Marina Mourtzakis, Lesley Moisey, Daren Heyland and Rosemary Kozar. We appreciate the interest of Binay Safer and Safer in our article.
Definition of sarcopenia
The EWGSOP recommends that sarcopenia be defined as low muscularity as well as loss of muscle strength or performance or both , necessitating advances in precisely measuring muscle mass. The authors correctly indicate that we did not measure muscle strength/performance. However, our study was performed on critically injured, older trauma patients at the time of admission, prohibiting any meaningful assessment of muscle strength. We present a novel approach that is highly accessible and practical as CT scans are frequently performed as part of routine care for the injured patient. Our aim goes beyond categorizing sarcopenic versus non-sarcopenic patients to show differences in various clinical outcomes (note that muscle, as a continuous variable, is also associated with mortality) . Our ultimate goal is to identify vulnerable patients who require specific nutritional or rehabilitative intervention and assess success or failure of this intervention by using CT. CT imaging is one of a few modalities that measure muscle specifically and is far superior to body weight (most commonly used to measure nutritional status in the ICU). Single-slice CT images in the lumbar region (including L3) have been strongly associated with whole-body muscle volume by using MRI in healthy people  and DXA in a clinical population , indicating that lumbar muscle is representative of whole-body muscle.
Computed tomography versus dual-energy x-ray absorptiometry
When CT and MRI are not available, DXA is recommended by the EWGSOP as an alternative method. DXA is not always available, accessible, practical, or feasible for critically injured patients. Transporting these patients for additional studies carries risk and does incur additional cost. Importantly, DXA measures lean body mass, which includes skeletal muscle as well as internal organs such as liver and spleen that may be confounding during critical illness. Isolating appendicular muscle is possible but requires some manual analysis and may introduce human error.