renal threshold for phosphate in mmol/L =
= (plasma phosphate in mmol/L) – ((urine phosphate in mmol/L) * (plasma creatinine in mmol/L) / (urine creatinine in mmol/L)) =
= 1 - (fractional phosphate excretion)
where:
• The urine collection period is usually 12 or 24 hours (720 or 1,440 minutes respectively).
• The serum specimen ideally should be collected midway in the collection period. For simplicity of collection in outpatients, the serum sample can be collected at the time the collection vessel is collected or on receipt in the laboratory (collecting a sample at both times and using the average may be more accurate).
• The urine sample should be collected in an acid-washed, detergent-free container, acidified with hydochloric acid after collection to pH < 3.
• The patient may be on an unrestricted diet or a constant daily diet.
Interpretation:
• The normal renal threshold is >= 1.10 mmol/L for infants < 12 months of age and >= 1.00 mmol/L for children >= 1 year old.
Limitations:
• The creatinine clearance should be > 40-50 mL/min for tests of urinary phosphorus excretion (and urinary calcium excretion) to be valid.
