Establishment of Reference Values for Estimated Kidney Volume Using Ultrasonography in Japanese Pediatric Patients

Background: Ultrasonography serves as a noninvasive and effective screening modality for renal evaluation. Assessing renal dimensions is crucial, given the established correlation between renal function and kidney size. In pediatric populations, renal size varies with growth, necessitating the establishment of growth-specific reference values. Renal length reference values derived from ultrasonography are widely utilized in clinical practice due to their ease of measurement. However, we believe that renal length alone is inadequate for a comprehensive assessment of kidney size due to the variability in renal morphology. Consequently, we hypothesize that renal volume provides a more precise metric for evaluating kidney size and have investigated the reference values for renal volume in Japanese children with normal renal function. Methods: We measured serum creatinine levels, body height and weight, and renal long and short diameters and depths via ultrasonography in pediatric patients under 18 years of age without renal disease. We included patients with eGFR within the reference range and excluded those with body height and weight significantly deviating from the reference values. We calculated kidney volume using the average of two ultrasound measurements of kidney size, approximated as an ellipsoid: long diameter × short diameter × thickness × π / 6. Results: Out of 42 candidates, 29 patients meeting the criteria for body height, weight, and eGFR were included in the study. The median age of participants was 12 years. The strongest correlation was found between renal volume and body surface area (r2 = 0.852), followed by age, body height, weight, or body mass index. Total kidney volume, defined as the sum of bilateral kidney volumes divided by body surface area, increased with growth, with multivariate analysis identifying age as the primary influencing factor. Thus, the reference value for total kidney volume can be expressed as total kidney volume/body surface area (mL/m2) = 140 + age × 2. This reference value for renal volume showed a stronger correlation with the previously reported reference value for renal length diameter in Japanese children. Furthermore, renal volume correlates more strongly with eGFR than renal length diameter. Conclusion: Renal volume is the more precise method for evaluating renal size.