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Hypophosphataemia (Low phosphate)

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Patients with hypocalcaemia should have their calcium corrected before replacing phosphate to prevent further hypocalcaemia. Parenteral phosphate is usually given IV when serum phosphate is < 0.32 mmol/L or severe symptoms. A low phosphate can precipitate the refeeding syndrome.

About

Normal range 0.8 - 1.4 mmol/L
Severe is < 0.4 mmol/L (1.24 mg/dL).

Aetiology

Phosphate reabsorption in renal PCT
Decreased uptake with PTH

Causes

Cause	Clinical Features	Investigations	Management
Refeeding Syndrome	Muscle weakness, respiratory failure, arrhythmias, confusion, usually occurring after initiation of feeding in malnourished patients.	Serum phosphate, calcium, potassium, and magnesium levels; close monitoring during refeeding.	Slow reintroduction of nutrition, oral or intravenous phosphate supplementation, close monitoring of electrolytes during refeeding.
Chronic Alcoholism	Muscle weakness, bone pain, confusion, tremors, seizures, often with concurrent hypomagnesemia and hypokalemia.	Serum phosphate, magnesium, and potassium levels, renal function tests, liver function tests.	Oral or intravenous phosphate supplementation, nutritional support, treatment for alcohol dependence, correction of associated electrolyte imbalances.
Diabetic Ketoacidosis (DKA)	Polyuria, polydipsia, dehydration, confusion, muscle weakness, may occur during the treatment phase when insulin is administered.	Serum phosphate, glucose, ketones, blood gases, serum potassium, and magnesium levels.	Intravenous fluids, insulin therapy, phosphate supplementation if levels are critically low, monitoring of electrolytes during treatment.
Hyperparathyroidism	Bone pain, muscle weakness, kidney stones, fatigue, increased urination, symptoms related to hypercalcemia.	Serum phosphate, calcium, PTH levels, urinary phosphate excretion, bone density scans.	Treatment of underlying hyperparathyroidism (e.g., parathyroidectomy), phosphate supplementation if necessary, monitoring of calcium and phosphate levels.
Chronic Malnutrition	Fatigue, muscle weakness, bone pain, confusion, increased susceptibility to infections.	Serum phosphate, calcium, magnesium, and albumin levels, nutritional assessment.	Oral or intravenous phosphate supplementation, nutritional support, addressing underlying causes of malnutrition.
Renal Tubular Disorders (e.g., Fanconi Syndrome)	Polyuria, polydipsia, bone pain, rickets or osteomalacia, growth retardation in children.	Serum phosphate, calcium, bicarbonate, and potassium levels, urine analysis for glucose, amino acids, and bicarbonate.	Oral phosphate supplementation, potassium and bicarbonate supplements, treatment of underlying disorder, monitoring of growth in children.
Medications (e.g., Antacids, Diuretics)	Weakness, bone pain, confusion, symptoms of hypophosphatemia, often related to long-term use of phosphate-binding antacids or diuretics.	Serum phosphate, calcium, and magnesium levels, review of medication history.	Discontinue or adjust the offending medication, oral phosphate supplementation, monitor and correct electrolyte imbalances.
Respiratory Alkalosis	Muscle cramps, tetany, paresthesias, confusion, related to hyperventilation from anxiety, sepsis, or mechanical ventilation.	Arterial blood gases (ABG), serum phosphate, calcium, and potassium levels, assessment of underlying cause.	Treat underlying cause of hyperventilation, phosphate supplementation if severe, monitor and correct other electrolyte imbalances.

Clinical

Asymptomatic, Muscle weakness, myalgia, rhabdomyolysis
Seizures, delirium, diaphragmatic weakness
Paraesthesia, Convulsions, Tremor, Haemolysis
Respiratory failure due to weak resp muscles
Cardiac failure from muscle weakness
Osteomalacia, Coma can be seen

Associations

Osteomalacia, bone pain, reduced insulin sensitivity
Glycosuria, hypercalciuria and hypermagnesaemia

Investigations

Check U&E Creatinine
Ca, Mg, PO4, ALP, VBG

Management below

Mild Hypophosphataemia (0.6-0.69mmol/L)
- Manage cause and monitor. Oral replacement may be considered see below. Manage as outpatient. Guidelines differ in threshold for treatment.
- Oral phosphate and diet rich in phosphate e.g. protein and dairy
Moderate Hypophosphataemia (0.4-0.59mmol/L)
- Consider phosphate replacement if the patient is symptomatic or following a consideration of the clinical risks and benefits.
- Consider Phosphate Sandoz 1-2 tablets PO TDS
- Symptomatic consider 25mmol (250ml) over 12 hours IV Phosphate Polyfusor
Severe Hypophosphataemia (<0.4 mmol/L)
- IV Phosphate Polyfusor IVI (100mmol PO4 in 500mL) given over 8-12 hours. Do not give IV phosphate to a patient who is hypercalcaemic or oliguric.

Cautions

Rapid rates of phosphate administration can lead to hypocalcaemia, hyperphosphataemia, and metastatic calcification.
Phosphate is renally cleared. Phosphate (especially via the intravenous route) should be used with caution in patients with renal impairment.

References

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