Psoas muscle index is not representative of skeletal muscle index for evaluating cancer sarcopenia

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/height(2), cm(2)/m(2)) and psoas muscle index (PMI = CSMA of psoas at L3/height(2), cm(2)/m(2)) 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 (<55cm(2)/m(2)) and for females (<39 cm(2)/m(2)). 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. RESULTS: Seven hundred and sixty‐six patients were analysed (mean age 65.0 ± 11.8 years, 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 <6.6cm(2)/m(2) in males and at <4.8 cm(2)/m(2) 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. CONCLUSIONS: 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.