Sonographic assessment of postvoid residual urine volumes in patients with benign prostatic hyperplasia.

OBJECTIVE

To derive a formula that defines the postvoid residual (PVR) urine volume more accurately in patients with prostatic gland enlargement.

DESIGN

Prospective.

SETTING

Department of Radiology, University of Ilorin Teaching Hospital, Ilorin.

SUBJECT

Fifty-two consecutive patients with benign prostatic hyperplasia. The mean age was 64.98+/-9.57 years.

METHOD

PVR urine was evaluated by ultrasonography. Each patient had two examinations, the first of which was with a full bladder and the second of which was immediately after voiding. Two orthogonal diameters were measured on each bladder section (longitudinal and transverse) in the supine position. Fifty-two paired sets of ultrasonic measurements were thus obtained. Catheterized postvoid urine residue was regarded as the gold standard.

RESULTS

Using these measurements, an equation--[PVR(CUBIC)=374.057+(-196.94+V1)+(32.5539+V1(2))+(-1.1480+V1(3)) where V1=average of the length (L), width (T), and the anteroposterior distance on transverse section (Dt) of the postvoid urinary bladder]--more accurate than previously existing ones was obtained by cubic regression analysis. Mean ultrasound estimated volume was 220.51 ml as against 220.76 ml after catheterization. The mean difference was 0.25 ml (not significant, p<0.01) with 95% confidence interval of +/-10 ml. With this equation, the ultrasonographic residual urine volume showed a higher correlation coefficient with the catheterized volume at p<0.01 (Pearson r=0.982, r2=0.96) than previously defined formulas. The standard error of the mean was 5.11 ml (mean=220.5+/-190.4 ml).

CONCLUSION

With the above equation, we consider conventional transabdominal ultrasonography a reliable method for assessing the residual urine volume in patients with benign prostatic hyperplasia. This equation, though complex when compared to some of the pre-existing formulas, can be integrated into the memory of modern ultrasound machines for easy and faster computation.