Background: Low skeletal muscle index (SMI) and sarcopenia adversely affect clinical outcomes in oncology patients. Study aims were to assess the agreement of bioelectrical impedance analysis (BIA), mid-arm muscle circumference (MAMC), and computed tomography (CT) at the third lumbar vertebra (L3), for the measurement of muscle mass and identification of sarcopenia, in patients with colorectal cancer (CRC).Method: A comparison study of low SMI and sarcopenia determined by BIA and MAMC, compared to CT. Sensitivity, specificity and area under the curve (AUC) were calculated.Results: CT scans were obtained for 100 participants. Low SMI was identified in 29%, 57%, and 20% of participants using CT at L3, BIA, and MAMC, respectively. For low muscle mass BIA showed 60% of participants were correctly classified (AUC 0.619, sensitivity 80%, specificity 52%, kappa 0.241, P = 0.009) and for MAMC, 73% of participants were correctly classified (AUC 0.625, sensitivity 38%, specificity 88%, kappa 0.286, P = 0.005). There were 14%, 31%, and 10% of participants identified as having sarcopenia from CT, BIA, and MAMC, respectively.Conclusions: Both BIA and MAMC show a poor level of agreement for measuring muscle mass compared to CT scans using L3 in patients with CRC.