**4. Conclusions**

Critically ill patients suffer significant LBM loss, much of it in the first 7–10 days of ICU stay, requiring adequate timing initiation and optimal nutritional support to slow the catabolic process and to minimize adverse events such as prolonged mechanical ventilation, longer ICU stay, and increased risk of death. Due to the correlation between SCr levels and muscle mass, SCr in the steady state has been used as a surrogate of muscle mass measurements. However, SI could be considered a useful tool with a superior performance compared with sCr alone in the estimation of muscle mass, while the clinical usability of UCR seems limited and influenced by other factors such as decreased effective blood volume, protein intake or gastrointestinal bleeding, and also acute kidney injury. However, muscle wasting, often present in critically ill patients, can influence SCr and mask a diagnosis of AKI, decreasing the sensitivity of SCr for the early detection of AKI. Future studies should address the effect of muscle wasting on the true SCr concentration.
