• Lucy J Modra, Alisa M Higgins, David V Pilcher, Ada S Cheung, Morgan N Carpenter, Michael Bailey, Sav Zwickl, and Rinaldo Bellomo.
• Department of Critical Care, University of Melbourne, Melbourne, VIC; Intensive Care Unit, Austin Health, Melbourne, VIC. Electronic address: lucy.modra@austin.org.au.
• Chest. 2024 May 1; 165 (5): 112011281120-1128.

BackgroundPatient sex affects treatment and outcomes in critical illness. Previous studies of sex differences in critical illness compared female and male patients. In this study, we describe the group of patients classified as a third sex admitted to ICUs in Australia and New Zealand.Research QuestionWhat are the admission characteristics and outcomes of ICU patients classified as belonging to a third sex group compared with patients classified as female or male?Study Design And MethodsRetrospective observational study of admissions to 200 ICUs, recorded in the Australian and New Zealand Intensive Care Society's Adult Patient Database from 2018 to 2022. We undertook mixed effect logistic regression to compare hospital mortality across the sex groups, adjusted for illness severity, diagnosis, treatment limitation, year, and hospital.ResultsWe examined 892,161 admissions, of whom 525 (0.06%) were classified as third sex. Patients classified as third sex were represented across all diagnostic categories, jurisdictions, and hospital types. On average, they were younger than the groups classified as female (59.2 ± 20.0 vs 61.3 ± 18.4 years; P = .02) or male (63.2 ± 16.7 years; P < .001), respectively. Patients classified as third sex were more likely to be admitted after orthopedic surgery (10.1% third sex admissions [95% CI, 7.7%-13.0%]; 6.2% female [95% CI, 6.1%-6.3%]; 4.8% male [95% CI, 4.7%-4.9%]) and drug overdose (8.8% third sex admissions [95% CI, 6.5%-11.5%]; 4.2% female [95% CI, 4.1%-4.2%]; 3.1% male [95% CI, 3.0%-3.1%]). There was no difference in the adjusted hospital mortality of patients classified as third sex compared with the other groups.InterpretationPatients classified as third sex composed a small minority group of adult ICU patients. This group had a different diagnostic case mix but similar outcomes to the groups classified as female or male. Further characterizing a third sex group will require improved processes for recording sex and gender in health records.Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.

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