Control-IQ pivotal trial (iDCL): t:slim X2 closed-loop vs. sensor-augmented pump

What was tested and why it matters

This trial — formally the International Diabetes Closed Loop (iDCL) pivotal study — tested whether an "artificial pancreas" that automatically adjusts insulin could keep blood sugar in a healthy range better than the best widely available alternative.¹ The closed-loop system was the Tandem t:slim X2 insulin pump running the Control-IQ algorithm, paired with a Dexcom G6 sensor: every five minutes it reads glucose, predicts where it is heading, and nudges insulin up or down — including automatic correction doses for highs.¹ The comparison was a sensor-augmented pump: the same kind of pump and sensor, but with no automation, so the person makes every dosing decision.²

Who it was for

Researchers enrolled people aged 14 and older who had lived with type 1 diabetes for at least a year.¹ Importantly, they accepted a broad spread of starting control (HbA1c from about 5.4% to 10.6%), so the findings apply to people who are already doing well and to those who are struggling.²

How it was designed

This was a 6-month randomized, multicenter trial at sites across the United States.² A total of 168 participants were assigned in a 2:1 ratio — 112 to the closed-loop system and 56 to the sensor-augmented pump.² The single most important measure, set in advance, was the share of time glucose stayed between 70 and 180 mg/dL, tracked continuously by the sensor.¹

Key results

Time in the target range rose in the closed-loop group from 61% at baseline to 71% over the six months, while the comparison group held steady at about 59% — an adjusted difference of roughly 11 percentage points (95% CI, 9 to 14; P<0.001).² That is about 2.6 more hours in range every day. Closed-loop also lowered average glucose, cut time spent high, modestly reduced time below 70 mg/dL, and lowered HbA1c by about 0.33 percentage points — all of which met the trial's pre-set statistical bar.² The system stayed in automated mode about 90% of the time, and all 168 participants completed the study.² On safety, there were no severe low-blood-sugar events; one case of diabetic ketoacidosis occurred in the closed-loop group, attributed to an infusion-set problem.² A later analysis showed the benefit held up whether people came in using a pump or daily injections, and whether or not they had used a sensor before.³

What it means and what's next

These results made the scientific case for Control-IQ and supported its U.S. regulatory clearance, helping move automated insulin delivery from research into everyday care.¹ The honest caveat: the trial ran six months at expert centers, so it speaks to short-to-medium-term glucose control rather than decades of real-world use, and a roughly 11-point gain — while large — still leaves room for the fully "hands-off" pancreas that remains the goal.²

References

1. University of Virginia / Jaeb Center for Health Research. The International Diabetes Closed Loop (iDCL) Trial: Clinical Acceptance of the Artificial Pancreas — A Pivotal Study of t:slim X2 With Control-IQ Technology. ClinicalTrials.gov, NCT03563313 (registry record, accessed 2026). https://clinicaltrials.gov/study/NCT03563313 ↩ ↩² ↩³ ↩⁴ ↩⁵

2. Brown SA, Kovatchev BP, Raghinaru D, et al. Six-Month Randomized, Multicenter Trial of Closed-Loop Control in Type 1 Diabetes. New England Journal of Medicine 381(18):1707-1717 (2019). https://doi.org/10.1056/NEJMoa1907863 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹

3. Ekhlaspour L, Raghinaru D, Forlenza GP, et al. Outcomes in Pump- and CGM-Baseline Use Subgroups in the International Diabetes Closed-Loop Trial. Journal of Diabetes Science and Technology 17(4):935-942 (2022). https://doi.org/10.1177/19322968221089361 ↩