Urodynamics is the gold standard in the diagnosis of bladder outflow obstruction (BOO) in patients with benign prostatic enlargement, but the clinical value of invasive urodynamic testing in daily practice has been criticized. In the past (<30 years ago), not much effort has been made to improve the clinical method for diagnosing BOO in older men, probably because of the acceptable success rate of the therapy available at the time. Prostate (transurethral) resection was performed alone or mainly in patients with clinically large prostates and alternatives to surgery were not available.
The number of alternative treatments currently available has grown rapidly and the barrier to seeking medical care for lower urinary tract symptoms (LUTS) appears to have diminished, possibly because of the availability of these alternatives.
Selection criteria are needed to take advantage of less invasive therapies and the low barrier to seeking care. Improving the clinical diagnosis can become useful for the stratification of patients. Patients who cannot be treated medically or conservatively may be eligible for surgery or one of the other invasive treatments. However, symptoms are not a good predictor of the existence of BOO. Clinical assessment in secondary and / or referred care includes, in addition to the bladder diary, assessment of flow rate and PVR and assessment of prostate size (as an optional test in most guidelines). We assessed the value of uroflowmetry and prostate size determination to predict BOO in a little invasive manner, using invasive pressure flow studies (PFS) as the comparator.

We performed cystometry with PFS in 759 men with LUTS flowrates and prostate-sizes were included in this retrospective data. All patients were referred to our clinic because of signs and symptoms of LUT dysfunction. Patients unable to void during urodynamics  due to situational inhibition and were not included in this analysis. All patients were considered neurologically normal based on history, symptoms and physical examination (no motor, sensory or reflex deficits). Urine sediment and culture were negative at the time of urodynamics. Transurethral double lumen cathter urodynamics  with water filled external pressures with reference to atmospheric pressure at the sympysis level was done and voiding was allowed immediately after strong desire to void at a volume that was comparable to the voiding diary volumes (taking PVR at earlier assessment into account). Most men have voided in standing position.

Mean age of all men was 69,9 y (sd 10,7y) and 35 men were <45y. Mean Qmax was 9,7mL/s (s.d. 5mL/s) with a mean voided volume of 296,5mL (s.d. 150mL) and PVR 93mL (s.d.150mL) 531 men voided with PVR <100mL. Mean prostate size (transrectal ultrasound LxWxHx.52) was 41,7cm3 with sd 24,5cm3. Mean IPSS was 16,7 (s.d 6,4) with QuOL 3,6 (s.d. 1,4). 
Statistical analysis demonstrated that Qmax and prostate volume had relevant weight to predict BOO with (Pearsons) correlation coeffecients of -.562 (p.000) and .343 (p.000) with BOOI respectively. Also PVR (and voided volume were significantly associated with BOOI (.158 (.000) and -.405 (.000) respectively). 
We have based on the statistics, derived a clinical prostate score that we based on measured prostate size and Qmax: Prostate size - 3xQmax. Further analysis demonstrated that with this index 226 of the 256 men without BOO (88,3%) could be predicted correctly when the index was negative. In other words: when the flowrate multiplied by 3 is larger than the prostate volume in cm3 (or mL); the chance that a man has no bladder outflow obstruciton is 88%. 39 were falsely predicted to have no BOO. If the score was positive (prostate size larger than 3 times flowrate) 36% had no BOO. The sensitivity to detect noBOO is high. The sensitivity of especialy IPSS to detect (no) BOO is very much lower. We show ROC curves of the score (CLIPS) and TRUS and Qmax in the figure the index (CLIPS) has an area under the curve of .850.

Combining prostate measured volume and Qmax provides a simple means to exclude BOO in men with symptoms and signs of LUTD. We assume that this is a useful rule of thumb to exlude patients from invasive management and or the necessity of invasive diagnostics (to diagnose the necessity of invasive treatment).

When a combination of prostate volume and Qmax was used, an 88% reliable prediction of the absence of bladder outflow obstruction is possible when 3x qmax is larger than the size of the prostate size in mL.