Low reproducibility of maximum urinary flow rate determined by portable flowmetry

To evaluate the reproducibility in maximum urinary now rate (Q(max)) in men with lower urinary tract symptoms (LUTSs) and to determine the number of f lows needed to obtain a specified reliability in mean Q(max), 212 patients with LUTSs (mean age, 62 years) referred to the University Hospital Niljmeg en, with various degrees of obstruction on pressure-flow studies, used a po rtable home-based uroflowmeter with 12 disposable beakers. Voided volume an d maximum flow rate were recorded continuously during micturition. Flows wi th voided volumes of at least 100 mt and without possible artifacts were in cluded. All analyses were repeated while excluding flows with voided volume s <150 mi. A coefficient of variation (CV) was calculated for each patient. The CV represents the standard deviation relative to the mean. All individ ual CVs were subsequently pooled into a population mean CV. This parameter was used to estimate the number of flows required to obtain a mean Q(max) w ith specified reliability for an individual patient. All analyses were repe ated, while successively excluding the first, the first two, and the first three flows, to assess a possible learning curve. A total of 1,854 flows wa s available for analyses, yielding an average of nine flows per patient. Me an Q(max) was 13.2 ml/sec; the mean CV was 24%. To allow, for instance, a 1 0% deviation from the true mean Q(max) (e.g., 15 ml/s +/- 1.5 ml/s), approx imately 25 flows are necessary. The actual number of flows needed is in fac t even higher due to the presence of small and artifactual flows. Using a 1 50 mi volume cutoff point, somewhat fewer flows are required, but the total number of flows needed (that is, valid, small, and artifactual flows) incr eases. There was no evidence of a learning curve. The boundaries of a confi dence interval around a single Q(max) measurement that is likely to contain the true mean Q(max), lie approximately 50% below or above that single Q(m ax) measurement. To reduce this proportion down to 10%, approximately 25 fl ows are needed. Thus, to obtain reliable mean Q(max) values, considerably m ore flows are required than are normally performed in urologic practice. (C ) 1999 Wiley-Liss,Inc.