External validation of Solomon-Greenwell nomogram for female bladder outlet obstruction

Abstract

Aim: There is no unified diagnostic standard for female bladder outlet obstruction (BOO) to date. The Solomon-Greenwell (S-G) nomogram was developed to indicate the probability of female BOO by performing a pressure-flow study, and the equation of the BOO Index in females (BOOIf) is PdetQmax - 2.2 × Qmax. We aimed to validate the diagnostic value of the S-G nomogram in female BOO.

Materials and methods: We retrospectively reviewed a videourodynamic study (VUDS) cohort in our institution. Between 2015 and 2020, 192 female patients underwent VUDS for lower urinary tract dysfunction (LUTD). We excluded patients with neurogenic LUTD (n = 30) and patients with no detrusor contraction and/or no void during VUDS (n = 51). The diagnosis of female BOO was based on the Nitti criteria (radiological evidence of urethral narrowing in the presence of a sustained detrusor pressure). BOOIf was calculated for each enrolled patient. The cutoff values of BOOIf were set at <0, >5, and >18 as the original S-G nomogram proposed. The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of each threshold to diagnose female BOO were calculated.

Results: Out of the 111 enrolled patients, 43 (38.7%) were diagnosed as having female BOO by VUDS. The most common etiology of female BOO was dysfunctional voiding (19/43, 44.2%), followed by primary bladder neck obstruction (PBNO, 15/43, 34.9%). When the cutoff value was <0 (low probability of obstruction), the sensitivity, specificity, PPV, and NPV were 90%, 91%, 92%, and 87%, respectively; when >5 (likely obstructed), the values were 79%, 96%, 92%, and 88%, respectively; and when >18 (obstruction almost certain), the values were 47%, 100%, 100%, and 75%, respectively. Fourteen of 15 PBNO patients would be classified as non-BOO if the cutoff value was >18. Six PBNO patients would not be diagnosed as female BOO if the threshold was >5.

Conclusion: A BOOIf <0 showed good diagnostic value for excluding female BOO. A BOOIf >18 had perfect specificity and PPV for diagnosing female BOO. However, the sensitivity of the S-G nomogram for detecting female BOO was unsatisfactory, especially for patients with PBNO. VUDS remains the examination of choice for patients with suspected female BOO.

Keywords: bladder outlet obstruction; diagnosis; female; nomograms; videourodynamic study.