Prevalence, duration, and clinical implications of Continuous Glucose Monitor (CGM) measurement limit capping in type 1 diabetes
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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.
Materials and 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 1,162 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 minutes and over one third exceeded 60 minutes. 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 patients, 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.
Novelty statement
What is already known about this subject?
CGM devices operate within fixed measurement limits (40-400 mg/dL), and truncate extreme values. Glucose distributions in type 1 diabetes are right-skewed, with frequent hyperglycaemic excursions approaching upper sensor limits.
What has this study found?
Sensor limit capping is near-universal (93.5-100% of participants) and produces sustained periods (often exceeding 60 minutes) during which CGM cannot quantify true hyperglycaemia severity. Capping is inversely associated with age, positively associated with HbA1c, and does not substantially distort summary glucose metrics.
What are the implications of this study?
CGM records can confirm that extreme glucose events occurred but cannot quantify their severity. Clinicians, trialists, and guideline authors should account for this when interpreting extreme-range metrics, particularly in younger patients and paediatric trial populations.
