Accuracy of renal ultrasound measurements for predicting actual kidney size

Purpose: We attempted to evaluate the accuracy of renal ultrasound measurements for determining actual kidney size in a growing renal unit. Materials and Methods: We evaluated 18 renal units in growing piglets weekly for 11 weeks on ultrasound. Measured parameters included length, transverse and anteroposterior dimensions, and volume. At 11 weeks final ultrasound measurements were obtained, animals were sacrificed and gross specimen measurements were made. Final ultrasound dimensions and serial measurements were correlated with actual size. Growth charts were constructed to evaluate variability between observations. Results: Mean difference between final ultrasound renal length and actual size was 3.8 mm. (correlation coefficient 0.74). Serial length measurements were not more accurate than isolated measurements for predicting final length (correlation coefficient 0.34). Volume measurement was not more accurate than renal length. Analysis of individual growth charts revealed a significant week-to-week variation. Conclusions: Error in ultrasound measured kidney length can equal 1 or more years of growth in a child. Our results show that caution should be used when incorporating ultrasound measurements into surgical decision making.