Prevalence, duration, and clinical implications of continuous glucose monitor measurement limit capping in type 1 diabetes

AIMS: CGM devices report glucose only within fixed limits (typically 40-400 mg/dL; 2.2-22.2 mmol/L), truncating extreme values to a boundary ('capping'). We characterised prevalence, duration, and consequences of capping in type 1 diabetes trial data. METHODS: We analysed 46,990,617 CGM readings from 948 participants across four publicly available clinical trial datasets (Dexcom G4 Platinum or G6 sensors). Capping prevalence, run duration, and associations with age, HbA1c, and sex were characterised across all datasets. In the 77 participants of the Replace-BG trial, CGM-plus-blood glucose monitor (BGM) arm, CGM-derived metrics were compared with contemporaneous BGM measurements across 1162 non-overlapping 14-day windows. RESULTS: Between 93.5% and 100% of participants had at least one capped reading, and capped values comprised 0.47-0.98% of all readings. In the three datasets for which duration could be calculated, over 70% of upper-cap runs exceeded 15 min and over one third exceeded 60 min. Upper-limit capping was inversely associated with age (Spearman ρ -0.20 to -0.47, p ≤ 0.002) in three of the datasets, and positively associated with baseline HbA1c (ρ 0.39-0.62, p < 0.001) in all four datasets. A within-participant analysis showed that capping burden did not predict CGM-BGM divergence in any summary metric (all p > 0.2), and a systematic CGM-BGM offset in mean glucose and time in range (TIR) reflected the physiological lag between blood and interstitial fluid rather than capping artefact. CONCLUSIONS: Sensor limit capping is near-universal in type 1 diabetes, produces sustained periods of right-censored glucose data disproportionately affecting younger participants, and does not substantially distort standard summary metrics at the population level. Clinicians and trialists should be aware that CGM data can confirm extreme glucose events but cannot quantify their severity.