Proposed criteria for identifying hyperabsorbers among normocalcaemic renal stone formers

Normocalcaemic male stone formers (n = 138), 31-51 years of age, were divided into a hypercalciuric group (n = 80), with calcium excretions exceeding 7.0 mmol/24 h, and a normocalciuric group (n = 58), with calcium excretions of 7.0 mmol/24 h or less. The hypercalciuric group of patients was further subdivided using two previously published methods for identifying hyperabsorbers--one based on fasting urinary calcium/creatinine ratios and renal threshold phosphate concentrations, Jongen and the other method based on an oral calcium load test, Pak et al. The first method identified 37 patients and the second method 12 patients as hyperabsorbers. However, of the latter 12 patients only 8 were identified as hyperabsorbers using the first method of classification. It is thus evident that the two methods used for subclassification may give surprisingly different results. In order to obtain better congruence, the consequences of changing the defining limits in the two different methods were investigated. On the basis of the results obtained a new model for identification of hyperabsorbers is presented and discussed. With these new selection criteria (TmPO4/GFR greater than or equal to 0.75, urinary fasting molar calcium/creatinine ratio less than 0.40, and an increase in urinary molar calcium/creatinine ratio after calcium load greater than or equal to 0.20), 27 of the hypercalciuric patients were identified as hyperabsorbers. This group included 23 of Jongen's 37 and all of Pak's 12 hyperabsorbers. No patient not identified as a hyperabsorber according to either Jongen's or Pak's method were found in this group. The suggested model for identifying hyperabsorbers seems to be practical at least in studies on the relation between hypercalciuria and metabolism of vitamin D3.