Abstract Background A common method for diagnosing sarcopenia involves estimating the muscle mass by computed tomography (CT) via measurements of the cross‐sectional muscle area (CSMA) of all muscles at the third lumbar vertebra (L3) level. Recently, single‐muscle measurements of the psoas major muscle at L3 have emerged as a surrogate for sarcopenia detection, but its reliability and accuracy remain to be demonstrated. Methods This prospective cross‐sectional study involved 29 healthcare establishments and recruited patients with metastatic cancers. The correlation between skeletal muscle index (SMI = CSMA of all muscles at L3/height2, cm2/m2) and psoas muscle index (PMI = CSMA of psoas at L3/height2, cm2/m2) was determined (Pearson's r). ROC curves were prepared based on SMI data from a development population (n = 488) to estimate suitable PMI thresholds. International low SMI cut‐offs according to gender were studied for males (