Use of residual fraction instead of residual volume in the evaluation of lower urinary tract symptoms.

Assessment of postvoid residual volume (PVR) has become a valuable routine investigation in the evaluation of bladder outlet obstruction. PVR has been shown to have interindividual and intraindividual variation and dependence on prevoid urinary volume, thus raising a question about its significance. The aim of this study was to investigate an alternate parameter more reliable than PVR, described as residual fraction (RF) and calculated as (PVR x 100)/prevoid volume. Ninety-three adult patients with lower urinary tract symptoms (LUTS) presenting to the urology outpatient clinic were evaluated for bladder outlet obstruction. Patients with urinary retention and neurological disorders were excluded. Evaluation was by clinical assessment, uroflowmetry, and ultrasound bladder for prevoid and postvoid urine volume estimation. The latter was compared with RF. Results were statistically analyzed using bivariate analysis and Spearman's test. In 93 evaluable patients, there were 87 (94%) males and 6 (6%) females. Residual volume ranged from 4 to 450 mL (mean 91.4+/-92.7 mL). Peak flow rate (Qmax) varied from 3 to 49 mL/s (mean 9+/-15 mLis). Two-tailed correlation between PVR and Qmax was significant at the .05 level, whereas a more significant R value at .01 was observed between RF and Qmax. There was a strong positive correlation between RF and PVR with Qmax. RF statistically correlated better with Qmax than PVR. It is recommended that RF be used instead of PVR in the routine noninvasive evaluation of LUTS.