Obtaining MSU culture has been a standard practice prior to any stone surgery. This is usually done at least a week prior to surgery and treated with appropriate antibiotics for 3–5 days. It is then repeated to confirm sterility of the urine. Antibiotic prophylaxis before endourological procedure is given in accordance with standard guidelines [5]. Even with proper pre-operative preparation and sterile urine cultures, patients still develop systemic and sometimes catastrophic infections [6, 7]. As per the literature, post-PCNL sepsis can occur in 10–15% of cases [8]. Various risk factors have been associated with sepsis with common ones including long duration of the procedure, urinary bacterial load, severity of obstruction by stone and infection in the stone [3]. Other risk factors described in the literature include use of nephrostomy tube, renal insufficiency, amount of irrigation fluid used and high fluid pressure during operative procedure [4, 9, 10]. It is a usual practice to obtain a pre-operative mid-stream urine culture and procedure is undertaken with antibiotics based on its culture results. However, several authors have reported a poor concordance between organism in the stone and bladder urine specimens [8, 11]. In the series by Fowler et al, stone culture was positive in 77% of the patients, whereas a simultaneous bladder urine sample was positive in only 12.5% of the patients [12]. Another study by Mariappan et al. also reported discordance between urine and stone culture with MSU culture being positive in only 11.1%, whereas stone culture positivity was in 35.2% cases [3]. Devraj et al in 2016 also reported stone culture positivity in 30.1% compared to MSU positivity in just 10.8% of the patients [13].
In our study, we found that the stone culture was positive in 50.82% of patients, whereas mid-stream urine culture was positive in 24.59% of patients. We also observed that urine culture has a poor sensitivity, specificity, PPV, NPV and diagnostic accuracy to predict stone culture positivity. Another interesting observation in our study was that urine cultures and stone cultures vary significantly in cases of renal stones where stone cultures were found to be positive in significantly larger patients (58.3%) as compared to pre-operative MSU cultures (29.16%) (p value < 0.0001). This, however, was not true for bladder stones. Since in patients of ureteric stones, none of cultures turned out to be positive, association cannot be made, thereby limiting our interpretation for the same. Thus, the above observation of difference between MSU and stone cultures appears to be most important in cases of renal stones.
Microbiology of stone and urine predominantly includes gram-negative bacterial infection with fewer infections by gram-positive organisms [14]. In our study, most common organism isolated in stone culture was Pseudomonas aeruginosa and Enterobacter and those in urine culture were E. coli and Pseudomonas. These results corroborate with the present literature [14]. However, we noted that most often identified organism in both samples differ which was also found in the study by MC Songra et al in 2015 [2]. In their study, they also found bacteriology of stone as a better predictor of post-operative sepsis than urine culture. Annerleim Walton‐Diaz et al in their study also demonstrated that urine culture and stone cultures were discordant and that post-operative sepsis correlated significantly with intra-operative stone cultures rather than pre-operative urine culture [8].
Thus, stones in kidney have higher chances of harbouring microorganisms when compared with urine and a negative urine culture does not rule out infection within the stone. Microbiological environment of the stone is also quite different from that found in urine. Thus, stones remain one of the most important sources of sepsis post-operatively. Various studies have demonstrated positive association of stone culture and post-operative sepsis specially when pre-operative urine cultures were negative [2, 3, 8, 13]. Stone cultures can be easily obtained at the time of surgery at minimal additional cost. This not only reduces the hospital stay but overall cost of treating an episode of post operative sepsis. This not only reduces the hospital stay but overall cost of treating an episode of post operative sepsis. This study thereby brings to light the importance of
intra-operative stone culture in addition to routine use of
urine culture.
