The usefulness of an oral calcium tolerance test in the choice of management of patients with idiopathic hypercalciuria.

We studied 40 patients with calcium urolithiasis and idiopathic hypercalciuria in an attempt to identify patients with an absorptive or renal type of hypercalciuria. An oral calcium tolerance test was performed in all patients, resulting in a rise in serum calcium in all cases (2.35 +/- 0.09 mmol/l vs 2.49 +/- 0.09 mmol/l; P less than 0.001). This was also true for serum phosphate (0.96 +/- 0.17 mmol/l vs 1.09 +/- 0.18 mmol/l; P less than 0.001), TmPO4/GFR (0.95 +/- 0.19 mmol/l vs 1.20 +/- 0.25 mmol/l; P less than 0.001) and fasting calcium excretion (3.14 +/- 1.16 mmol/100 l GF vs 6.17 +/- 2.02 mmol/100 l GF; P less than 0.001). All patients showed a drop in nephrogenous cAMP excretion (1.33 +/- 0.95 nmol/dl GF vs 0.74 +/- 0.72 nmol/dl GF; P less than 0.001). iPTH levels declined significantly (2.70 +/- 1.50 pmol/l vs 2.11 +/- 1.19 pmol/l; P less than 0.001). However, discordant individual changes in suppression of nephrogenous cAMP excretion, and rises in fasting calcium excretion prohibited a distinction between the absorptive or renal type of hypercalciuria. It is concluded that an oral calcium tolerance test is not helpful in the choice of management of patients with idiopathic hypercalciuria.