Short-Lived Hyperglycaemia Contributes to Continuous Glucose Monitoring Metrics Used for Therapeutic Decision-Making

Background: Continuous glucose monitoring (CGM) metrics, including time in range (TIR) and median glucose, guide therapeutic decisions in type 1 diabetes (T1D) but do not distinguish sustained from transient hyperglycaemia. We investigated the prevalence of short-lived hyperglycaemia (SLH), i.e. brief isolated hyperglycaemic episodes, and its contribution to CGM metrics in T1D children and adolescents. Methods: In this single-centre retrospective cohort study, T1D patients aged 6 months-20 years, enrolled in 2025-2026, with 5-minute CGM data, >=70% coverage and median glucose 6.1-9.4 mmol/L within a 45-day window were included. SLH were hyperglycaemic episodes lasting 10-60 minutes surrounded by normoglycaemia, classified by onset as daytime or night-time. Their contribution to CGM metrics was estimated by replacing SLH with plausible trajectories; sensitivity analyses varied construction parameters. Findings: Among 135 participants, 2,874 of 15,556 hyperglycaemic episodes (18.5%) met daytime SLH criteria (median 19.0 per participant). Removing SLH increased daytime TIR by 1.74 percentage points (95% CI 1.59-1.89) and reduced daytime median glucose by 0.18 mmol/L (95% CI 0.17-0.20). Whole-day effects were modest, but local impact was substantially greater: at maximal divergence, glucose profiles differed by 1.30 mmol/L (95% CI 1.21-1.38), around meals. Patients with near-identical conventional metrics could show markedly different SLH frequency. Interpretation: SLH are common in this paediatric T1D cohort and alter routinely reported CGM metrics: modest whole-day effect contrasts with a substantially greater impact at specific times of day. Distinguishing transient from sustained hyperglycaemia provides clinically relevant information beyond aggregate metrics and may guide behavioural or educational interventions rather than insulin intensification. Funding: Francqui Foundation Seed Grant to LS; no funder role.