Overview #
An electrolyte disturbance that is due to low levels of serum phosphate.
The normal range for serum phosphate is 0.81-1.50 mmol/L (this number can vary between laboratories so always check your local reference range).
Definition
Mild hypophosphataemia1
Serum phosphate 0.50-0.80 mmol/L
Moderate hypophosphataemia1
Serum phosphate 0.30-0.49 mmol/L
Severe hypophosphataemia1
Serum phosphate <0.30 mmol/L
The values provided can vary between laboratories so always check your local reference ranges to avoid errors in management. #
References #
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Triage #
Severe, moderate and symptomatic hypophosphataemia
- Attend to the patient urgently (within 30 minutes) due to the risk of:
- Respiratory failure secondary to diaphragm weakness
- Neurological symptoms (seizures, coma)
- Reduction in myocardial contractility2.
Mild or asymptomatic hypophosphataemia #
- Attend to the patient by the end of the shift or flag the issue for review on the morning round.
Causes #
The major causes of phosphate depletion can be divided into internal redistribution, decreased intestinal absorption and renal losses.
Internal redistribution
- Acute respiratory alkalosis3
- Refeeding syndrome4
- Hungry bone syndrome5
Reduced intestinal absorption
- Inadequate intake of phosphate (usually needs a prolonged period of reduced intake)
- Medications
- Phosphate binders
- Aluminium and magnesium antacids6
- Niacin7
- Chronic diarrhoea
- Steatorrhoea
Renal losses
- Primary and secondary hyperparathyroidism8
- Vitamin D deficiency
- Iron infusion
- Fanconi syndrome9
- Primary renal phosphate excretion (due to several rare syndromes)
- Tumour-induced osteomalacia
Clinical features #
Clinical features are rarely seen unless serum phosphate is below 0.64mmol/L, whilst the more severe features are more likely to be seen when serum phosphate is below 0.30mmol/L10.
Cardiorespiratory2
- Impairment of myocardial contractility
- Higher incidence of ventricular arrhythmias in patients who have a recent acute myocardial infarction
- Impaired diaphragmatic contractility
Neurological symptoms2
- Paraesthesia
- Agitation and delirium
- Generalised seizures
- Dysphagia
- Proximal myopathy
- Coma
Investigations #
Initial investigations
| Investigation | Significance |
| ECG | In severe and symptomatic hypophosphataemia, particularly if the patient has had a recent myocardial infarction. To look for ventricular arrhythmias. |
| UEC, Calcium | Due to the risk of concurrent electrolyte abnormalities such as hypocalcaemia. |
Further investigations
| Investigation | Indication |
| 24 hour urinary phosphate excretion | If the cause of hypophosphataemia is not evident from the history. This test will help determine if there is primary renal wasting of phosphate and a referral to nephrology may be considered if this is the case. |
| Serum vitamin D level | If the calcium level is low then consider sending a vitamin d level as this could be the underlying cause. |
Management – mild, moderate or asymptomatic #
Identify and treat the underlying cause of the hypophosphataemia.
Unlikely to need treatment unless serum phosphate is less than 0.6mmol/L except in patients with respiratory failure, recovery from DKA, renal phosphate wasting, malnutrition, refeeding syndrome and alcoholism/withdrawal1.
Replace phosphate with oral supplementation1
| Effervescent phosphate* 500-1000 mg oral up to three times a day.Duration dependant upon clinical response and treatment of the underlying cause. Caution – Do not give alongside oral calcium replacement as it will act as a phosphate binder. Discuss with senior clinician regarding withholding calcium supplements. Caution – Repeated doses in patients with renal impairment increases the risk of hyperphosphataemia.Caution – Prescribe with caution in patients with hyperkalaemia as some phosphate formulations contain potassium. |
Management – severe or symptomatic #
Identify and treat the underlying cause of the hypophosphataemia.
Replace phosphate intravenously1
| Sodium dihydrogen phosphate* 10mmol in 250mL of sodium chloride 0.9% over 2-6 hours IV.Caution – Consider giving repeated doses in patients with renal impairment due to the risk of hyperphosphataemiaCaution – Prescribe with caution in patients with hyperkalaemia as some phosphate formulations contain potassium.Note – Can give more concentrated solutions in critically ill patients preferably through a central line however this should be discussed with a senior clinician. |
Monitor plasma phosphate, calcium levels and renal function every 12 to 24 hours1.
Contributors
Reviewing Consultant/Senior Registrar
Dr Ajinkya Bhonsle
Dr Rahul Barmanray
