting full-calorie feeds in a malnourished patient after prolonged starvation.
Pitfall
Using albumin-corrected calcium to decide on calcium therapy in shock or massive transfusion.
Pearl
Unexplained failure to wean: check phosphate and magnesium.

Calculators for this topic

Albumin-corrected calcium

Ca (mg/dL) + 0.8 × (4.0 − albumin g/dL). Prefer ionised calcium in ICU.

References

  1. 1

    da Silva JSV, Seres DS, Sabino K, et al. ASPEN consensus recommendations for refeeding syndrome. Nutr Clin Pract. 2020;35(2):178-195

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