To study the causes for the variation between measurements, which is considerable, in maximal flow rate (Qmax) and the associated detrusor pressure (PdetQmax). Because of the central role of Qmax and PdetQmax in the diagnosis of bladder outlet obstruction, this is reason for concern.

Spectral analysis was carried out on two consecutive urodynamic measurements in 131 patients. The parameters for bladder outlet resistance and bladder contraction strength were determined, and difference plots were made to study the systematic variations. Logistic regression analysis was used to study whether the differences represent true changes of the function of the lower urinary tract.

Signal components in the detrusor pressure and the flow rate signal with frequencies of 1 Hz or greater may be considered noise. Filtering out these frequencies changes the estimates of Qmax and PdetQmax, but not the between-measurement difference in them. Bladder contractility and bladder outlet resistance were systematically lower in the second measurement. Both the systematic and nonsystematic between-measurement variations were statistically significant predictors for postvoid residual urine volume.

The nonsystematic between-measurement variability in Qmax and PdetQmax apparently reflects true variability in the physiologic state of the bladder outlet. It therefore does not discredit the pressure-flow study as the preferred method in the diagnosis of bladder outlet obstruction. Rather, the pressure-flow study is the only currently available method to study and quantify the apparent within-patient variability in bladder outlet resistance and bladder contractility.