renal phosphorus clearance in mL/min =
= (urine phosphorus in mg/dL) * (urinary volume in mL) / ((serum phosphorus in mg/dL) * (time of collection in minutes))
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 clearance can be adjusted for body surface area (per 1.73 square meters).
• 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.
Limitations:
• The creatinine clearance should be > 40-50 mL/min for tests of urinary phosphorus excretion (and urinary calcium excretion) to be valid.
