Diagnostic Use
Causes of low phosphate:
3 primary mechanisms – decreased GI absorption, transcellular shift and renal loss.
- Decreased absorption/ low intake
- Malnutrition, vomiting, diarrhoea
- Malabsorption, vitamin D deficiency, phosphate binding antacids containing Ca, Mg, Al
- Alcoholism
2. Transcellular shift, rapid uptake into cells
- Refeeding syndrome
- Dextrose/Glucose infusion
- Insulin infusion
- Recovery phase of DKA (with Dextrose/insulin infusion)
- Acute respiratory alkalosis
- Catecholamines e.g. in acute severe illness
- ‘hungry bones’ – e.g. post parathyroidectomy
3. Increased renal tubular loss
- Primary or secondary hyperparathyroidism
- Fanconi syndrome
- Mesenchymal tumours
- Hypophosphataemic rickets
- Medications
- Diuretics: acetazolamide, thiazides, loop diuretic
- Anti-retroviral (tenofovir, adefovir)
- Iron infusion (ferric carboxymaltose). Hypophosphataemia peaks at 2 weeks post infusion and can cause severe hypophosphataemia (<0.3 mmol/L). Ferric carboxymaltose impairs the degradation of FGF23, resulting in an elevated FGF23 which leads to reduced renal reabsorption of phosphate.
- Cisplatin, tetracyclines, aminoglycosides
Extremely low serum/plasma phosphate level (usually less than 0.3 mmol/L) can cause respiratory insufficiency, muscle weakness, rhabdomyolysis, intravascular haemolysis, seizure, myocardial depression, myopathy and haemolytic anaemia.
Causes of high phosphate:
- Spurious cause: Paraproteinaemia
- Storage of unseparated blood
- Increased intake/absorption: phosphate containing laxatives, enema, vitamin D intoxication
- End Stage Renal Failure
- Cell lysis – haemolysis, tumour lysis syndrome, rhabdomyolysis
- Acidosis: DKA and lactic acidosis
- Hypoparathyroidism
Reference Intervals
| AGE |
PHOSPHATE |
| 0 – 1 year |
1.40 – 2.50 mmol/L |
| 1 year – < 4 years |
1.10 – 2.20 mmol/L |
| 4 years – < 15 years |
0.90 – 2.00 mmol/L |
| 15 years – < 18 years |
0.80 – 1.85 mmol/L |
| > 18 years and older |
0.75 – 1.50 mmol/L |
|
Conversion factors: phosphorus (mg/100 mL) x 0.323 = phosphate (mmol/L) or
phosphate (mmol/L) x 3.096 = phosphorus (mg/100 mL)
Plasma phosphate reference intervals were updated on 31/08/2021 following a review of literature, local data and paediatrician agreement (the summary document can be provided upon request). The lower limit for infants has increased from 1.00mmol/L to 1.40mmol/L. For all other age groups, the changes are minor. The new ranges have been adopted by all clinical labs in Auckland.
The increase in the lower limit for infants is largely driven by the literature findings, including the local ProVIDe RCT findings (1). This study has shown that in extremely low birth weight neonates, serum phosphate <1.40mmol/L was associated with an increased risk of mortality and severe hypophosphataemia (<0.9mmol/L) was associated with five times higher odds of severe intraventricular haemorrhage. Based on this study, local NICU teams maintain serum phosphate >1.40mmol/L in the extremely low birth weight neonates. This lower limit, which is mostly specific to ELBW neonates, is also consistent with literature findings and guidelines for normal infants (2-6).
Test Method
Principle: Molybdate UV
Analyser: Roche Diagnostics Cobas c703/c503
Reagent: PHOS2