Diagnosis and Grading of Vesicoureteral Reflux

Editor: We read with interest the article by Dr Berrocal and colleagues in the November 2001 issue of Radiology (1). Although we agree with the overall results, we would like to raise several issues. In the Results section of the abstract, the authors state that “twenty-nine of the 94 units showed VUR [vesicoureteral reflux] at only cystosonography, and 10 units at only VCUG [voiding cystourethrography].” In fact, on the basis of results presented in table 3, the 29 units identified with cystosonography are separate from the 94 that were identified with both cystosonography and VCUG. The statement should read: Ninety-four units showed VUR with both methods; 29 units, with only cystosonography; and 10 units, with only VCUG. Furthermore, the authors state that “the McNemar test for patients showed no significant difference. . . .” Although this is true with our calculation (P .23), this result is not found in the article. Finally, in the Conclusion section of the abstract, the sentence should read: Depiction of VUR at cystosonography with SH U 508A is more sensitive than that with conventional VCUG. In the Materials and Methods section, the authors state “For statistical analysis, VCUG was considered the reference standard. Cases in which VUR was detected at only cystosonography were not included.” However, in their statistical analysis, the authors compared the detection rate of cystosonography with that of VCUG and did not exclude the cases detected with only cystosongraphy. Cystosonography was compared with VCUG by using the McNemar test for symmetry and was found to be superior in the detection of VUR, which was not found with VCUG. Sensitivity and specificity of cystosonography were not reported with VCUG as the standard, and VCUG was not used as a reference standard in the analysis. The use of the McNemar test for kidney-ureter units is problematic. Four hundred forty kidney units were present in 216 patients. Each patient contributed two or three kidney units. Since the detection of VUR depends on the filling of the bladder, as well as the timing of imaging relative to micturition, detection of VUR in each of the multiple kidney units within a single patient is not an independent observation. Thus, there are not 440 independent observations in this study. A corrected form of the McNemar test is required in this situation. Furthermore, the use of McNemar test for comparison of the grade of VUR implies that the McNemar test was performed with a 5 5 table (since there are five grades of VUR). When the McNemar test is performed with a table that is larger than 2 2, one should specify how discordant pairs are weighted—when a discordant pair is further off of the diagonal, a greater weight should be assigned to its deviation from true agreement. A standard weighting scheme is the quadratic technique, but we do not know what method was used in this study. It is interesting that results of the McNemar test for the comparison of grades of VUR by patient were significant (P .05), whereas those for the detection of VUR by patient were not significant. Finally, we do not understand why the authors report the statistic with an SD. The statistic is used as a measure of agreement and is generally presented as a single number in the range of 0–1. We have not seen other articles in which SD is presented for a statistic. Furthermore, since random agreement will send the value to zero, it is not obvious that the value of is normally distributed. The SD of does not appear to add any useful information to this article.