BACKGROUND/PURPOSE: There have been no studies evaluating how body mass index (BMI) impacts on waitlist and post-liver transplant (LT) mortality in acute-on-chronic liver failure (ACLF) by sex. We aimed to determine these impacts using the United Network for Organ Sharing (UNOS) database. METHODS: Adults listed for LT with estimated ACLF (Est-ACLF) (2005-2023) were identified and subdivided by sex and BMI (high/middle/low). Competing-risk analyses evaluated impacts on waitlist mortality. Kaplan-Meier analyses assessed post-LT survival. Multivariable Cox regression identified risk factors. RESULTS: Of 37 251 Est-ACLF patients, 14 534 (39.0%) were female. Females had higher 90-day waitlist mortality than males (subhazard ratio [sHR]: 1.20, p <
.01). High/low BMI patients had higher mortality than middle (sHR: 1.08/1.11, p <
.01). In females, high BMI was associated with higher mortality than low (sHR: 1.10, p = .02)
in males, low BMI was associated with higher mortality than high/middle (sHR: 1.16/1.16 vs. high/middle, p <
.01). Multivariable analyses showed in females, high BMI was a significant risk factor for waitlist mortality (sHR:1.21, p <
.01), while low was not
in males, high/low BMI was significant, with low having higher sHR (1.17) than high (1.09). Post-LT survival showed no significant difference in females
in males, low BMI showed worse post-3-/5-year-LT survival (p <
.01). Multivariable Cox regression showed for females, neither low nor high BMI was significant for post-LT survival
for males, low BMI was significant for 1-/3-/5-year-LT survival (HR: 1.30/1.30/1.22, p <
.01). CONCLUSIONS: Our analysis of BMI's impact on LT outcomes in ACLF by sex enables risk stratification and provides a basis for adjusting BMI.