AI-ESTIMATED "KIDNEY AGE" FROM RENAL ULTRASOUND: A NOVEL DIGITAL BIOMARKER ASSOCIATED WITH RENAL OUTCOMES

 

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AI-ESTIMATED "KIDNEY AGE" FROM RENAL ULTRASOUND: A NOVEL DIGITAL BIOMARKER ASSOCIATED WITH RENAL OUTCOMES

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Yu
Hara
Yu Hara harayu1210@gmail.com Institute of Science Tokyo Department of Nephrology Tokyo Japan *
Masako Fujita mfujita7212@gmail.com Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Nobuhisa Morimoto nmorkid@tmd.ac.jp Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Ken Ikenouchi ikenouchi.ken@tmd.ac.jp Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Tamami Fujiki tfujkid@tmd.ac.jp fan Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Hiroaki Kikuchi hkikuchi.kid@tmd.ac.jp Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Yutaro Mori y-mori.kid@tmd.ac.jp Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Fumiaki Ando fandkidc@tmd.ac.jp Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Shintaro Mandai smandai.kid@tmd.ac.jp Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Takayasu Mori tmori.kid@tmd.ac.jp Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Koichiro Susa ksuskid@tmd.ac.jp Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Soichiro Iimori siimori.kid@tmd.ac.jp Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Shotaro Naito snaikid@tmd.ac.jp Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Eisei Sohara esohara.kid@tmd.ac.jp Institute of Science Tokyo Department of Nephrology Tokyo Japan -
Shinichi Uchida suchida.kid@tmd.ac.jp Institute of Science Tokyo Department of Nephrology Tokyo Japan -

Renal ultrasound visualizes cortical and medullary structures and is widely used worldwide, including in low-resource settings, because it is noninvasive, low-cost, and portable. In chronic kidney disease (CKD), beyond size reduction, qualitative morphological changes such as irregular renal contour and loss of corticomedullary differentiation are seen, but standardized, objective quantification has not been established. CKD is often regarded as pathologically accelerated aging of the kidney, and many of its morphological features resemble age-related changes. Building on this premise, we used deep learning to quantify such changes as “kidney age” and examined its biological and clinical validity.

We curated 142,401 abdominal ultrasound exams from 27,109 individuals (2007–2024) at Science Tokyo Hospital, each paired with an eGFR within ±3 months, and extracted long-axis kidney images. To avoid patient-level leakage, we split the data by patient into a development cohort (training/validation/test; used for model training and tuning) and an independent clinical evaluation cohort of 5,498 patients (nephrology outpatients, biopsy recipients, and patients with serial ultrasounds) used for pathology, prognosis, and treatment-response analyses. We trained an ImageNet-pretrained EfficientNet regression model with a single long-axis renal ultrasound image as input and the chronological age at the ultrasound date (years) as the target. At inference, the model output—the age predicted from an image—is referred to as “kidney age.” We defined ΔAge = kidney age − chronological age and tested its associations with renal pathology, kidney outcomes, and treatment response. 

On the test set, age estimation achieved R²=0.528 with a mean absolute error of 7.0 years. Occlusion sensitivity indicated that predictions mainly relied on the renal cortex, consistent with known cortical predominance of aging-related changes. In cases with biopsy shortly after ultrasound, ΔAge correlated with interstitial fibrosis grade, supporting biological validity. During follow-up, greater ΔAge predicted faster eGFR decline; in multivariable Cox models adjusted for sex, age, diabetes, hypertension, proteinuria, and baseline eGFR—with the event defined as a 30% eGFR decline—ΔAge remained an independent predictor of renal outcomes. In the IgA nephropathy subset, higher ΔAge was associated with less reduction in proteinuria after steroid therapy. Estimates were consistent across planes, time points, devices, and operators, with an intraclass correlation coefficient of 0.99.

AI-derived kidney age and ΔAge from renal ultrasound align with pathology, predict kidney function decline, and help stratify treatment response. Given their high reproducibility, these measures may serve as noninvasive digital biomarkers suitable for clinical use.

Kewords