The Liverpool nomogram [1] is widely used to give an indication of lower urinary tract (LUT) voiding dysfunction.  Since the nomogram percentile curves are not horizontal and maximum flow rates depend on volume voided, a simple cut-off for maximum flow rate is not useful. A method for calculating the percentile on the nomogram (which accounts for the volume voided) has been developed [2].  Interpretation of uroflowmetry is also influenced by the post-void residual volume, included in the parameter Void% (= Volume Voided / Bladder Volume), but which is not a factor included in the construction of the Liverpool nomogram, as that was compiled from asymptomatic participants.  We investigated the proportion of diagnoses from urodynamic investigations of symptomatic patients within each cell of a matrix of Liverpool nomogram percentile against Void% in a single tertiary level clinic.

A database of 6,700 anonymised male urodynamic patients yielded 4,113 records that had full records of uroflow immediately before a urodynamic pressure flow test with urodynamic diagnosis.  The Liverpool nomogram percentile and Void% from free uroflow was calculated for each patient who was able to void, and their subsequent voiding diagnosis from their pressure flow study was recorded. For each cell of the matrix (Liverpool nomogram percentile against Void%), the prevalence of pressure flow diagnoses of bladder outflow obstruction (BOO, with BOO index ≥40, =pdetQmax-2*Qmax) and detrusor underactivity (DU, with detrusor contractility index <100, =pdetQmax + 5*Qmax) were calculated.

For each range of free uroflow results grouped by Liverpool percentile (representing LUT voiding function) and Void% (representing LUT efficiency), the prevalence of BOO and DU was recorded in a matrix (Figure 1). The matrix enables clinical staff and patients to examine the patient’s free uroflow position in the matrix and be aware of the likelihood of each condition in their case.
The correlation between percentiles for a given patient’s free uroflow and pressure flow studies was 0.38 (weak). The correlation between Void% for free uroflow and pressure flow study was 0.55 (moderate).

It is intended that the matrix could lead to better counselling of patients after a free uroflow test to help them realise the need for any further investigations.  It could also serve as a tool for training clinicians in the uncertainties inherent in uroflowmetry testing. The weak correlation (0.38) between free flow and pressure-flow percentiles for each patient indicates checks on the stability of Liverpool percentile per patient should be investigated. Some variability, however, could be explained by the presence of the pressure measurement catheter, the invasive nature of the investigation, the clinical environment and staff being sometimes present during the voiding pressure flow study.
Further work will involve using separate uroflowmetry clinic data rather than pre-urodynamic flow data to give more representative flows, revising the % ranges used in each matrix cell to improve clinical utility and introducing colour coding for the prevalence data to assist comprehension.  Separate tables could also be considered, one each for prevalence of BOO and for DU. However, it is likely to be more useful for patient counselling with the data in one single matrix.

The matrix presented here enables better counselling of patients after a free uroflow test to help them realise the need for any further investigations.  It also serves as an educational tool for training clinicians to emphasise the uncertainties inherent in uroflowmetry testing. Further refinements of the matrix will mitigate against the limitations recognised above.

Haylen, B.T. et al. Maximum and Average Urine Flow Rates in Normal Male and Female Populations—the Liverpool Nomograms. British Journal of Urology 1989; 64: 30-38.
A Sutton et al. Using two nomogram percentiles and post void residual in the analysis of a large male uroflowmetry database. Continence 2023; 7(S1):100920