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Key summary points: Aim: To investigate the impact of acute muscle wasting on 90-day mortality in older patients with severe community-acquired pneumonia using ultrasound and chest computed tomography. Findings: Acute muscle wasting assessed by ultrasound and chest CT independently predicted all-cause 90-day mortality in older patients with severe community-acquired pneumonia. A higher muscle loss had an increased risk of 90-day mortality. Message: Acute muscle wasting is an independent predictor of adverse outcomes in older patients with severe community-acquired pneumonia. Purpose: To investigate the impact of acute muscle wasting on 90-day mortality in older patients with severe pneumonia using ultrasound and chest computed tomography (CT). Methods: Quadriceps muscle layer thickness was measured via ultrasound on days 1, 7, and 14, and cross-sectional area of the erector spinae muscle was assessed using chest CT on days 1 and 14 in patients aged ≥ 65 years old. The primary outcome was all-cause 90-day mortality. Receiver operating characteristic curves were conducted for muscle loss to predict 90-day mortality. Cox proportional hazard models and Kaplan–Meier survival curves were employed to evaluate the association between muscle loss and 90-day mortality. Results: Sixty-two patients were enrolled with median age of 80.2 years, 29 (46.8%) were men and 28 (45.2%) patients died. Muscle mass measured using ultrasound and CT decreased significantly from baseline to day 14 in the non-survivor group. Muscle loss assessed by ultrasound (with minimum and maximum pressure) and CT independently predicted all-cause 90-day mortality (adjusted hazard ratios = 1.497, 1.400 and 1.082; P < 0.001, P = 0.002, and P = 0.004; respectively), and cutoff values of muscle loss were 0.34 cm, 0.11 cm and 4.92 cm2, correspondingly. A higher muscle loss had an increased risk of 90-day mortality. Conclusions: Acute muscle wasting assessed by ultrasound and chest CT persisted for 14 days and was an independent predictor of adverse outcomes in older patients with severe pneumonia. A greater decline in muscle mass was associated with a higher 90-day mortality risk. [ABSTRACT FROM AUTHOR]