Serum creatinine is used to measure the estimated glomerular filtration rate (eGFR); however, it is influenced by muscle mass and may therefore overestimate renal function in patients with sarcopenia. We examined calf circumference (CC) as a convenient muscle mass evaluation tool that can potentially indicate the need to test for cystatin C instead of creatinine in elderly inpatients. We retrospectively reviewed the electronic health record of 271 inpatients aged 65 or over. CC was determined by measuring the thickest part of the nondominant calf. eGFRcys and eGFRcr were calculated using cystatin C and creatinine levels, respectively. We evaluated optimum CC cutoff values using the eGFRcys/eGFRcr ratio for detecting hidden renal impairment (HRI, defined as eGFRcr ≥ 60 mL/min/1.73 m² but eGFRcys < 60 mL/min/1.73 m²). CC showed a significant positive correlation with the eGFRcys/eGFRcr ratio in both sexes. The areas under the receiver operating characteristic curve were 0.725 and 0.681 for males and females, respectively. CC cutoffs with a sensitivity or specificity of 90% or 95% might be used to detect HRI in males. In conclusion, utilizing the optimum cutoff, CC could be a cost-effective screening tool for detecting HRI in elderly male patients using cystatin C as an add-on test.

Sunghwan Lee, M.D., Yu Jin Park, M.D., Ph.D., Saeam Shin, M.D., Ph.D., Eun-Ki Min, M.D., Yongseop Lee, M.D., Mi Ri Park, B.S., Dongju Won, M.D., Deok-Gie Kim, M.D., Ph.D., Myoung Soo Kim, M.D., Ph.D., Dong Jin Joo, M.D., Ph.D., Jong Rak Choi, M.D., Ph.D., Su Jin Jeong, M.D., Ph.D., Jae Geun Lee, M.D., Ph.D., and Seung-Tae Lee, M.D., Ph.D.. Ann Lab Med -0001;0:. https://doi.org/10.3343/alm.2026.0068