BACKGROUND: In the SafeBoosC-III trial, treatment guided by cerebral oximetry monitoring for the first 72 hours after birth did not reduce the incidence of death or severe brain injury in extremely preterm infants at 36 weeks' postmenstrual age, as compared with usual care. Despite an association between severe brain injury diagnosed in the neonatal period and later neurodevelopmental disability, this relationship is not always strong. The objective of the SafeBoosC-III follow-up study is to assess mortality, neurodevelopmental disability, or any harm in trial participants at 2 years of corrected age. One important challenge is the lack of funding for local costs for a trial-specific assessment. METHODS: Of the 1601 infants randomised in the SafeBoosC-III trial, 1276 infants were alive at 36 weeks' postmenstrual age and will potentially be available for the 2-year follow-up. Inclusion criteria will be enrollment in a neonatal intensive care unit taking part in the follow-up study and parental consent if required by local regulations. We aim to collect data from routine follow-up programmes between the ages of 18 and 30 months of corrected age. If no routine follow-up has been conducted, we will collect informal assessments from other health care records from the age of at least 12 months. A local co-investigator blinded to group allocation will classify outcomes based on these records. We will supplement this with parental questionnaires including the Parent Report of Children's Abilities-Revised. There will be two co-primary outcomes: the composite of death or moderate or severe neurodevelopmental disability and mean Bayley-III/IV cognitive score. We will use a 3-tier model for prioritisation, based on the quality of data. This approach has been chosen to minimise loss to follow-up assuming that little data is better than no data at all. DISCUSSION: Follow-up at the age of 2 years is important for intervention trials in the newborn period as only time can show real benefits and harms later in childhood. To decrease the risk of generalisation and data-driven biased conclusions, we present a detailed description of the methodology for the SafeBoosC-III follow-up study. As funding is limited, a pragmatic approach is necessary. TRIAL REGISTRATION: ClinicalTrials.gov NCT05134116 . Registered on 24 November 2021.

2 Department of Neonatology Poznan University of Medical Sciences Poznań Poland

Centre for Clinical Intervention Research Copenhagen Trial Unit The Capital Region Copenhagen University Hospital Rigshospitalet Copenhagen Denmark

Department of Clinical Sciences and Community Health University of Milan Milan Italy

Department of Intensive Care Copenhagen University Hospital Rigshospitalet Copenhagen Denmark

Department of Neonatology Children's Hospital of Fudan University Shanghai China

Department of Neonatology Children's University Hospital of Zürich Zurich Switzerland

Department of Neonatology Copenhagen University Hospital Rigshospitalet Blegdamsvej 9 Copenhagen Ø 2100 Denmark

Department of Neonatology Gazi University Hospital Yenimahalle Ankara Turkey

Department of Neonatology La Paz University Hospital Madrid Spain

Department of Neonatology Oslo University Hospital Oslo Norway

Department of Neonatology Royal Hospital for Children Glasgow UK

Department of Neonatology University Hospital Leuven Louvain Belgium

Department of Neonatology University Hospital Motol Prague Czech Republic

Department of Paediatrics and Adolescent Medicine Copenhagen University Hospital Hilleroed Denmark

Department of Pediatrics Copenhagen University Hospital Hvidovre Denmark

Department of Pediatrics Division of Newborn Medicine Mountainside Medical Center Montclair NJ USA

Department of Pediatrics Medical University of Graz Graz Austria

Department of Pediatrics NICU University General Hospital of Patras Patras Greece

Department of Regional Health Research The Faculty of Health Sciences University of Southern Denmark Odense Denmark

Division of Neonatology and Pediatric Intensive Care Medicine Center for Pediatrics and Adolescents Medicine Medical Center University of Freiburg Freiburg Germany

Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico Milan Milan Italy

Infant Research Centre and Department of Paediatrics and Child Health University College Cork Cork Ireland

St Johns Medical College Hospital Bengaluru India

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