Assessment of renal mobility using ultrasound-guided body-surface projection in asymptomatic males aged 16–19 years: a cross-sectional study

Abstract Background A practical workflow for quantifying postural renal mobility could improve its description during ultrasound examination. We evaluated ultrasound-guided body-surface projection and estimated side-specific upper reference limits in asymptomatic males aged 16–19 years. Methods This cross-sectional study included 338 participants undergoing health screening. Renal poles and anatomical landmarks were marked in the supine and standing positions, with renal pole localization at the end of quiet inspiration. Two sonographers jointly verified the marks before a single measurement set was recorded. The primary index was the change in lower-pole-to-costal-margin distance (bc − BC). Analyses comprised renal-length agreement, right–left comparisons, percentile estimation with bootstrap confidence intervals, descriptive threshold comparisons, and exploratory correlations with participant characteristics. Results Median bc − BC was 2.4 cm (interquartile range, 1.4–4.0) for the right kidney and 3.5 cm (2.0–4.5) for the left kidney ( P < 0.001). The 95th-percentile (P95) upper reference limits were 5.5 cm (95% confidence interval, 5.1–6.0) and 5.7 cm (5.5–6.0), respectively. A bc − BC value ≥ 5.0 cm in either kidney was observed in 84 participants (24.9%); 35 (10.4%) met the corresponding side-specific P95 criteria. Renal-length agreement coefficients ranged from 0.682 to 0.775. Several body-size measures were inversely associated with displacement. Conclusions Ultrasound-guided body-surface projection provides an anatomically interpretable description of postural renal mobility and preliminary side-specific reference estimates for this cohort. These findings support further evaluation of the workflow; reproducibility and external clinical validation remain necessary.