Remnant kidney volume-to-weight ratio and point score predict post-donor kidney function: the hypertrophy paradox and structure-demand model

BACKGROUND: Most living kidney donors recover 60-70% of baseline renal function, yet some show suboptimal compensation despite a favorable baseline eGFr. The remnant kidney volume to weight ratio (rKV/w) reflects anatomical reserve relative to metabolic demand, but its predictive value has not been tested in Middle eastern populations, and it is unknown whether volumetric hypertrophy guarantees functional gain. METHODS: we retrospectively analyzed 368 living kidney donors who underwent left sided laparoendoscopic single site nephrectomy (2017-2024). Preoperative CT volumetry measured rKV/w. suboptimal recovery was defined as eGFr < 60% of baseline at 1 year. Multivariate logistic regression, roC analysis, and a point score were developed. In 124 donors with follow up imaging, we compared compensatory hypertrophy (percent volume increase) between recovery groups. RESULTS: suboptimal recovery occurred in 38 donors (10.3%). rKV/w independently predicted suboptimal recovery (adjusted or 2.94; 95% CI 1.85-4.67; p < 0.001). A threshold of 2.2 mL/kg yielded 84.2% sensitivity and 76.1% specificity (AUC for rKV/w alone 0.82). rKV/w correlated modestly with MAG3 split function (rho = 0.31; 95% CI 0.20-0.41). A 4 item point score (age ≥ 35, BMI ≥ 27, baseline eGFr ≥ 95, rKV/w < 2.2) achieved an AUC of 0.84. Novel finding: compensatory hypertrophy was similar between groups (18.2% vs 19.5% volume increase, p = 0.52), yet functional recovery differed significantly, a "hypertrophy paradox." CONCLUSIONS: rKV/w independently predicts post donation functional compensation, supporting a structure-demand model. The hypertrophy paradox challenges the assumption that an increase in volume alone equates to adaptation. The 2.2 mL/kg threshold and point score (AUC 0.84) offer practical risk stratification tools, but external validation is required before clinical use.