Post-void residual (PVR) of urine is a debated part in the assessment of patients with lower urinary tract symptoms (LUTS) due to its not standardized threshold.
Aim of this study was to measure post void residual of urine in young and healthy males and females.

This is an observational prospective multicenter ongoing study started on January 2018, involving volunteers both men and women with age 18-35 yrs. For each All the subject a detailed history and physical examination were recorded. Volunteers were asked to perform a uroflowmetry (UF) whit a normal micturition desire. A VAS scale was marked by each volunteer indicating the subjective evaluation of the voiding at UF compared to the habitual micturition at home. Peak-flow (Qmax), voided volume and the curve of the UF were registered. Post void residual of urine was measured immediately after the UF with a bladder scan. PVR-ratio, as the ratio of PVR to bladder volume (BV: voided volume + PVR) was also calculated. Males filled the International Prostate Symptoms Score (IPPS) questionnaire, while women both W-IPSS and ICIQ-FLUTS. Population was also divided according with IPSS/WIPSS severity scores: (i) scores 0-7; (ii) scores 8-19; (iii) scores 20-39. UF data was also plotted on Liverpool nomograms. 
Statistical analysis was performed using T test and Mann-Whitney test. Exclusion criteria were: lower urinary tract symptoms, urological and neurological diseases, surgery of the urinary tract or the pelvis or genitalia, pharmacological therapies, previous urethral catheterization, radiation therapy of the pelvis. Informed consent was obtained by all the volunteers.

A total amount of 114 volunteers were enrolled in the study, 45.6% (n 52) males and 54.4% (n 62) females. Mean age was 26 years old (18-35). Table 1 reports data on Qmax, PVR, PVR-ratio IPSS/WIPSS of the population. Mean ICIQ-FLUTS score was 2.8 ± 3.9, and the median 2 (1-3). According to IPSS-WIPSS scores 98.2% (n 110/112) of the volunteers reported a score <8, and 2 males (1.8%) referred an IPSS score between 8 and 19. VAS scale of the subjective evaluation of voiding was <6 in 5.3% (n 6/114) of the population. Table 2 reports outcomes according to Liverpool nomograms. A PVR of 0mL was found in 60.5% of the population (69/114), in 63% of the males (34/54) and in 56.4% of the females (35/62). A PVR> 50 ml was found in 12.3% of the cohort (14/114): 7.7% of males (4/52%) and 16.1% (10/62). A PVR-ratio >10% was found in 21% of the volunteers (24/114), in 17.3% in males (9/52) and 24.2% in females (15/62).

In a young and healthy population with regular UF parameters only 60% had no PVR, while 1/10 volunteers showed a PVR >50ml, with a double ratio in the female volunteers. 
PVR-ratio was two times higher in the female population. The PVR-ratio >10% was present in 1/5 volunteer, and more common in females.
Although our population reported no lower urinary tract symptoms at the medical history, with normal urinary symptom scores, surprisingly 17% of the volunteers showed an abnormal score at the Liverpool nomograms. Volunteers of this subgroup showed significantly lower Qmax without significant difference in PVR volumes. This data may indicate that PVR could be a poor reliable parameter of pathological bladder emptying.

In a non-negligible part of young and healthy population a post void residual of urine was documented. In females PVR was a more frequent finding with volumes two times higher. Liverpool nomograms evaluation recognized a significant part of the volunteers as pathological. This group had a significant lower Qmax, but no significant higher PVR. This data may confirm the controversial role of post void residual of urine in the evaluation of patients with voiding dysfunction.