Phosphate

Physiology

• Most important for aerobic energy production
• Predominantly intra-cellular

Hyperphosphataemia

• Massive muscle or bowel necrosis or tumour lysis syndrome, or renal impairment
• Patients with renal failure - dialysis
• Otherwise can promote phosphate binding in upper GI tract with sucralfate or aluminium-containing antacids

Hypophosphataemia

• Commonly seen during recovery from critical illness
• Causes (most commonly refeeding syndrome in surgical patients)

Internal redistribution

Increased insulin secretion, particularly during refeeding

Acute respiratory alkalosis

Hungry bone syndrome

Decreased intestinal absorption

Inadequate intake

Inhibition of phosphate absorption (eg, antacids, phosphate binders, niacin)

Steatorrhea and chronic diarrhoea

Vitamin D deficiency or resistance

Increased urinary excretion

Primary and secondary hyperparathyroidism

Vitamin D deficiency or resistance

Hereditary hypophosphatemic rickets

Oncogenic osteomalacia

Fanconi syndrome

Other - acetazolamide, tenofovir, IV iron, chemotherapeutic agents

Removal by kidney replacement therapies

• Symptoms mostly occur below serum 0.32mmol/L
  • Metabolic encephalopathy, impaired myocardial contractility, respiratory failure to diaphragmatic weakness, proximal myopathy, dysphagia, ileus
• Asymptomatic patients with serum phosphate <0.64mmol/L, give oral phosphate
• Symptomatic patients
  • Serum phosphate 0.32 to 0.63mmol/L: oral phosphate
  • Serum phosphate <0.32: IV phosphate, switching back to oral when serum levels >0.48
• Stop repletion when serum level >0.64 unless there is an indication for chronic therapy
• Oral repletion should be given as a combined preparation of sodium and potassium phosphate; sodium phosphate is preferred for IV therapy
• Oral dosing: 30-80mmol per day in divided doses
  • Serum >0.48: 1mmol/kg of elemental phosphorus (min 40, max 80) can be given in 3-4 divided doses over 24 hours
  • Serum <0.48: 1.3mmol/kg of elemental phosphorus (max 100mmol) in 3-4 divided doses over 24 hours
  • If reduced GFR, give half the dose
  • Recheck 2-12 hours post the last of the divided doses
• IV dosing
  • Be very careful - can precipitate with calcium and produce adverse effects including hypocalcaemia, kidney failure, arrhythmias
  • Serum >0.4: 0.08-0.24mmol/kg over 6 hours (max total dose 30mmol)
  • Serum <0.4: 0.25-0.5mmol/kg over 8-12 hours (max total dose 80mmol)
• If eGFR < 30, give half the usual dose