Assessing Compensatory Renal Hypertrophy in Children with a Solitary Kidney: Utility of Ultrasound-Based Renal Volume Measurement

Background: In children with a solitary kidney, compensatory renal hypertrophy frequently occurs, helping preserve renal function. However, these patients are at increased long-term risk of chronic kidney disease, necessitating ongoing surveillance. Because renal size correlates with nephron number, it serves as a surrogate marker of renal function. Nonetheless, standardized criteria to quantify compensatory hypertrophy in these patients remain undefined. Having previously established reference values for total renal volume in healthy children, we aimed to evaluate renal volume in children with a solitary kidney using ultrasound. Methods: We retrospectively analyzed children with a solitary kidney who underwent renal ultrasound at our institution. Clinical parameters included underlying diagnosis, age, height, body surface area (BSA), renal volume, and estimated glomerular filtration rate (eGFR). Renal volume was calculated using the ellipsoid formula based on renal length, width, and thickness. A total of 108 ultrasound examinations were performed in 21 patients. Measured renal volumes were compared with age- and BSA-adjusted reference values derived from healthy children. Results: The underlying etiologies were multicystic dysplastic kidney (MCDK) in 12 patients, renal agenesis in 7, and post-nephrectomy status in 2. The median age at examination was 4.6 years (IQR: 2.0–8.0 years). Renal volume increased with age until approximately 2 years, then plateaued, with a median value of 82% (IQR: 76–88%) of the total renal volume in healthy peers. In patients with available eGFR data, renal volume was positively correlated with eGFR. Conclusion: Our findings suggest that compensatory hypertrophy in children with a solitary kidney is largely completed by the age of two, after which renal growth parallels somatic growth. The renal volume in these patients averaged approximately 80% of the total renal volume observed in healthy controls, corresponding to roughly 1.6 times the volume of a single normal kidney. Serial ultrasound-based renal volume assessment may provide a non-invasive means of identifying patients at increased risk for future renal function decline. This approach offers a simple and effective method for evaluating the adequacy of compensatory hypertrophy in this population.