An indirect method for urinary oxalate estimation.

Oxalate was first coprecipitated at pH 5 as calcium oxalate precipitate (COP) with excess calcium ions and ethanol. Interference in the COP, mainly calcium phosphate, was successfully removed by washing with 0.1 M acetic acid equilibrated with calcium oxalate monohydrate crystal. Since the calcium content of the washed COP showed a high correlation with the original oxalate concentration, both in artificial (r = 0.998, P <0.001) and real (r = 0.951, P < 0.001 for colorimetric; r = 0.982, p < 0.001 for enzymatic methods) urines, it could be used to predict indirectly the urinary oxalate concentration. The mean percentage recovery of the predicted oxalate was 98.95, S.D. = +/-4.77% (n = 6). The method is simple, reproducible and relatively precise. It therefore could be set up as a routine method of urinary oxalate assessment in general clinical laboratories.