A new nomogram of urinary flow rate and volume based on multiple measurements per healthy adult Japanese men using a portable uroflowmeter (P-Flowdiary®)

We successfully developed new nomograms of urinary volume and flow based on the data of a sufficient number of Japanese men without LUTS and multiple flows per participant with clear characteristics. Our nomograms are almost consistent with previous nomograms, with some differences. The 2 previous nomograms were fit for nonlinear regression models, similar to ours [10, 11]. In those nomograms, including ours, the increase in MFR is very low, at approximately ≥ 400 mL. However, the Liverpool nomogram was fit for a linear regression model. MFR gradually increased with VV up to 600 mL [12], which can be attributed to the data of single micturition per participant who was instructed to attend with full bladder. Our nomogram compensates for the shortcomings of previous nomograms and may be the most ideal nomogram to screen men aged < 60 years for voiding dysfunction.

The urinary flow may be different among races and/or ages. Although the mean MFR was around 25 mL/s at approximately 400 mL in the previous Japanese nomogram [11] and ours, the mean MFR was around 30 mL/s at approximately 400 mL in the Canadian [10] and Liverpool nomograms [12]. One of the reasons might be the difference in body mass index, which may affect bladder outlet condition and detrusor contraction. BMI is higher in Western populations than in Asian populations [16]. A higher BMI correponds to a higher prevalence of metabolic syndrome, which may cause peri-urethral fibrosis that contributes to bladder outlet obstruction [17]. If bladder outlet obstruction occurs, the bladder goes through three stages: an initial hypertrophy phase, a subsequent compensation, and a late decompensation [18]. In the initial hypertrophy phase, enhanced detrusor contraction may increase urinary flow. Hence, Western populations may be more likely to be in this state than Japanese populations. In the study of the Liverpool nomogram, the MFRs decreased with age (1.0–1.6 mL/s/10 years) [12]. In previous studies that enrolled relatively younger participants including teenagers, the mean MFRs were relatively higher. In a Thai study, the participants were 18–30 years old, and the mean MFR was 31.2 ± 9.0 mL/s [19]. In an Austrian study, the participants were 18 years old, and the mean MFR was 28.4 ± 9.7 mL/s [20]. In an Indian study, the participants were 15–40 years old, and the mean MFR was 27.3 ± 6.7 mL/s [21]. However, in another Indian study, the participants were 15–50 years old, and the mean MFR was 22.8 ± 9.3 mL/s, which was similar to our study [22]. In the present study, the mean MFR was significantly lower in the 50–59-year age group (~ 22 mL/s) than in the younger groups (~ 24 mL/s), but the mean MFRs were not significantly different among the 20–29-, 30–39-, and 40–49-year age groups. It might be because of the prostate volume. A previous cohort study in Japan demonstrated that the proportion of men older than 50 years with prostate larger than 20 mL was 35%, whereas that of men aged 40–49 years was 20.0% [23].

MFR gradually increased with VV to a certain threshold, followed by a decrease in 66 out of 101 participants (167 out of 894 data sets). In these participants, the MFR decreased by 3.08 ± 2.5 mL/s at more than the threshold VV (~ 300 mL) in comparison with the MFR at the threshold. In the Indian study, the mean MFR was lower at > 750 mL than at ≤ 750 mL [21]. In the Austrian study, the threshold was 550 mL [20]. One of the reasons may be that bladder overdistension leads to weaker detrusor contraction, which was demonstrated in the animal experiment using foxhounds [24].

The present study has some limitations that need to be addressed. First, we did not collect data from women, but we have a plan to study the use of P-Flowdiary® in women, who can utilize the optional portable toilet seat. Second, we did not collect data from men older than 60 years. However, defining “healthy” micturition is not easy because the simple consistency between LUTS and urinary flow may be lower with age [14]. Lower urinary tract dysfunction due to benign prostatic hyperplasia, overactive bladder, and underactive bladder influences consistency in aged men. The “healthy” flow curve of the 50–59-year age group or a treatment target of aged men with lower urinary tract dysfunction may be an ideal. Therefore, the nomograms of men younger than 60 years might be enough.

In conclusion, the quadratic regression model was fit for the nomogram of the relationship between MFR and VV. The nomogram, which can predict MFR by VV in Japanese adult men aged 20–59 years without LUTS, is a useful tool to assist in diagnosing voiding dysfunction.

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