Hypophosphataemia

Hypophosphataemia is defined as a serum phosphate concentration lower than 0.8 mmol/L (normal range is 0.8 to 1.5 mmol/L).

Treatment is not always necessary, depending on the severity and the rate of decline of serum phosphate concentration.

Moderate hypophosphataemia (serum phosphate concentration 0.5 to 0.8 mmol/L) is often seen with primary hyperparathyroidism, tumour-induced osteomalacia, vitamin D deficiency, iron infusions and antacid abuse. It usually resolves when the cause is treated or stopped, so oral phosphate replacement is rarely needed.

Severe hypophosphataemia (serum phosphate concentration lower than 0.5 mmol/L) can be caused by alcohol withdrawal syndrome, refeeding syndrome (anorexia nervosa, starvation), trauma and sepsis, and occurs in around 1% of hospitalised patients. It is often asymptomatic, but presenting symptoms can be:

• neuromuscular—ranging from progressive myopathy to paralysis, confusion and seizures
• cardiorespiratory—respiratory muscular failure, left ventricular dysfunction with heart failure and arrhythmias
• haematological—haemolysis, thrombocytopenia and impaired leucocyte function.

Management of acute hypophosphataemia in critically ill patients is guided by the severity of the deficiency. For patients with severe hypophosphataemia and normal kidney function, use:

Measure serum calcium and phosphate concentrations hourly and adjust the dose as necessary. Monitor cardiac and kidney function during the infusion. Phosphate replacement is usually required until the cause has been treated.

Chronic hypophosphataemia can occur in patients with conditions such as familial X-linked hypophosphataemic rickets and tumour-induced osteomalacia. It can be treated with a combination of:

• oral elemental phosphorus (sodium acid phosphate, monobasic sodium phosphate)
• calcitriol (usually only used for children and adolescents).

The aim of treatment is to maintain a normal serum phosphate concentration. Use:

Elemental phosphorous can cause diarrhoea, which may limit the dose.

Milk is rich in phosphate and regular consumption can reduce the dose requirement of elemental phosphorous.