Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1121 page)

   Nonrenal acidosis (increased phosphate excretion as renal buffer)

Decreased In

   Hypoparathyroidism

   Pseudohypoparathyroidism

   Secondary hyperparathyroidism (renal rickets)

   Rickets and osteomalacia

   Parathyroidectomy

   Limitations

   Interpretation of urinary phosphorus excretion is dependent on the clinical situation and should be interpreted in conjunction with the serum phosphorus concentration.

   There is significant diurnal variation in excretion, with values highest in the afternoon.

   Urinary excretion depends on diet.

   Hypophosphatemia with normal serum calcium, high alkaline phosphatase, hypercalciuria, and low urinary phosphorus occurs with osteomalacia from excessive antacid ingestion. Children with thalassemia may have normal phosphorus absorption but high renal phosphaturia, leading to a deficiency of phosphorus.

   Increasing dietary intake of potassium has been reported to increase serum phosphate concentrations apparently by decreasing renal excretion of phosphate. 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 infants.