See also 

Electrolyte abnormalities
Hyperphosphataemia
Intravenous fluids

Key points

1. Oral/enteral is the preferred route of phosphate administration
2. Hypophosphataemia may be asymptomatic. Phosphate levels should be monitored in high-risk clinical scenarios
3. Acute severe hypophosphataemia can be a life-threatening medical emergency
4. Assess for other electrolyte abnormalities and renal function

Background

• Approximately 85% of body phosphate is located in bones and teeth, 14% in cells and <1% in extracellular fluid
• Phosphate requirements depend on bone formation and growth rate. They are higher in childhood, especially in premature infants
• Phosphate balance is closely related to calcium homeostasis
• The terms serum phosphate and serum phosphorus are often used interchangeably, it is the phosphorus content of anionic plasma phosphate PO4(3-) that is measured

Definition

• The lower limit of normal for serum phosphate changes over childhood, from around 1.5mmol/L in early infancy to 0.8mmol/L by late adolescence
• Reference ranges vary substantially by age and assay
• Severe acute hypophosphataemia ( <0.5 mmol/L) may be associated with weakness, lethargy and paraesthesia
• Serum phosphate below 0.3mmol/L represents a life-threatening medical emergency

Causes of hypophosphataemia

*For advice about implicated medicines, discuss with a pharmacist or check adverse reactions in a drug prescribing resource

Assessment

• Assess renal function (urine output, serum urea, electrolytes and creatinine).
• Check baseline serum calcium and albumin levels. Severe hypophosphataemia may cause hypercalcemia
• Identify underlying cause and correct where possible
• hypophosphataemia may be asymptomatic
• lethargy may be the only symptom present
• if underlying cause not evident, consider checking parathyroid hormone level, urine phosphate excretion and creatine kinase (for severe hypophosphataemia)
• Assess for signs/symptoms of hypophosphataemia
• Gastrointestinal: anorexia
• Respiratory: respiratory muscle dysfunction, decreased oxygen delivery
• Cardiac: arrhythmias, chest pain
• Neuromuscular: muscle weakness, bone pain, altered mental status, seizure, arthralgia, coma
• Haematological: haemolytic anaemia, impaired leukocyte and platelet function
• Endocrine: rickets eg limb bowing or radiological finding

Management

Investigations

Serum phosphate levels should be monitored, with frequency depending on the degree of the abnormality and whether the child is symptomatic

Treatment

Treatment should be directed at the underlying cause with an aim to return serum phosphate levels to the normal range

• Reduce the dose in children with renal impairment. Seek specialist advice
• Adjust dose as needed for response and tolerance

Oral/enteral dosing

Consider for mild to moderate asymptomatic hypophosphataemia

Dose

• Neonate: 1mmol/kg/day in 2-4 divided doses
  • higher doses (up to 3mmol/kg/day) are used for osteopenia of prematurity
• Child: 2-3mmol/kg/day in 2-4 divided doses
  • Usual maximum dose 
    • Age <5 years: 3 tablets daily 
    • Age >5 years: 6 tablets daily
• Adolescent/adult: 1-2 tablets 2-3 times per day

Medication Forms

Administration
For doses less than a whole tablet: disperse one tablet in 20mL of water to make a concentration of 0.8mmol/mL (25mg/mL) of phosphate. Mix until an even dispersion is formed and measure the required dose immediately 

Monitoring
Adverse effects may include:

• abdominal pain, diarrhoea, nausea and vomiting
• hyperphosphatemia (over-correction), hypocalcaemia, hypernatraemia
• renal nephrocalcinosis (acute phosphate nephropathy) leading to acute renal failure

Intravenous Dosing

Indications

• Intravenous phosphate replacement should be considered for
• Symptomatic or severe hypophosphataemia
• Contraindication to oral dosing (eg severe malabsorption or nil by mouth)

Precautions

• Seek specialist advice for intravenous phosphate replacement
• Cardiac monitoring should be considered due to potassium content, rapid intravenous administration or overdose has risk of arrhythmia/cardiac arrest  
• Check serum phosphate and potassium before infusion and approximately 2 hours after completion
• Do not infuse via same IV access point or lumen as magnesium or calcium-containing intravenous fluids including parenteral nutrition 

Dose
0.36mmol/kg as slow infusion

• Doses greater than 10mmol must be approved by a senior clinician
• The dose may be repeated, if required, up to a maximum of 2mmol/kg/day (maximum 70mmol/day)
• Consider the potassium content of intravenous phosphate preparation (see table below)
• For patients with potassium levels near upper end of normal range or above, use phosphate formulation without potassium where available
• Reserve potassium dihydrogen for patients requiring intravenous potassium replacement  

Medication Forms

Administration

• Dilute before use
• Peripheral line: dilute to 0.05mmol/mL or weaker
• Central line: dilute to 0.12mmol/mL or weaker
• Give via slow infusion over 6 hours
• The rate of administration must not exceed 0.2mmol/kg/hour or 10mmol/hour
• Careful and thorough mixing after dilution is essential to prevent pooling of phosphate solutions
• Refer to local guidelines for more detailed administration information

Monitoring
Monitor for risk of hyperphosphataemia (due to over-correction)
Adverse effects may include:

• Bradycardia, hypotension, arrhythmias
• Hyperkalaemia, hypocalcemia
• Acute kidney injury
• Dyspnoea, oedema
• Extravasation injury

Consider consultation with local paediatric team when

The child is symptomatic or there is a significant abnormality

Consider consultation with renal team when

Renal dysfunction is present

Consider transfer when

The child is symptomatic or requires close monitoring not available in your centre

Consider discharge when

• The child is clinically stable with an appropriate management plan in place
• Any acute abnormality has resolved

Last updated January 2023