An improved method for the routine biochemical evaluation of patients with recurrent calcium oxalate stone disease.

By means of a computerized calculation program, a simplified estimate of the ion-activity product of calcium oxalate was derived (AP(CaOx)-index), based on the 24-h urinary excretion of calcium (Ca), oxalate (Ox), magnesium (Mg), citrate (Cit) and the urine volume (V): (formula: see text) With urinary electrolyte values within the normal range, there was a good correlation between the AP(CaOx)-index and the more laboriously obtained ion-activity product (r = 0.997). To express the biochemical risk of CaOx stone formation a CaOx-risk index was designed, which also includes the inhibition of calcium oxalate crystal growth (I) and with all variables related to urinary creatinine (Cr): (formula: see text) The mean CaOx-risk index (+/-SEM) in urine from 100 normal men and 156 male stone formers were 648 +/- 27 and 1019 +/- 38 respectively (p less than 0.001). A risk index without inhibition index, had the corresponding values 366 +/- 14 and 527 +/- 17 (p less than 0.001).