[Sonographic volumetry of the urinary bladder in paediatric patients. value of different calculation formulae taking into account a simple formula that is calculable in one's head].

BACKGROUND

Volumetry of the urinary bladder is a frequently performed sonographic procedure. Usually the formula of the prolate ellipsoid is applied for this purpose. In the literature however a variety of different formulas is proposed.

PURPOSE

The aim of this study was to evaluate different simple formulae used for the calculation of the bladder volume in children, taking into account a formula that is easily calculable in one's head.

MATERIAL AND METHODS

139 children were examined by sonography, 6 different formulae were applied. The bladder volume was confirmed by micturition. Only children without residual volume were included in the study.

RESULTS

82 children (mean age: 8.5 years) could be included with a mean bladder volume of 92 mL (0 to 227 mL). The mean error ranged from 20.2 % (cuboid formula) to 38.0 % (prolate ellipsoid). With decreasing bladder volume all formulae showed an increasing error and a tendency for underestimation of the bladder volume. For small volumes best results were achieved by the cuboid formula with a mean error of 26.4 %. The formula of the prolate ellipsoid yielded the worst results. The proposed truncation formula, easily calculable in one's head, showed better results than both the formula of the prolate ellipsoid and the cuboid formula.

CONCLUSION

The formula of the prolate ellipsoid should not be used for bladder volumetry in children. Small bladder volumes should be calculated by multiplying the three diameters of the bladder (cuboid formula). If the calculated volume lies above 60 mL a correction factor of 0.8 should be used to avoid overestimation of the true bladder volume.