Calcium oxalate dihydrate crystal growth.

Calcium oxalate dihydrate (COD), although thermodynamically unstable with respect to calcium oxalate monohydrate (COM), is more commonly seen in human urine. For the first time, a COD-seeded crystal growth assay has been developed. Seed crystals of COD were precipitated from filtered urine by the addition of ammonium oxalate and were stored dry and added to a supersaturated calcium oxalate solution to initiate an experiment. The growth rate in the COD-seeded system was 22 per cent of that for a COM-seeded system, for equivalent surface areas. Urine samples from normal subjects have similar inhibitory activity in the COD- and COM-seeded systems, as do pyrophosphate and heparin, which are known inhibitors of COM crystal growth. These results indicate that the technically simpler COM-seeded growth assay is suitable for measurement of inhibitors of calcium oxalate growth in urine.