Tandem t:slim X2 with Control-IQ

Control-IQ’s logic can be read as a set of predicted-glucose bands. Each band tells the algorithm what to do with the basal rate, and whether the Autobolus is on the table.

• Predicted above 10.0 mmol/L (180 mg/dL): Autobolus territory. Once an hour the algorithm can deliver an Autobolus at 60% of the calculated correction dose. The calculation uses the correction factor, current insulin on board, and the predicted glucose. The Autobolus is capped at six units in any single delivery and is delivered on top of the basal increase, not in place of it. It fires without confirmation; the insulin goes in, then appears in the pump history.
• Predicted above 8.9 mmol/L (160 mg/dL): basal lift. The algorithm can increase basal delivery up to four times the programmed basal rate, capped at 15 units per hour. The Autobolus stacks on top from 10.0 mmol/L upwards.
• Predicted between 6.25 and 8.9 mmol/L (112 to 160 mg/dL): hold. The pump delivers the programmed basal rate unchanged.
• Predicted below 6.25 mmol/L (112 mg/dL): basal reduce. The pump trims the basal rate downward.
• Predicted below 3.9 mmol/L (70 mg/dL): basal suspend. The pump stops basal delivery. In Exercise Activity the suspend trigger lifts to 4.4 mmol/L (79 mg/dL).

The hold band of 6.25 to 8.9 mmol/L is where most steady-state running happens; the action sits at the edges.

What you actually feel day to day on Control-IQ is the algorithm working two channels at once on a familiar 5-hour clock, with two named modes that change how it behaves overnight and during exercise. The four pieces below are what makes it Control-IQ rather than any other system.

• Dual-channel automation. Both basal and bolus are adjusted. The algorithm has more to work with than basal alone.
• 30-minute predictive horizon. Every delivery decision is made on where glucose is heading, not where it currently sits.
• Fixed 5-hour AIT. On-screen IOB tracks physiological insulin reasonably well. Useful for exercise planning.
• Sleep Activity and Exercise Activity modes. Sleep tightens the target band to 6.25 to 6.7 mmol/L (112 to 121 mg/dL) and starts increasing insulin as predicted glucose approaches 6.7, instead of waiting until 8.9. Exercise raises the target band to 7.8 to 8.9 mmol/L (140 to 160 mg/dL), suspends the Autobolus, and lifts the basal-suspend trigger from 3.9 to 4.4 mmol/L.

Control-IQ does not learn or adapt its parameters to individual patterns the way some other AID systems do. Behaviour is determined by the built-in logic plus the programmed settings: basal rates, carb ratios, correction factor. Getting those settings well-tuned matters more on Control-IQ than on the more adaptive systems.

UK MHRA / UKCA status for the Mobi is not yet confirmed as at this writing. The 200-unit reservoir with a 30-unit minimum fill makes it well suited to very low total-daily-dose users (insulin wastage on larger reservoirs is significant for low-TDD users). Verify availability with Tandem UK.

Laurel Messer, Tandem Global Medical Affairs, GNL Podcast episode 42: “The Control-IQ technology lives in the Tandem Mobi pump itself. It directly talks to the CGMs. So automation occurs all the time. It never kicks you out of automation. As long as you have a CGM running, automation is running. Doesn’t matter if your phone is in the other room.”

The biggest lever. Messer 2023 found the strongest CF quartile achieved 14% more time in range than the weakest. The algorithm uses CF to size every automated correction; too weak and it under-doses, too strong and it stacks. The Optimiser expresses CF as a TDD-anchored rule: 80 / TDD at the strongest level, 110 / TDD at the most permissive, 90 / TDD at Level 3.

Laurel Messer, Tandem Global Medical Affairs, GNL Podcast episode 42: “While I think all the levers are really important, especially knowing that we have a robust protection against hypoglycemia, correction factor is the piece that I think of as the secret sauce, the secret tuning for better outcomes with Control-IQ.”

And: “Or even if you’re not bolusing, if you’re not able to consistently deliver boluses, that correction factor is that magical piece that’s going to help with the missed boluses.”

Not a target to hit, a calculation for setting programmed basal rates. For a delivered TDD of 20 u/day at Level 3 (55% basal), that is 11 u/day of basal, around 0.46 u/h. The ladder moves this from 45% at Level 1 to 65% at Level 5, giving the algorithm more dynamic range at higher strength.

The manual-bolus lever. The pump uses it to size a meal bolus when carbs are announced. The TDD-anchored rule runs from 500 / TDD at Level 1 to 300 / TDD at Level 5. Bolus percentage was retired from the ladder on Laurel Messer’s advice in April 2026; ICR is the actual pump setting and is what people and clinicians adjust.

Sleep Activity is not a level axis. It tightens the target (around 6.25 to 6.7 mmol/L), strengthens the every-5-minute delivery, and disables the Autobolus while active. Two modes:

• Sleep Activity 24/7 when meal boluses are being given consistently. The 5-minute delivery does the work; the Autobolus is not needed because you handle meals.
• Sleep Activity night-only when daytime meal boluses are regularly missed. This keeps the daytime Autobolus active so it can cover the meal excursions you have not bolused for.

Running Sleep Activity 24/7 without consistent bolusing removes the daytime Autobolus and tends to produce more hyperglycaemia, not less.

The five-level ladder was reviewed and refined with input from CamAPS, MiniMed, Tandem and Insulet global medical leads (Tandem covers both Control-IQ and Mobi, same algorithm family). The levels themselves have not been validated against any manufacturer’s internal simulator. The Optimiser is a Grade D educational synthesis on a Grade A and B evidence base (Breton 2020 NEJM, Messer 2023 DT&T n=20,764, Beck 2023 pooled analysis, Shah 2026 ADjust, ISPAD 2024 Ch23). It carries a declared bias toward insulin-on-board visibility; this is the educational frame GNL has chosen, not a clinical override. Any deviation from manufacturer-recommended starting settings is a conversation with your diabetes care team.

Paediatric target floors are an open evidence gap on Control-IQ. The Optimiser does not apply a manufacturer-published paediatric floor on Control-IQ; only the MiniMed 780G has a manufacturer-published paediatric floor (Cohen 2024). Discuss paediatric configuration with your diabetes team.

Control-IQ+ is the next-generation Control-IQ algorithm. The behaviour is the same Control-IQ at heart; the headline change is the range it can be tuned across.

• Total-daily-dose range: 5 to 200 units. Previously the algorithm was constrained to people requiring between 10 and 100 units of insulin per day. Control-IQ+ runs from 5 units at the bottom to 200 units at the top. This brings two populations into scope at once: very young children with very low total daily insulin needs, and adults requiring high doses (often where insulin resistance is in play).
• FDA age indication: 2 years and greater. Control-IQ+ is FDA-cleared from age two. That extends the system into the preschool band, four years lower than the standard Control-IQ floor of age six. Combined with the wider TDD range, this opens the system to a population previously without an on-label closed-loop option in the US.
• Correction-factor cap: up to 33 mmol/L (600 mg/dL). Standard Control-IQ capped a programmed correction factor at 11 mmol/L (200 mg/dL) when the algorithm was active; if a clinician set a weaker CF than that, the system would still treat it as 11. Control-IQ+ lifts that cap to 33 mmol/L (600 mg/dL). Practically: clinicians working with very-insulin-sensitive children can now programme genuinely weak correction factors for the overnight period without the algorithm overriding them, while still having the system step in if a rise becomes a real excursion.

Laurel Messer, Tandem Global Medical Affairs, GNL Podcast episode 42: “Before it was 10 to 100 units, and now it’s 5 to 200. … The algorithm with Control-IQ+ now has been optimized for very low TDI users. So people who require five units or more can now use the system. And also on the very high end, people who are using up to 200 units of insulin a day.”

Pregnancy with type 1 diabetes asks for time-in-range to be measured against a tighter band than usual. The pregnancy-specific time-in-range target is 3.5 to 7.8 mmol/L (63 to 140 mg/dL). CIRCUIT is the CamAPS-style randomised trial for Control-IQ; the parent results landed in JAMA in 2025, and a prespecified secondary analysis covering labour, delivery, and the first six weeks postpartum landed in Diabetes Care in 2026. Control-IQ is not licensed for pregnancy in Europe; the CIRCUIT trial used it off-label under research protocol with the diabetes-in-pregnancy team.

CIRCUIT, the gestation-window result (Donovan 2025, JAMA). Ninety-one pregnant women with type 1 diabetes across 14 academic sites in Canada and Australia were randomised before 16 weeks gestation to Tandem t:slim X2 with Control-IQ or to their existing insulin delivery with CGM. From 16 to 34 weeks gestation, the closed-loop group spent approximately three hours per day more time in pregnancy-specific range (63 to 140 mg/dL, 3.5 to 7.8 mmol/L) than standard care. The full citation is Donovan LE et al., JAMA 2025;334:2176-2185.

CIRCUIT, the intrapartum and postpartum result (Donovan 2026, Diabetes Care). The prespecified follow-up looked at the 24 hours before delivery, the first postpartum week, and the six-week postpartum window. 89% of those randomised to closed loop (39 of 44) continued Control-IQ intrapartum. In the 24 hours before delivery, time in pregnancy range was 79.6% on Control-IQ against 64.8% on standard care, an adjusted difference of 13.2 percentage points (95% CI 5.2 to 21.2; P = 0.002), with no rise in time below 63 mg/dL. Intravenous insulin was used by 1 of 44 (2%) closed-loop participants against 20 of 44 (45%) standard-care participants. In the first postpartum week, time below 70 mg/dL was 1.7% on Control-IQ against 3.2% on standard care, an adjusted reduction of 1.8 percentage points (about 26 minutes per day less hypoglycaemia). The benefit was sustained through six weeks postpartum. No maternal severe hypoglycaemia or DKA occurred intrapartum in either group. Citation: Donovan LE et al., Diabetes Care 2026;49:1-9, doi: 10.2337/dc26-0470.

The protocol CIRCUIT used. Sleep Activity (target 6.25 to 6.7 mmol/L) was recommended intrapartum and was actually used 91% of the time in the 24 hours before delivery; 77% of Control-IQ users ran Sleep Activity 100% of the time, with the option to exit if low glucose was a concern. A preprogrammed postpartum profile was activated just prior to delivery: basal rates, correction factors, and insulin-to-carb ratios approximately 50% weaker than the end-of-pregnancy settings, or approximately 33% weaker than prepregnancy pump settings where available. No routine manual adjustments were made for user weight or total daily dose, in contrast to some protocols recommended elsewhere.

In pregnancy. The AiDAPT trial of CamAPS FX in pregnancy reported about a 10.5 percentage-point gestation-window advantage over standard care (Lee TTM et al., NEJM 2023;389:1566-1578). For the labour-and-delivery window specifically, AiDAPT’s intrapartum abstract reported a 7-percentage-point effect (95% CI minus 3 to 16) and the CRISTAL trial of MiniMed 780G reported a 9-percentage-point effect (72% vs 63%, P = 0.030). The Control-IQ intrapartum effect from Donovan 2026 is the largest of the three, though the trials differ in baseline HbA1c, insulin delivery before randomisation, and how much intravenous insulin was used in the standard-care arm. The honest position: all three commercial closed-loop systems work in pregnancy, all three are safer than standard care intrapartum on the available data, and the choice between them is best made with your diabetes-in-pregnancy team on grounds other than expected time-in-range.

Outside pregnancy. Control-IQ and CamAPS FX share the same fundamental approach: increase algorithm-delivered insulin by pushing basal higher (up to 65% of TDD) and setting weaker carb ratios so the remaining bolus is divided by actual carb intake. Both track IOB more physiologically than the alternative approach. That alternative is the Medtronic 780G and Omnipod 5 path, which lower IOB visibility by using a shorter AIT. The shorter AIT lets the algorithm assume insulin clears faster, which enables more frequent auto-corrections, but it costs you IOB visibility, particularly during and after exercise where hidden active insulin can drive an unexpected hypo. The IOB physiology mismatch is explored in the IOB Guide.