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

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.

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.

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.

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.