Hypophosphataemia
Summary
Hypophosphataemia is defined as serum phosphate <0.8 mmol/L. Phosphate is essential for ATP production, so deficiency affects all energy-dependent processes. The most important clinical scenario is refeeding syndrome, where starved patients develop acute hypophosphataemia when fed carbohydrates, leading to potentially fatal respiratory failure, arrhythmias, and heart failure. Other causes include alcoholism, diuretics, and primary hyperparathyroidism. Severe hypophosphataemia (<0.3 mmol/L) is a medical emergency requiring IV phosphate replacement with cardiac monitoring.
Key Facts
- Definition: Serum phosphate <0.8 mmol/L
- Critical Level: <0.3 mmol/L (severe)
- Key Scenario: Refeeding syndrome
- Major Consequences: Respiratory failure (diaphragm weakness), Heart failure, Arrhythmias, Rhabdomyolysis
- Mechanism: ATP depletion (no adenosine tri-PHOSPHATE)
- Treatment: IV Phosphate (severe); Oral Sandoz Phosphate (mild)
Clinical Pearls
"Refeeding = Re-PHOSPHATE": When feeding a starved patient, insulin drives phosphate (and K, Mg) into cells. Always start feeds slowly and supplement phosphate.
"No Phosphate, No ATP": Phosphate is literally part of ATP. Severe deficiency causes cellular energy failure.
"Feed Low, Go Slow": Start refeeding at 10kcal/kg/day in high-risk patients. Give Pabrinex BEFORE feeding.
"IV Phosphate Needs Monitoring": IV phosphate can cause hypocalcaemia and cardiac arrhythmias. Always give with cardiac monitoring.
Incidence
- 2-5% of hospitalised patients
- Up to 30% in ICU patients
- Very common in malnourished/alcoholic patients
At-Risk Groups for Refeeding Syndrome
- Anorexia nervosa
- Chronic alcoholism
- Oncology patients
- Elderly/malnourished
- Post-bariatric surgery
- Prolonged fasting (>5-7 days)
- Uncontrolled diabetes (DKA treatment)
Causes
| Mechanism | Examples |
|---|---|
| Redistribution | Refeeding syndrome, Insulin, DKA treatment, Respiratory alkalosis |
| GI losses | Malabsorption, Antacids (phosphate binders), Chronic diarrhoea |
| Renal losses | Primary hyperparathyroidism, Fanconi syndrome, Diuretics |
| Poor intake | Alcoholism, TPN without phosphate |
Phosphate Functions
- Component of ATP, ADP, AMP (energy transfer)
- DNA and RNA structure
- Phospholipid membranes
- Bone mineralisation
- Oxygen delivery (2,3-DPG in RBCs)
Refeeding Syndrome Mechanism
- Starvation: Body depletes intracellular stores but serum PO4 remains normal
- Carbohydrate feeding: Insulin release
- Insulin drives PO4 (and K, Mg) into cells for glycolysis and protein synthesis
- Serum PO4 crashes → ATP depletion
- Clinical consequences: Respiratory failure, cardiac failure, arrhythmias
Why Respiratory Failure?
- Diaphragm is highly metabolically active
- ATP depletion → Muscle weakness
- Can cause respiratory arrest
Symptoms by Severity
| Severity | Serum PO4 | Symptoms |
|---|---|---|
| Mild | 0.6-0.8 | Usually asymptomatic |
| Moderate | 0.3-0.6 | Weakness, malaise |
| Severe | <0.3 | Respiratory failure, heart failure, confusion, seizures |
Symptoms
| System | Features |
|---|---|
| Neuromuscular | Weakness, myopathy, rhabdomyolysis |
| Respiratory | Diaphragm weakness → Respiratory failure |
| Cardiac | Arrhythmias, heart failure, cardiomyopathy |
| CNS | Confusion, irritability, seizures (severe) |
| Haematological | Haemolysis (ATP depletion in RBCs), WBC dysfunction |
General
- Weakness
- Weight loss / cachexia (if malnourished)
Respiratory
- Tachypnoea
- Shallow breathing
- Accessory muscle use
Neuromuscular
- Generalised weakness
- Hyporeflexia
First-Line
| Test | Purpose |
|---|---|
| Serum Phosphate | Confirm diagnosis (<0.8 mmol/L) |
| Potassium | Often low (same mechanism) |
| Magnesium | Often low (refeeding) |
| Calcium | May be low (IV PO4 can worsen) |
| U&E | Renal function |
| Glucose | Hyperglycaemia drives PO4 low |
Additional
- CK (rhabdomyolysis)
- PTH (if hyperparathyroidism suspected)
- Vitamin D
- ABG (respiratory alkalosis causes PO4 shift)
Refeeding Syndrome Prevention
┌──────────────────────────────────────────────────────────┐
│ REFEEDING SYNDROME PREVENTION │
├──────────────────────────────────────────────────────────┤
│ │
│ IDENTIFY HIGH-RISK PATIENTS (NICE CG32): │
│ • BMI <16 or >15% weight loss in 3-6 months │
│ • Little/no food intake for >10 days │
│ • Low K, Mg, PO4 before feeding │
│ • History of alcohol/drug misuse │
│ │
│ BEFORE FEEDING: │
│ • Check and correct PO4, K, Mg │
│ • Give Pabrinex (B vitamins; thiamine prevents │
│ Wernicke's) │
│ │
│ START FEEDS SLOWLY: │
│ • 10 kcal/kg/day (very high risk: 5 kcal/kg/day) │
│ • Increase slowly over 4-7 days │
│ │
│ MONITOR DAILY: │
│ • PO4, K, Mg (BD in first 3 days) │
│ • Fluid balance, weight │
│ │
└──────────────────────────────────────────────────────────┘
Phosphate Replacement
| Severity | Treatment |
|---|---|
| Mild (0.6-0.8) | Oral Phosphate Sandoz 2 tabs TDS |
| Moderate (0.3-0.6) | Oral if asymptomatic; IV if symptomatic |
| Severe (<0.3) | IV Phosphate Polyfusor (9mmol in 250mL over 6-12h) |
IV Phosphate Cautions
- Cardiac monitoring required
- Risk of hypocalcaemia (Ca-PO4 precipitation)
- Risk of arrhythmias
- Recheck levels after infusion
Of Hypophosphataemia
- Respiratory failure (diaphragm weakness)
- Cardiac arrhythmias and heart failure
- Rhabdomyolysis
- Haemolysis
- Seizures
- Death (if severe/untreated)
Of Treatment
- IV Phosphate: Hypocalcaemia, arrhythmias, hypotension
With Treatment
- Correctable if identified early
- Severe cases can be fatal (especially refeeding syndrome)
Mortality
- Refeeding syndrome: 10-20% mortality if not recognized
- With proper prevention: Excellent outcomes
Key Guidelines
- NICE CG32: Nutrition Support for Adults
- ASPEN Guidelines: Refeeding Syndrome
Key Evidence
Refeeding Risk
- Studies show high mortality in unrecognized refeeding syndrome
- Slow feeding and electrolyte monitoring reduce risk dramatically
What is Hypophosphataemia?
Hypophosphataemia means you have low phosphate levels in your blood. Phosphate is an essential mineral your body uses to make energy (ATP).
Why Does It Happen?
The most common cause in hospital is refeeding syndrome - when someone who hasn't eaten for a while starts eating again, phosphate moves into cells rapidly, causing blood levels to drop dangerously.
What Are the Symptoms?
- Muscle weakness
- Tiredness
- In severe cases: difficulty breathing, confusion, heart problems
How is it Treated?
- Oral supplements for mild cases
- IV phosphate for severe cases (with heart monitoring)
- Prevention is key: high-risk patients are started on food very slowly
Primary Guidelines
- NICE. Nutrition Support for Adults (CG32). 2006, updated 2017.
Key Studies
- Mehanna HM, et al. Refeeding syndrome: what it is, and how to prevent and treat it. BMJ. 2008;336(7659):1495-1498. PMID: 18583681