Residual fraction as a parameter to predict bladder outlet obstruction in men with lower urinary tract symptoms

To determine whether noninvasive tests including the residual fraction are reliable for the diagnosis of bladder outlet obstruction (BOO).

A total of 212 men (median age 68, range 44-89 years) were included in the present study. The median serum prostate-specific antigen level and prostate volume were 1.3 ng/mL (range 0.2 to 9.4) and 37.9 mL (range 11.3 to 148.0), respectively.

Among the variables analyzed in the multivariate model, the likelihood of BOO varied by the total prostate volume, with a 3.6-fold higher odds for >or=40 mL than for <40 mL (odds ratio [OR], 3.616; 95% confidence interval [CI], 1.217-10.749; P = 0.021). In the same model, a low maximal flow rate (Qmax) (OR, 2.840; 95% CI, 1.260-6.401; P = 0.012) and high residual fraction (OR, 7.103; 95% CI, 1.924-26.225; P = 0.003) were associated with an increased likelihood of BOO. The sensitivity and specificity for predicting BOO using a total prostate volume of 40 mL or greater were 73.7% and 65.2%, respectively. Using a Qmax cut-off of 12 mL/s or less for predicting BOO, the sensitivity and specificity were 77.2% and 54.2%, respectively. Prediction of the BOO by the residual fraction only had a sensitivity and specificity, for a residual fraction of less than 20%, of 75.4% and 67.7%, respectively.

The presence or absence of BOO might be predicted using non-invasive methods. The residual fraction may help with patient management by better predicting the likely patient classification from pressure-flow studies.