Phosphorus, 24-Hour Urine

Evaluate calcium/phosphorus balance.

High urinary phosphorus (ie, increased renal losses) occurs in primary hyperparathyroidism, vitamin D deficiency, renal tubular acidosis, diuretic use. Phosphates are among the substances which may be lost in the Fanconi syndrome. Renal loss of phosphate may itself lead to rickets or osteomalacia.

Low in hypoparathyroidism, pseudohypoparathyroidism, vitamin D intoxication

Evaluate nephrolithiasis. Hypophosphatemia with normal serum calcium, high alkaline phosphatase, hypercalciuria, low urinary phosphorus occur with osteomalacia from excessive antacid ingestion. The relationship between serum PO4 and phosphaturia, the phosphate excretion index, is described.1 Largely, however, urine phosphate simply reflects phosphate intake in patients not on phosphate binding medications.

Children with thalassemia may have normal phosphorus absorption, but high renal phosphaturia, leading to a deficiency of phosphorus.2 Increasing dietary intake of potassium has been reported to increase serum phosphate concentrations apparently by decreasing renal excretion of phosphate.3 During the last trimester of pregnancy there is a sixfold increase in calcium and phosphorus accumulation as the fetus triples its weight. Plasma phosphorus concentrations and increased urinary phosphate may provide a useful means to assess response to phosphate supplements in the premature infant.4

1. Black DAK, Cameron JS. Renal function. In: Brown SS, Mitchell FL, Young DS, eds. Chemical Diagnosis and Disease. Amsterdam, Holland: Elsevier/North Holland; 1979:453-524.

2. Lapatsanis P, Sbyrakis S, Vertos C, Karaklis BA, Dosiadis S. Phosphaturia in thalassemia. Pediatrics. 1976 Dec; 58(6):885-892. PubMed 995519

3. Sebastian A, Hernández RE, Portale AA, Colman J, Tatsuno J, Morris RC Jr. Dietary potassium influences kidney maintenance of serum phosphorus concentration. Kidney Int. 1990 May; 37(5):1341-1349. PubMed 2345430

4. Mayne PD, Kovar IZ. Calcium and phosphorus metabolism in the premature infant. Ann Clin Biochem. 1991 Mar; 28(Pt 2):131-142. PubMed 1859151