High carbonate level of apatite in kidney stones implies infection, but is it predictive?

Abstract

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, 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 which 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 percutaneous nephrolithotomy.

Evaluating factors that dictate struvite stone composition: A multi-institutional clinical experience from the EDGE Research Consortium

“…Furthermore, classically described urease producing bacteria were isolated in only 30% of cases indicating that even urinary bacterial culture is not a reliable predictor for the presence of infection stones during preoperative evaluation. Similar to previous reports in the literature, we found significant heterogeneity among the bacterial isolates, [17][18][19] with the two most common being E. coli and Proteus. While this may suggest an increase in the number of urease-positive E. coli strains, it is more likely that patients are suffering from multi-species urinary tract infections, and E. coli is the most commonly isolated species due to the fact that its short replication rate results in it outcompeting and outgrowing other bacterial species.…”

Preoperative Bladder Urine Culture as a Predictor of Intraoperative Stone Culture Results: Clinical Implications and Relationship to Stone Composition

“…However, Englert et al showed that among positive stone cultures, urease producing organisms were cultured in 71% of cases with no difference observed among stones of different compositions. 23 They concluded that mineral content is an imperfect indicator of the presence of infectious organisms in kidney stones, an observation consistent with the data reported here. It may be that some struvite stones developed in the presence of bacteriuria but no longer contain viable organisms.…”

Evaluation of Biofilm Induced Urinary Infection Stone Formation in a Novel Laboratory Model System

“…Carbonate apatite is commonly found in struvite stones. 7 Similarly In addition to mineral composition, the in vitro and in vivo samples showed crystals of similar shape and as typical for struvite ( fig. 3, A and D).…”