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. 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 (<55cm2 /m2 ) and for females (<39cm2 /m2 ). Youden's index (J) and Cohen's kappa (κ) were calculated to estimate the test's accuracy and reliability. PMI cut-offs were validated in a validation population (n=243) by estimating the percentage concordance of sarcopenia diagnoses with the SMI thresholds. Seven hundred and sixty-six patients were analysed (mean age 65.0±11.8years, 50.1% female). Low SMI prevalence was 69.1%. Correlation between the SMI and PMI for the entire population was 0.69 (n=731, P<0.01). PMI cut-offs for sarcopenia were estimated in the development population at <6.6cm2 /m2 in males and at <4.8cm2 /m2 for females. The J and κ coefficients for PMI diagnostic tests were weak. The PMI cut-offs were tested in the validation population where 33.3% of the PMI measurements were dichotomously discordant. A diagnostic test employing single-muscle measurements of the psoas major muscle as a surrogate for sarcopenia detection was evaluated but found to be unreliable. The CSMA of all muscles must be considered for evaluating cancer sarcopenia at L3.