About bgcast

What this is

bgcast is an open-loop notification pump. A real pump runs an algorithm continuously and acts on its own. bgcast runs the algorithm continuously and, when action is warranted, notifies you — and you inject. The software is the controller; you are the actuator.

It pulls Freestyle Libre readings from LibreLinkUp into a database on this server, shows them in real time, and suggests correction doses based on parameters you set.

How the suggestion works

Insulin on board is modelled with an exponential curve parameterised by peak time and duration of action — the same shape used by Loop and AndroidAPS. Logged carbs are modelled too, absorbing over a time you set, so the suggestion is based on where your glucose is heading once the insulin already administered has finished working and the food you have logged has absorbed — not simply where it is now:

carb impact = COB × (ISF ÷ ICR)
eventual BG = current BG − (IOB × ISF) + carb impact
suggestion  = (eventual BG − target) ÷ ISF

Because logged food raises the eventual BG, one suggestion covers both a meal and a correction; a meal matched by its bolus cancels out to nothing. A suggestion is suppressed entirely — with the reason shown — if glucose is low or predicted to go low, if data is stale, if a dose was taken too recently, or if glucose is rising faster than the insulin and logged carbs together explain, which usually means food that bgcast does not know about.

The eventual BG is only the endpoint. Because rapid insulin peaks and clears faster than food absorbs, a dose that lands on target at the end can still dip low in the middle. bgcast walks the whole curve forward: if the dose is projected to fall below your urgent-low line on the way, it is refused outright; if it only grazes the low band, the suggestion still stands but the projected dip is shown so you can split the dose or keep fast carbs to hand.

Limitations you should know about

• The data source is unofficial. LibreLinkUp has no public API. bgcast uses a reverse-engineered one that Abbott can change or block at any time.
• It only knows what it is told. Insulin or carbs taken without being logged are invisible to the calculation, and the suggestion will be wrong as a result.
• Carb absorption is a rough model. Real absorption varies with the meal, activity and more; bgcast assumes a simple, steady absorption over the time you set. Treat a suggestion that leans on carbs on board with extra caution.
• Overnight suggestions are suppressed by default. A pump can undo a correction it gave at 3am by cutting basal. You cannot undo an injection.