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.