Sensitivity of noncontrast helical computerized tomography and plain film radiography compared to flexible nephroscopy for detecting residual fragments after percutaneous nephrostolithotomy

Purpose: We prospectively compared plain film radiography and noncontrast, thin cut helical computerized tomography (CT) to flexible nephroscopy for detecting residual stones after percutaneous nephrostolithotomy. Materials and Methods: We prospectively evaluated 36 patients (41 renal units) undergoing percutaneous nephrostolithotomy for large (greater than 3 cm., 23 renal units) or staghorn (18 renal units) calculi. All patients underwent postoperative imaging with plain film of the kidneys, ureters and bladder and noncontrast helical CT, and flexible nephroscopy on postoperative day 2 or 3. The size and location of residual fragments determined radiographically and identified by flexible nephroscopy were compared. Results: Plain film radiography and CT detected an average of 0.7 and 3.4 stones per renal unit, respectively. With a mean operating time plus or minus standard deviation of 77.3 ± 35 minutes and a mean fluoroscopy time of 7.6 ± 6.7 minutes, an average of 2.3 stones per renal unit were retrieved at flexible nephroscopy. In 90.2% of renal units all calices could be directly inspected. The sensitivity and specificity were 46% and 82% for plain film radiography, and 100% and 62% for CT, respectively, using flexible nephroscopy as the gold standard for detecting residual stones. The overall stone-free rate after flexible nephroscopy was 92.6%. The cost of this procedure is $5,625.13 compared to $220 for CT, including the interpretation fee, at our institution. Conclusions: Selective use of flexible nephroscopy after percutaneous nephrostolithotomy based on positive CT findings will avoid an unnecessary operation in 20% of patients. The rate of unnecessary procedures is 32% if all patients undergo flexible nephroscopy, regardless of radiographic findings. At our institution this strategy will result in a cost savings of $109,687 per 100 patients.