[Usefulness of oral calcium test in renal lithiasis]

We report the results of an oral tolerance test performed in 317 patients with kidney stones. In order to avoid PTH or AMPc measurements, and therefore to reduce costs and time to get the results, we measured the tubular maxima of phosphate per glomerular filtration rate (TmP04/GFR, the phosphate threshold). Urine collections from 7 to 9 h and from 9 to 13 h were obtained. The samples were analyzed for calcium, creatinine and phosphorus content. All patients ingested 1 g of calcium mixed in a meal at 9 o'clock. Venous blood samples were obtained for calcium, creatinine and phosphorus measurements, previous to the calcium ingestion. Urinary calcium to creatinine ratio, before and after the calcium-load, as well as TmP04/GFR were calculated. In 97 subjects (30.8%) there were no calcium metabolism abnormalities. Idiopathic hypercalciuria was present in 183 (57%) and primary hyperparathyroidism in 37 (11.7%). Idiopathic hypercalciuria was classified in four subgroups: absorptive hypercalciuria with normal serum phosphorus, absorptive hypercalciuria with low serum phosphorus (renal phosphate leak), renal hypercalciuria with normal phosphorus and renal hypercalciuria with low serum phosphorus.