To characterize patients who form nonstruvite stones associated with infection (secondarily infected calculi), and to define the bacteria associated with these.

Patients undergoing percutaneous nephrolithotomy were prospectively recruited. Medical records were reviewed, and stones were analyzed using micro computed tomography and infrared spectroscopy. A fragment of each stone was sent for bacterial culture. Patients were categorized by stone culture results (SC±) and the presence of struvite (ST±). The Fisher exact test was used for comparison of proportion. Sterility of intraoperative SC was established with independently collected controls.

In total, 125 patients were enrolled: 24 SC+/ST−, 19 SC+/ST+, and 82 SC−/ST−. Proportions of patients with prior urologic surgery, diabetes, and immunodeficiency were similar between groups. Patients with neurogenic bladder were more likely to have SC+/ST+ stones or SC+/ST− stones than SC−/ST− stones (26% vs 8% vs 0%, respectively; P <.01). Among patients with metabolic evaluations, hypocitraturia was found in 31.6% (6 of 19) SC+/ST− patients, 46.7% (7 of 15) SC+/ST+ patients, and 26.0% (19 of 73) of SC−/ST− patients (P = .28). Approximately 40% of cultured organisms in the secondarily infected calculi possessed urease and another 40% citrate lyase activities.

Secondarily infected stones were detected in approximately 20% of this surgical cohort and may be more common than previously appreciated. Neurogenic bladder appeared to predispose patients to either struvite or secondarily infected stones. The role of bacterial infection in stone formation is unclear but may include alteration of urinary components, acting as a nidus for crystallization, or inducing inflammation.