Flow resistive forces index (QRF): Development and clinical applicability assessment of a novel measure of bladder outlet resistance, aiming to enhance the diagnostic performance of uroflowmetry

Low Urin Tract Symptoms. 2020 Sep;12(3):190-197. doi: 10.1111/luts.12301. Epub 2020 Jan 30.


Background: Currently, the diagnostic ability of uroflowmetry, the most widely used urodynamic test available for initial assessment of patients with lower urinary tract symptoms (LUTS), is considered limited by its inability to accurately discriminate between the underlying mechanisms of this condition. To improve the diagnostic accuracy of uroflow, we developed a mathematical formula that calculates the flow resistive forces index (QRF), a novel measure of bladder outflow/urethral resistance, and assessed its clinical applicability compared to the maximum flow rate (Qmax ).

Materials and methods: A cross-sectional observational study was conducted in a cohort of 61 adult men presenting with voiding dysfunction symptoms, who all underwent free uroflowmetry followed by pressure flow study. The development of the mathematical formula which contains five key uroflowmetry variables (voided volume, flow time, Qmax , average flow rate, and peak flow time) was based on the assumption that urine volume momentum changes during voiding, the concept of diphasic uroflow pattern (acceleration/deceleration), and the urethral resistance factor (URA) equation. Study subjects were classified either as obstructed or nonobstructed according to established urodynamic criteria (linearized passive urethral resistance relation, LinPURR; Abrams-Griffiths number, AGN [also called bladder outlet obstruction index, BOOI]; and URA). Univariate linear correlations, binary logistic regression model, and receiver operating characteristic (ROC) curve statistical analysis were employed (SPSS-22, MedCalc, GraphPad [P < .05]).

Results: Outflow obstruction was diagnosed in 50.8% (1 in 2) patients. Univariate analysis, and bivariate linear correlation, binary logistic regression, and ROC curve analyses showed that the QRF was a strong independent predictor of bladder outlet/outflow obstruction (BOO), significantly outperforming Qmax .

Conclusions: QRF index accurately predicts BOO, significantly outperforming the currently widely used bladder outlet obstruction estimator Qmax . Despite potential study limitations (mainly small cohort size and lack of control group), we anticipate that with further study and proper clinical validation, QRF could become a valuable complement to uroflowmetry.

Keywords: bladder outflow resistance; bladder outlet obstruction; flow resistive forces index (QRF); maximum flow rate (Qmax); uroflowmetry.

Publication types

  • Observational Study

MeSH terms

  • Aged
  • Cross-Sectional Studies
  • Humans
  • Linear Models
  • Male
  • Middle Aged
  • ROC Curve
  • Rheology / methods*
  • Rheology / statistics & numerical data*
  • Sensitivity and Specificity
  • Urethra / physiopathology
  • Urinary Bladder / physiopathology*
  • Urinary Bladder Neck Obstruction / diagnosis*
  • Urinary Bladder Neck Obstruction / physiopathology*
  • Urination / physiology*
  • Urodynamics