Crystalline composition of urinary stones in recurrent stone formers

175 stones from 70 recurrent stone formers with each at least two stones available for analysis were studied crystallographically. The chance that a subsequent stone belonged to the same of five major stone groups was 65% but rose to 91% if three of these groups were considered as one (calcium stones). Other findings, however, indicate that distinction between the three groups of calcium stones is of interest. Thus, pure calcium oxalate stones (Ox) did not occur in the same patients as pure calcium phosphate stones (P). In comparable stones containing both the above types of components (OxP), the calcium phosphate content was significantly lower (average apatite content per stone 9%) in stones from patients who also had Ox stones than (37%) in those who had associated P stones. Also, the stone nucleus was frequently Ox in the OxP stones from the former and usually P in the OxP stones of the latter patients. Brushite occurred in 10% of the stones, which is more frequent than in unselected stone materials and largely due to a tendency for it to recur in a few patients. Patients who provide more than one stone for analysis appear to differ considerably in stone composition from stone patients in general, and inclusion of several stones from some patients will lead to bias in materials purporting to reflect stone composition in a population. When available, several stones from each patient should preferably be analysed. Exceptions may be made for cystine and, possibly, pure uric acid stone patients.