Differences in Paediatric intensive care mortality by biological sex: a study of English PICUs using linked administrative datasets.

Objective: To determine if there is a causal relationship between sex and mortality in Paediatric Intensive Care Units (PICUs). Methods: A multi-centre, population-based cohort study of English PICUs, designed using the potential outcomes framework and doubly robust methods. Participants were children aged 0 to 8 years with at least one admission to PICU between January 2010 and December 2019, with linked PICU and hospital records. The exposure was biological sex, and the main outcome was the causal difference in mortality between females in PICU and their counterfactual mortality if these females were hypothetically male (Average Treatment effect in the Treated; ATT). Results: There were 48,500 children, of whom 20,412 (42.1%) were females and 28,088 (57.9%) were males and with a median age at first PICU admission of 0.5 (IQR 0.08-2.02) years. During the first admission, 1,760 (3.63%) children died; of whom, 777/20,412 (3.81%) were female and 983/28,088 (3.50%) were male. The doubly robust estimated causal mortality risk difference captured by the ATT (i.e. excess mortality for girls in PICU), was 0.31% (95% CI 0.01-0.61). Conclusions: Although boys have a higher rate of death than girls by age in the general population, this mortality trend is reversed in PICU. Evidence of this increased female mortality persists after applying doubly robust causal methods, indicating a causal link. The supposition that all pre-pubertal children have the same responses to critical illness and their management should be re-examined. Supplementary Information: The online version contains supplementary material available at 10.1007/s44253-026-00130-8.