The value of multiple free-flow studies in men with lower urinary tract symptoms

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To assess the variability of free-flow studies in men presenting with lower urinary tract symptoms (LUTS) suggestive of benign prostatic obstruction (BPO) and to determine the sensitivity, specificity and predictive values of consecutive measurements of maximum flow rate for the presence of bladder outlet obstruction (BOO) at several threshold values.

Patients and methods

The value of multiple free-flow studies was assessed in 165 men presenting with LUTS suggestive of BPO. Each patient was requested to void four times into a uroflowmeter and the voided volume and post-void residual urine volume (PVR) were also measured. The variability of the maximum flow rate (Qmax), voided volume and PVR between consecutive voids was assessed. In addition, the sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of various thresholds of Qmax for the presence of BOO were calculated for each consecutive void.


The mean Qmax on void 1 was 10.2 mL/s and the mean maximum value for Qmax between voids 1 and 2 was 12.5 mL/s. For voids 1, 2 and 3, the mean maximum Qmax was 13.9 mL/s and for voids 1 to 4 it was 15.2 mL/s. There were no significant changes in PVR among any of these voids. There was a statistically significant, although small, decrease in voided volume between voids 1 to 3 and voids 1 to 4. The specificity and PPV of Qmax for BOO increased with each subsequent void, such that using a threshold value for Qmax of 10 mL/s on the fourth void, the specificity and PPV for BOO were 96% and 93%, respectively.


There was a significant increase in Qmax with each successive void when men with LUTS suggestive of BPO performed multiple free-flow measurements and consequently, single free-flow measurements substantially underestimated the maximum Qmax that these patients achieved. The specificity and PPV of Qmax for BOO can be improved considerably by performing multiple free-flow studies and by carefully selecting an appropriate threshold value (although whether pressure-flow studies are unnecessary will depend on what level of specificity and PPV is deemed acceptable in clinical practice). These findings should be considered if free-flow studies are to be used as the basis for deciding the clinical management of men with LUTS and may be particularly useful for urologists with limited facilities for pressure-flow studies.

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