Infected stones frequently missed by urine cultures

June 1, 2010

Perioperative urine cultures yield a high number of false-negative and false-positive results and do not accurately identify infective organisms in a large percentage of percutaneous nephrolithotomy patients.

San Francisco-Perioperative urine cultures yield a high number of false-negative and false-positive results and do not accurately identify infective organisms in a large percentage of percutaneous nephrolithotomy (PNL) patients, according to study results presented at the AUA annual meeting here.

Previous studies have suggested that the risk of septic events following PNL correlates best with stone culture and worst with voided urine culture, said senior author James E. Lingeman, MD, adjunct clinical professor of urology at Indiana University School of Medicine and co-director of the International Kidney Stone Institute, Indianapolis.

He added that because many PNL patients have received antibiotic treatment from primary care physicians before surgery, the urine may be negative.

"You can be easily misled into thinking the urine is sterile when it is not and treating people with the wrong antibiotic prior to surgery," Dr. Lingeman said.

Another reason to consider routine stone culture is that the stone may be infected with a different organism than that infecting the bladder and urine, said co-author James C. Williams, MD, professor of anatomy and cell biology at Indiana University. This is particularly important in women because the predominant bacteria in urine may be something other than what exists in the stone.

Dr. Lingeman, Dr. Williams, and colleagues reviewed intraoperative stone culture results from 548 patients undergoing PNL between April 1999 and May 2009, among whom 498 had a preoperative urine culture for comparison. A positive stone culture was recorded for 205 of the 498 patients, and of these, 152 (74.1%) also had a positive urine culture, amounting to a false-negative result for 25.9% of patients. There were also 235 patients (47.2%) with a positive urine culture, and in 83 (35.3%) of these cases there was no infection found upon stone culture.

Urine cultures miss bacteria

Staphylococcus species were found in 25.7% of stone cultures but in only 16.2% of urine cultures, Enterococcus species were present in 14% of stone cultures versus 8.9% of urine cultures, and Candida organisms were identified in 8.4% of stone cultures compared with 5.5% of urine cultures, Dr. Williams reported. Bacteria identified with urine cultures that were not present in the patients' stone cultures included Escherichia coli and Proteus, Klebsiella, Pseudomonas, and Streptococcus species.

"Doing a culture of the stone is not something that urologists traditionally do," said Dr. Lingeman. However, it is an inexpensive, easy test, and PNL is an opportunity to culture the stone so if the patient has a fever or is sick postoperatively, the correct antibiotic can be prescribed.

"Do not pick up or handle the stone with gloved hands because the gloves touch the patients' skin and often are contaminated with Staphylococcus epidermidis," Dr. Lingeman advised. "We found if we picked the stone up and put it in the culture tube, we had quite a number of S. epidermidis infections."

Instead, Dr. Lingeman recommends using grasping forceps through the nephroscope to transfer a stone fragment directly into the culture cup.

"Then we grind it with a hemostat or something similar that we have not handled or touched previously," he said. "If the stone culture is positive, that gives us the information we need to keep the patient on antibiotics for 3 months after surgery."

Data on whether routine intraoperative stone cultures reduce postoperative septic events is being gathered by the Endourology Society, which has an international database comprising approximately 100 institutions.

"One of the questions we will be asking is whether our rate of septic events is better than that seen in institutions that don't use stone cultures," Dr. Lingeman said.