Cystine crystalluria and urinary saturation in cystine and non-cystine stone formers

It has been suggested recently that the first step in the formation of calcium oxalate stones appears to be crystallisation. This step is said to depend on the state of saturation of the urine. This hypothesis was checked in cystine stone formers. Cystine crystalluria was found in 83% of 24 urine samples from cystine stone formers (CSF) but in one of the 400 control samples and appears to be a good guide in the diagnosis of cystine lithiasis. Urinary cystine saturation was constantly higher in CSF than in non-cystine stone formers (NCSF) who exhibited undersaturated urine with respect to cystine. There was almost no overlap between these 2 groups. Crystals were never found in undersaturated urine and were always present when the saturation was above 1. There appears to be a good correlation between the level of urinary saturation and the presence of crystalluria and there is no need for any additional factor such as a defective inhibitor. The study underlines the limits of a therapeutic regimen of a high fluid intake and alkalinisation of the urine.