The effect of a urethral catheter on the measurement of maximum urinary flow rate.

The maximum urinary flow rates of 147 male subjects referred for voiding difficulties were measured in the presence and absence of a fine urethral catheter. A comparison of these flow rates showed that those measured with the catheter in situ were lower than would be anticipated for the same voided volumes in the absence of a catheter. Although the reduction in flow rate caused by the catheter was only of the order of several ml./second, it was sufficient to alter the diagnostic categorisation of almost a third of the subjects classified as obstructed, unobstructed or equivocal on the basis of a plot of detrusor voiding pressure versus maximum flow rate. The effect of the urethral catheter on diagnostic classification was most pronounced in the borderline patients on whom urodynamics would usually be performed to clarify the clinical assessment. It was concluded that the routine performance of full urodynamic studies for the assessment of outflow obstruction is unwarranted.