Validation of the sarcopenia index to assess muscle mass in the critically ill: A novel application of kidney function markers

Background & aims: Adverse outcomes for hospitalized patients with sarcopenia are well documented, and identification of patients at risk remains challenging. The sarcopenia index (SI), previously defined as (serum creatinine/serum cystatin C) × 100, could be an inexpensive, readily accessible, objective tool to predict muscle mass and risk for adverse clinical outcomes. The aim of this study was to assess the validity of the SI as a predictor of muscle mass. Methods: Retrospective study of critically ill adults admitted to Mayo Clinic from 2012 to 2015 with suspected sepsis and an available creatinine and serum cystatin C. Muscle surface area was quantified at the L3/4 vertebral level in patients with an abdominal CT scan (CTMSA). Multivariable regression modeling was used to assess the relationship between SI and CTMSA, as well as short-term clinical outcomes. Results: The 171 included had a mean weight and body mass index (BMI) of 75.2 ± 16.4 kg and 26.0 ± 4.6 kg/m ² and abdominal CT scans were available for 81 (47%) patients. The SI correlated with CTMSA (r = 0.40). After adjustment for age, sex, severity of illness, and BMI, SI was independently associated with muscle mass (P = 0.001). A decrease in the SI (indicative of lower muscle mass) was also associated with frailty and worse short-term clinical outcomes. Conclusion: The SI, a simple calculation from kidney function markers, is a significant predictor of muscle mass in this validation cohort of ICU patients. A low SI was associated with longer hospital length of stay and frailty. Future studies could explore whether the use of SI assists with identifying patients likely to benefit from pharmacotherapy-, nutrition-, or physical therapy-based interventions.