RADIATION DOSES IN PRONE VERSUS SUPINE POSITION DURING PERCUTANEOUS NEPHROLITHOTOMY, RESULTS WITH AN ANTHROPOMORPHIC MODEL

INTRODUCTION AND OBJECTIVES: Percutaneous nephrolithotomy (PNL) remains an effective treatment for patients with a large stone burden. When a nephrostomy tube (NT) is left in place postoperatively, antegrade flow of urine is often confirmed with antegrade nephrostography (ANG) as edema from a chronically obstructing stone can resolve in variable amounts of time. Alternatively, antegrade flow can be estimated by injecting methylene blue dye into the NT or performing a NT capping trial. We compared the methylene blue dye test and capping trial against ANG to assess antegrade urine flow. METHODS: Consecutive patients undergoing PNL at 2 hospitals were prospectively enrolled between July and October, 2014. A cap was placed on the NT on the morning of postoperative day 1 (POD1). Capping trial failure was defined as need to uncap NT for any reason including increased pain or fever. 2 hours after capping, 7cc of methylene blue was injected into the NT and the tube recapped. Positive test was defined as the presence of blue per urethral Foley. Later that afternoon, ANG was performed to radiographically document antegrade urine flow. Sensitivity, specificity, positive (PPV) and negative predictive values (NPV) were calculated comparing capping and methylene blue tests against ANG. RESULTS: 34 subjects underwent PNL. 55.9% were left sided and 73.5% were lower pole punctures (mean age 54.9 13.9 years; mean BMI 31.7 12.7kg/m2; mean stone size 2.86 1.42cm). Capping trial was successful in 86.7% of patients. Compared to ANG, it had a sensitivity of 94.4% (CI 83.9-100%), specificity 25.0% (CI 0.5-49.5%), PPV 65.4% (47.1-83.7%), and NPV 75.0% (CI 32.6-100%) to predict antegrade urine flow. The methylene blue test was positive in 41.4% of patients. Compared to ANG, it had a sensitivity of 52.9% (CI 29.2-76.7%), specificity 75.0% (CI 50.5-99.5%), PPV 75.0% (CI 50.5-99.5%), and NPV 52.9% (CI 29.2-76.7%) to predict antegrade urine flow. CONCLUSIONS: Compared to ANG, a capping trial and methylene blue test are 94% and 53% sensitive and 25% and 75% specific respectively for confirming antegrade urine flow following PNL. In clinical practice, these tests may potentially be used in combination to obviate the need for ANG, which can be redundant and timeconsuming.