The presence of infectious microorganisms in
urinary stones is commonly inferred from stone composition, especially by the presence of
struvite in a stone. The presence of highly carbonated
apatite has also been proposed as a marker of the presence of bacteria within a stone. We retrospectively studied 368 patients who had undergone
percutaneous nephrolithotomy (PCNL), and who also had culture results for both stone and urine. Urine culture showed no association with stone
mineral content, but stone culture was more often positive in
struvite-containing stones (73 % positive) and majority
apatite stones (65 %) than in other stone types (54 %, lower than the others, P < 0.02). In 51 patients in whom the
carbonate content of
apatite could be measured,
carbonate in the
apatite was weakly predictive of positive stone culture with an optimal cutoff value of 13.5 %
carbonate (sensitivity 0.61, specificity 0.80). In positive cultures of stones (all
mineral types combined), organisms that characteristically produce
urease were present in 71 % of the cases, with no difference in this proportion among different types of stone. In summary, the type of
mineral in the stone was predictive of positive stone culture, but this correlation is imperfect, as over half of non-
struvite, non-
apatite stones were found to harbor culturable organisms. We conclude that
mineral type is an inadequate predictor of whether a stone contains infectious organisms, and that stone culture is more likely to provide information useful to the management of patients undergoing PCNL.