Background <p>Individuals with anorexia nervosa are at increased risk of renal impairment, but assessing renal function in this population remains challenging. Cystatin-C, which is less affected by nutritional status than creatinine, may provide a more reliable biomarker. However, equations based on cystatin-C (CKiD‑U25, EKFC, and CKD-EPI) have not yet been evaluated against isotopic scintigraphy in individuals with anorexia nervosa.</p> Objective <p>This study aimed to compare the performance of creatinine-based and cystatin-C-based equations (CKiD-U25, EKFC, CKD-EPI) against 99mTc-DTPA renal scintigraphy as the reference standard in adolescents with restrictive anorexia nervosa.</p> Methods <p>Glomerular filtration rate was measured using renal scintigraphy and estimated with creatinine-based and cystatin-C-based equations. Serum cystatin-C and creatinine were measured at hospital admission (day 0), whereas renal scintigraphy was performed as soon as feasible after admission (mean of 5&#xa0;days), in adolescents with restrictive anorexia nervosa. Correlation coefficients, mean bias, root mean square error, and Bland–Altman plots were assessed.</p> Results <p>Across all equations, cystatin-C-based formulas consistently outperformed creatinine-based ones in terms of correlation, bias, and precision. The CKiD-U25 using cystatin-C equation showed the strongest correlation with renal scintigraphy (r = 0.77; bias = – 11.5&#xa0;mL/min/1.73 m²).</p> Conclusions <p>At admission for restrictive anorexia nervosa, cystatin-C-based equations, particularly the CKiD-U25, demonstrate superior agreement with renal scintigraphy compared to creatinine-based equations, making cystatin-C-based equations more reliable in this context.</p>