Phosphate (Blood)

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Auckland Te Toka Tumai

Test Guide

Phosphate (Blood)

Testing Laboratory

Auckland Te Toka Tumai | Auckland Hospital

Department

Chemical Pathology

Delphic Registration Code (Alpha Code)

SPO4

Test Code (Numeric Code)

0111

Laboratory Handling

Sample Stability

24 hours at 15-25°C, 4 days at 2-8°C, 1 year at -20°C

Associated Tests

Synonyms

Plasma/Serum Phosphate, Phosphorus, PO4

Turnaround Time

3 hours

Specimen Type

Plasma
Serum

Specimen Requirements

Tube/Container Type:

Preferred

Lithium Heparin PST- Gel (Light Green Cap)

Acceptable

Lithium Heparin - Non-gel (Green Cap)
Plain (Red Top)
SST II (Gold Cap)

Preferred Sample Volume (ml)

Minimum Sample Volume (ml)

3.5

No of Samples Required

Paediatric Specimen Requirements

Tube/Container Type:

Preferred

Lithium Heparin PST- Gel (Light Green Cap)

Acceptable

Plain (Red Top)

Preferred Sample Volume (ml)

0.5

Minimum Sample Volume (ml)

0.5

Additional Specimen Information

0.5 mL Paediatric Microtainer PST Blood (Preferred)
0.5 mL Paediatric Microtainer Plain Blood

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

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

Increased renal tubular loss

Primary or secondary hyperparathyroidism
Fanconi syndrome
Mesenchymal tumours
Hypophosphataemic rickets
Medications:
- Diuretics: acetazolamide, thiazides, loop diuretics
- 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

Units: mmol/L

Age Group <12 months	1.40 – 2.50
1 – <4 years	1.10 – 2.20
4 – <15 years	0.90 – 2.00
15 – <18 years	0.80 – 1.85
18 years and older	0.75 – 1.50

Conversion factors
phosphorus (mg/100 mL) x 0.323 = phosphate (mmol/L)
phosphate (mmol/L) x 3.096 = phosphorus (mg/100 mL)

Reference intervals were updated on 31/08/2021.

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).

There is a diurnal variation in plasma phosphate levels – levels can be higher in the evening. For plasma phosphate, sample is preferably taken after overnight fast.

Test Method

Performed in: LabPlus Automation
Principle: Colorimetric
Reagents: Roche assay
Analyser: Roche Cobas

Uncertainty of Measurement

Last updated: 07/08/2026

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Testing Laboratory

Department

Delphic Registration Code (Alpha Code)

Test Code (Numeric Code)

Laboratory Handling

Sample Stability

Associated Tests

Synonyms

Turnaround Time

Specimen Type

Specimen Requirements

Tube/Container Type:

Preferred

Acceptable

Preferred Sample Volume (ml)

Minimum Sample Volume (ml)

No of Samples Required

Paediatric Specimen Requirements

Tube/Container Type:

Preferred

Acceptable

Preferred Sample Volume (ml)

Minimum Sample Volume (ml)

Additional Specimen Information

Diagnostic Use

Causes of low phosphate

3 primary mechanisms – decreased GI absorption, transcellular shift and renal loss

Causes of high phosphate

Reference Intervals

Reference intervals were updated on 31/08/2021.

Test Method

Uncertainty of Measurement

Contact Information

Enquiry Form

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Phosphate (Blood)

Overview

Testing Laboratory

Department

Delphic Registration Code (Alpha Code)

Test Code (Numeric Code)

Laboratory Handling

Sample Stability

Associated Tests

Synonyms

Turnaround Time

Specimen

Specimen Type

Specimen Requirements

Tube/Container Type:

Preferred

Acceptable

Preferred Sample Volume (ml)

Minimum Sample Volume (ml)

No of Samples Required

Paediatric Specimen Requirements

Tube/Container Type:

Preferred

Acceptable

Preferred Sample Volume (ml)

Minimum Sample Volume (ml)

Additional Specimen Information

Referral

Clinical Information/Test Method

Diagnostic Use

Causes of low phosphate

3 primary mechanisms – decreased GI absorption, transcellular shift and renal loss

Causes of high phosphate

Reference Intervals

Reference intervals were updated on 31/08/2021.

Test Method

Uncertainty of Measurement

Additional Information

Contact Information

Contact Information

Enquiry Form

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