The last vocabulary day. The variable that explains more confusing CGM moments than any other — and that most people don't actively track.
Today's idea
IOB stands for "insulin on board" — the amount of insulin you've already taken that is still working. Rapid insulin's effect lasts 3–5 hours after a dose, with most of the action happening in the first 2–3. If you bolused 4 units 90 minutes ago, maybe half of those units are still in front of you. That's IOB.
Why it matters: a correction made without considering IOB is often a double-dose in disguise. The reason "I corrected and then I crashed" is one of the most common stories in Type 1 isn't bad luck. It's an invisible variable working in the background.
Pumps and many insulin delivery apps calculate IOB for you. If you're on MDI without an app, the math is approximate but doable. Either way, the skill is just remembering it exists.
The Pro Tip series puts it bluntly:
A drop with active insulin still on board is a very different situation than a drop without it. So is a high with active insulin versus a high without. The variable is invisible — but it's almost always in play.
From Pro Tip 1007
What I gave 90 minutes ago is still in play. I'm allowed to remember that.
Today, before any action you might consider, notice how much insulin is still working from earlier doses. Even a rough estimate. Just bring the variable into your awareness.
Small Sip #1434 — Are You Stacking Insulin
Did remembering IOB change how I read any moment today?