Test Details: This test is used for evaluation of hypo- or hyperphosphatemic states and evaluation of patients with nephrolithiasis. A variety of factors influence renal tubular phosphate reabsorption and consequent urine excretion. Factors that increase urinary phosphorus excretion include high phosphorus diet, parathyroid hormone, low dietary potassium intake and proximal tubule defects (e.g. Fanconi syndrome, X-linked hypophosphatemic rickets, tumor-induced osteomalacia). Factors that decrease or are associated with decrease in urinary phosphorus excretion include low dietary phosphorus intake, insulin, high dietary potassium intake and decreased intestinal absorption of phosphorus (e.g. phosphate-binding antacids, vitamin D deficiency, malabsorption states). A renal leak of phosphate has also been implicated as contributing to kidney stone formation in some patients.