Clinical background:
Urine phosphate is measured together with serum phosphate, and urine and serum creatinine, to enable calculation of the renal tubular reabsorption of phosphate (TmP/GFR), which can be used to investigate the cause of hypophosphataemia.
Specimen container paediatric:
Random urine sample (plain universal container)
Specimen container adult:
Random urine sample (plain universal container) or 24 hour urine collection (plain or acid rinsed bottle)
Minimum volume paediatric:
1 mL
Minimum volume adult:
1 mL
Special requirements:
Urine samples should be fresh or acidified to reduce the formation of insoluble calcium phosphate complexes.
A 24 h collection may be taken to reduce the effect of diurnal variation and diet on phosphate excretion.
If the sample is to be used for calculation of renal tubular reabsorption of phosphate (TmP/GFR), it should be taken into a plain bottle, fasting, together with a serum sample, to allow simultaneous measurement of serum phosphate,
and urine and serum creatinine.
Sample stability:
Unknown at 15-25C,
6 months at 2-8C,
unknown at -20C
Interpretation:
Urine phosphate is measured together with serum phosphate, and urine and serum creatinine, to enable calculation of the renal tubular reabsorption of phosphate (TmP/GFR), which can be used to investigate the cause of hypophosphataemia. A low TmP/GFR is found in several disorders of renal phosphate handling that result in hyperphosphaturia, hypophosphataemia and osteomalacia.
Reference ranges:
24hr urine:
Adult: 15 – 50 mmol/24hr
Random urine: (mmol/mmol creatinine)
6 – 12 months: 1.2 – 19.0
1 – <2 years: 1.2 – 14.0
2 – <3 years: 1.2 – 12.0
3 – <5 years: 1.2 – 8.0
5 – <7 years: 1.2 – 5.0
7 – <10 years: 1.2 – 3.6
10 – <14 years: 0.8 – 3.2
14 – <17 years: 0.8 – 2.7
Factors affecting result:
Urine phosphate excretion varies with age, time of day, renal function, plasma PTH concentration, muscle mass and diet. Renal tubular reabsorption of phosphate (TmP/GFR) is considered to be a better measure of renal phosphate handling.