OBJECTIVE
Little is known about the influence of residual islet function on glycemic control in type 1 diabetes (T1D). We investigated the associations between residual β-cell function and metrics of continuous glucose monitoring (CGM) in individuals with T1D.

RESEARCH DESIGN AND METHODS
In this cross-sectional cohort comprising 489 individuals (64% female, age 41.0 ± 14.0 years), T1D duration was 15.0 (interquartile range [IQR] 6.0–29.0) years. Individuals had a time in range (TIR) of 66% (IQR 52–80%) and a urinary C-peptide-to-creatinine ratio (UCPCR) of 0.01 (IQR 0.00–0.41) nmol/mmol. To assess β-cell function, we measured UCPCR (detectable >0.01 nmol/mmol), and to assess α-cell function, fasting plasma glucagon/glucose ratios were measured. CGM was used to record TIR (3.9–10 mmol/L), time below range (TBR) (<3.9 mmol/L), time above range (TAR) (>10 mmol/L), and glucose coefficient of variance (CV). For CGM, 74.7% used FreeStyle Libre 2, 13.8% Medtronic Guardian, and 11.5% Dexcom G6 as their device.

RESULTS
The percentage of patients with T1D who had a detectable UCPCR was 49.4%. A higher UCPCR correlated with higher TIR (r = 0.330, P < 0.05), lower TBR (r = −0.237, P < 0.05), lower TAR (r = −0.302, P < 0.05), and lower glucose CV (r = −0.356, P < 0.05). A higher UCPCR correlated negatively with HbA1c levels (r = −0.183, P < 0.05) and total daily insulin dose (r = −0.183, P < 0.05). Glucagon/glucose ratios correlated with longer TIR (r = 0.234, P < 0.05).

CONCLUSIONS
Significantly longer TIR, shorter TBR and TAR, and lower CV were observed in individuals with greater UCPCR-assessed β-cell function. Therefore, better CGM-derived metrics in individuals with preserved β-cell function may be a contributor to a lower risk of developing long-term complications.