The IOB Guide for T1D, Part 3

Choosing Your Device-Specific IOB Settings: What Are You Optimising For?

In clinic, the team asks: “Your AIT is set to four hours. Do you want faster corrections, or better exercise safety?” You realise the question has no neutral answer. Choosing your settings is choosing what you are optimising for.

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Bolus calculator apps

Standalone bolus apps vary in how they handle IOB. Tap each to expand.

mySugr / Accu-Chek Bolus Advisor

Model: Expected glucose after-meal behaviour (offset + meal rise + acting time); correction IOB deducted for corrections. Optimises for: trajectory-aware correction timing and stacking governance. Limitation: live insulin exposure is not displayed. Best fit: personalised after-meal control where you are comfortable without a visible IOB number.

Diabetes:M, mylife App, InPen, RapidCalc

Model: linear or curvilinear decay (manufacturer-modelled, exact shape proprietary); the IOB mode varies by app (no IOB recognised for corrections, correction IOB only, or meal + correction pooled). Optimises for: visible active insulin and straightforward stacking control. Limitation: behaviour depends heavily on AIT setting and whether active insulin is enabled. Best fit: you want to see total active insulin and are comfortable managing the correction-exposure trade-offs yourself.

Standard pumps (without AID)

Standard pumps pool meal and correction insulin into a single IOB number. Shorter AIT lets corrections through faster; longer AIT slows them down. Tap a manufacturer for its specific architecture.

Medtronic Bolus Wizard, pre-AID (530G family)

Model: curvilinear decay (manufacturer-modelled, exact shape proprietary); meal + correction pooled and deducted from corrections. User setting: AIT/DIA adjustable 2 to 8 hours in 15-minute increments. Note: this is the pre-AID Bolus Wizard family. The AID 780G SmartGuard uses a narrower AIT range and is covered in the AID section below.

Insulet Omnipod (DASH, Omnipod 5 Manual Mode)

Model: linear decay (manufacturer-modelled, exact shape proprietary); meal + correction pooled and deducted from corrections. User setting: AIT adjustable 2 to 6 hours in 30-minute increments.

Tandem t:slim X2 (Manual Mode)

Model: linear decay (manufacturer-modelled, exact shape proprietary); meal + correction pooled and deducted from corrections. User setting: AIT adjustable 2 to 8 hours in 15-minute increments.

Ypsomed mylife YpsoPump (without AID)

Model: linear decay (manufacturer-modelled, exact shape proprietary); meal + correction pooled and deducted from corrections. User setting: AIT/DIA adjustable 2 to 8 hours in 15-minute increments.

The trade-off, plainly stated. Shorter AIT gives you correction freedom and hides physiological insulin from view. Longer AIT shows the insulin and slows your corrections. Choosing your AIT is choosing which problem you would rather solve; and recognising the workarounds (shortening AIT to unlock corrections, repeated small corrections after a meal, using IOB as an exercise green light) are signals that the problem you have not chosen is biting back.

AID and hybrid closed-loop systems

AID adds an algorithm layer on top of the bolus calculator. The two layers may use different insulin accounting, so what shows as IOB rarely matches total exposure. Tap a system for its specific architecture.

How the four UK AID systems calculate insulin on board Four columns showing the IOB calculation approach for Tandem Control-IQ, MiniMed 780G, CamAPS FX, and Omnipod 5. Each column names the AIT model, the bolus tracking, and what shows on screen, with the Optimiser declared bias toward IOB visibility named at the foot. Insulin on board, four systems, four calculations Each system makes a different decision about how to model the insulin still working in the body. CONTROL-IQ Tandem t:slim X2 + Mobi AIT model Fixed at 5 hours. Not adjustable. Bolus tracking All boluses (manual + Autobolus) tracked over 5h linear decay. On screen Total IOB on the home screen. A reasonable physiological proxy. IOB visibility High Useful for exercise planning around IOB. MINIMED 780G Medtronic SmartGuard AIT model User-set, 2 to 4 hours. AIT 2h is the Cohen recommended start. Bolus tracking Manual + auto-corrections tracked over the chosen AIT. On screen Active insulin on screen, scaled to the chosen AIT. Shorter AIT under-shows IOB. IOB visibility Variable Depends on AIT setting. 2h hides IOB the most. CAMAPS FX Cambridge Adaptive AIT model Adaptive. Algorithm learns insulin clearance over time. Bolus tracking Personalised pharmacokinetic model, not a fixed AIT. On screen Two displays: portrait (simple) and landscape (IOB curve over time). IOB visibility High in landscape Most IOB-detailed display of any UK system. OMNIPOD 5 Insulet SmartAdjust AIT model Fixed by the algorithm. Not adjustable. Bolus tracking Manual boluses tracked on screen. Algorithm corrections handled internally. On screen IOB from manual boluses only. Algorithm IOB sits behind the curtain. IOB visibility Partial Manual-bolus IOB only. The GNL Optimiser declares an IOB-visibility bias: the more the system surfaces, the easier it is to plan exercise, late meals, and cycle-context decisions around what is still on board.
Each UK AID system makes a different decision about how to model the insulin still working. The Optimiser declared bias is toward higher IOB visibility on screen.
Medtronic 780G SmartGuard (AID)

Bolus Wizard and SmartGuard: curvilinear decay (manufacturer-modelled, exact shape proprietary), meal + correction IOB. User setting: AIT adjustable 2 to 4 hours. Algorithm: automated basal adjustments and micro-corrections, total IOB included.

At AIT 2 hours, the manufacturer-recommended starting configuration per the Cohen letter, the system runs at its highest algorithm strength: shortest assumed insulin life, lowest IOB carry, most frequent corrections. At AIT 4 hours, the upper bound of the SmartGuard AIT range, the algorithm sits at its most cautious: longer assumed insulin life, higher IOB carry, fewer corrections. The educationally useful starting band is the 2 to 3 hour slice where the Cohen-recommended configuration sits. Pre-AID Bolus Wizard nuance (AIT 5 to 8 hour ghost-carbohydrate workarounds) belongs to the earlier “Standard pumps” section.

Omnipod 5 (Automated Mode)

Bolus calculator: linear decay (manufacturer-modelled, exact shape proprietary), meal + correction IOB. User setting: AIT adjustable 2 to 6 hours in Manual Mode; in Automated Mode the SmartAdjust algorithm uses a fixed internal insulin model. Algorithm: manages delivery via a separate internal IOB pool (not publicly defined). Negative IOB values can appear when basal has been suspended.

The algorithm looks around 60 minutes ahead, so suspension fires earlier than on systems with longer prediction horizons. A shorter user-defined AIT (2 to 2.5 hours) usually works better in Manual Mode; longer AIT tends to produce prolonged hyperglycaemia. Many people land at AIT 2 to 2.5 hours, a stronger correction factor, and reverse correction disabled.

Tandem t:slim X2 with Control-IQ

Bolus calculator: linear decay (manufacturer-modelled, exact shape proprietary), meal + correction IOB. User setting: AIT fixed at 5 hours. Algorithm: automated basal modulation and hourly automated corrections based on the correction factor; user IOB incorporated via a fixed 5-hour insulin action curve.

The algorithm needs basal headroom to work in both directions, so basal at 50 to 60% of TDD is helpful. Some people use slightly weaker carb ratios and stronger correction factors so more insulin flows through automation rather than large boluses. The fixed 5-hour AIT means displayed IOB is a reasonable approximation of physiological insulin exposure for exercise planning.

CamAPS FX

Bolus calculator: proprietary internal insulin action model (pharmacodynamic kinetics, manufacturer-modelled, exact shape proprietary). User setting: no meaningful user adjustment of the insulin action model. Algorithm: pairs carbohydrate entry and insulin delivery within its internal model.

Highly specialised but opaque; behaviour is largely fixed and not transparent. Boost mode strengthens the algorithm response without requiring extra carbohydrate entry, which most people prefer because it avoids manipulating carb logging or confusing the data.

Part 3 of 7

Choosing Your Device-Specific IOB Settings

← Part 2 ↑ Hub Part 4: The Future →

Read more on GNL

The IOB Trade-Off: how four AID systems make different bets on insulin → AID Systems and Exercise (EASD/ISPAD 2025 position statement) → Exercise IOB Hypoglycaemia Risk Explorer → The AID Guide: per-system IOB defaults and how to read them →
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