Simplified protocol for biochemical evaluation of recurrent renal calcium stone disease

Groups comprising 42 recurrent and 38 single calcium stone formers, and 48 controls underwent a standardized study protocol. During 12 h of fasting along with a standardized water load urine was collected for the last 4 h. One blood sample was drawn midway during urine collection. As compared with controls recurrent stone formers demonstrated significantly raised values of plasma ionized calcium, 1,25-dihydroxyvitamin D [1,25-(OH)2D], urinary ionized calcium, urinary pH, and standardized excretion rate of calcium [ECa(U)]. The tubular reabsorption of sodium and chloride, the maximal tubular reabsorption of phosphate, nephrogenous cyclic AMP (NcAMP), glomerular filtration rate (GFR) and plasma albumin were all significantly depressed. Data from the single stone former group were intermediate between controls and recurrent stone formers. For the pathogenesis of recurrent calcium stone formation these data suggest a general, though variably expressed, defect in the proximal tubular reabsorption of calcium, phosphate, bicarbonate, sodium and chloride leading to hypersecretion of 1,25-(OH)2D, hyperabsorption of calcium from the gut and a modest increase in plasma ionized calcium. Computer analysis of data do not support the concept that stone patients can be classified as either intestinal hyperabsorbers or renal losers. A stepwise discriminant analysis selected plasma albumin, urinary pH, ECa(U), plasma phosphate, GFR and NcAMP as the most important variables contributing to the difference between controls and recurrent stone formers. By this procedure we obtain a sensitivity of 93% and a specificity of 96% respecting proper classification of the recurrent stone former. The analysis may therefore be of value predicting the risk of recurrent stone formation in the single stone former.