Comparison of repeatability of non-invasive and invasive urodynamics

We have developed a method for the non-invasive measurement of urinary bladder pressure, and we apply this method in a longitudinal study of changes in bladder contractility in response to prostatic enlargement. In each volunteer in this study, we measure the bladder pressure twice. In the present study we have used this data to compare the repeatability of the non-invasive method to that of pressure-flow studies in a comparable population of patients.

Difference plots were made of non-invasive bladder pressure measurements in 457 volunteers and of pressure-flow studies in a comparable population of 397 male patients. To compare the repeatability of two different methods for clinical measurement, the standard deviation of differences between repeated measurements in one individual needs to be normalised. Often a normalisation by dividing by the mean is done. We show that that normalisation may lead to erroneous results. We have normalised the standard deviations by dividing by the difference between the 97.5th and 2.5th percentile of the mean of the two observations in each subject.

Normalised repeatability of the non-invasive method was 0.15, that of the various parameters derived from the pressure-flow studies varied from 0.11 to 0.22.

We conclude that the repeatability of the tested non-invasive urodynamic method is comparable to, or slightly better than, that of pressure-flow studies. We further conclude that normalising standard deviations of differences by dividing by the difference between the 97.5th and 2.5th percentile of the mean is a suitable method to compare the repeatability of different methods for clinical measurement.