Association of nutritional status, frailty, and rectus femoris muscle thickness measured by ultrasound and weaning in critically ill elderly patients Introduction Sarcopenia and frailty are critical factors linked with poor clinical outcomes among elderly individuals. This study aims to investigate the association between nutritional assessment tests and frailty with muscle thickness measured by ultrasound and their relationship with weaning among crtically ill elderly patients. Materials and Methods Patients who were over 65 years old and required invasive ventilation were assessed for nutritional status and clinical frailty scale upon admission to the intensive care unit. Additionally, the thickness of their rectus femoris and vastus intermedius muscles were measured by ultrasound within 48 hours of intubation. Correlation analysis was conducted to examine the relationship between screening tests, frailty, and ultrasound results. The association between these parameters and weaning success was also evaluated. Results Between May and August 2022, 32 consecutive patients were enrolled in the study. The mean age was 79.3 ∓ 7.9, and 18 (56.3%) of them were female. Median APACHE-II- and first-day SOFA scores were 22.5 (16.2-29.7) and 7 (5-10.75), respectively. There was a moderate negative correlation between the thickness of the rectus femoris and frailty (r= -0.41, p= 0.036), and there was a moderate positive correlation between the rectus femoris and geriatric nutritional risk index (r= 0.45, p= 0.017). Of them, 18 (56.3%) patients were classified as weaning failure in which the mean frailty score was higher (7.6 ∓ 0.9 vs 6.5 ∓ 1.7, p= 0.035), sepsis (18 vs 7, p< 0.001) and use of vasopressor (17 vs 6, p= 0.004) more common, and in-hospital mortality were higher (18 vs 5, p< 0.001). Conclusion Bedside ultrasound could be beneficial for detecting nutritional high-risk patients. Frailty was associated with muscle thickness, and it was also associated with weaning failure.