LAUSR

Background We aimed to evaluate the feasibility of the upper thigh level as a landmark to measure muscle area for sarcopenia assessment on computed tomography (CT). Methods In the 116 healthy subjects who performed CT scans covering from mid-abdomen to feet, the skeletal muscle area in the upper thigh level at the inferior tip of ischial tuberosity (SMA UT ), the mid-thigh level (SMA MT ), and L3 inferior endplate level (SMA L3 ) were measured by two independent readers. Pearson correlation coefficients between SMA UT , SMA MT , and SMA L3 were calculated. Inter-reader agreement between the two readers were evaluated using intraclass correlation coefficient (ICC) and Bland-Altman plots with 95% limit of agreement (LOA). Results In readers 1 and 2, very high positive correlations were observed between SMA UT and SMA MT ( r  = 0.91 and 0.92, respectively) and between SMA UT and SMA L3 ( r  = 0.90 and 0.91, respectively), while high positive correlation were observed between SMA MT and SMA L3 ( r =  0.87 and 0.87, respectively). Based on ICC values, the inter-reader agreement was the best in the SMA UT (0.999), followed by the SMA L3 (0.990) and SMA MT (0.956). The 95% LOAs in the Bland-Altman plots indicated that the inter-reader agreement of the SMA UT (− 0.462 to 1.513) was the best, followed by the SMA L3 (− 9.949 to 7.636) and SMA MT (− 12.105 to 14.605). Conclusion Muscle area measurement at the upper thigh level correlates well with those with the mid-thigh and L3 inferior endpoint level and shows the highest inter-reader agreement. Thus, the upper thigh level might be an excellent landmark enabling SMA UT as a reliable and robust biomarker for muscle area measurement for sarcopenia assessment.