Background and aims The optimal target range for pulse oximeter saturation (SpO₂) in extremely preterm infants is unknown. BOOST-II UK is one of 5 international studies that have recently investigated this.

Methods Preterm infants born before 28 weeks’ gestation were randomised within 24 h of birth to an SpO₂ target range of 85–89% or 91–95%. The intervention used masked offset oximeters and was continued until 36 weeks’ gestation. The primary outcome was a composite of death or serious neurosensory disability (SND) in survivors at age 2 corrected for prematurity, evaluated in 745 infants cared for using updated trial oximeters. A sensitivity analysis restricted to infants assessed by a Bayley III examination and a secondary analysis including a further 228 infants who were studied before the oximeters were updated were also performed.

Results The primary outcome was determined for 722 (96.9%) of infants.

The sensitivity analysis showed similar results. In the secondary analysis the mortality difference was 4.9% (p = 0.05). Severe visual loss did not differ between groups.

Conclusions The higher SpO₂ target group had 8.5% greater survival with no increase in serious neurosensory disability. For infants born before 28 weeks’’ gestation, SpO₂ targets below 90% are not recommended.