How the numbers work

Insulin Pal turns the mealtime insulin doses you log into two estimates and a graph. This page explains how those estimates are worked out, where the numbers come from, and what they leave out. Insulin Pal never calculates or recommends a dose.

The two numbers

Insulin on board is all the insulin from your recent doses that is still in your body: the part already working, plus the part still waiting to be absorbed. It only goes down as time passes, so it shows how much insulin is still to come.

Active insulin is the part of your insulin on board that has been absorbed and is working right now. It starts near zero when you inject, rises to a peak, then fades. It is the height of the curve on your graph.

How a dose becomes a curve

Every type of mealtime insulin follows a typical pattern after a dose: it starts working, reaches a broad peak, then slowly fades out. Insulin Pal describes that pattern for each group of insulins with three timings: when the rise is complete, when the peak starts to fade, and when the dose has finished.

From that pattern, the app works out for any moment how much of a dose is still to come (insulin on board) and how much is working right now (active insulin). When you log several doses, their curves are added together. The graph shows each dose as a dashed line and the combined total as a solid one.

Which curve your insulin uses

You choose your insulin by the name on the pen or vial. Brands that contain the same insulin share one curve. Insulin Pal has four:

If you choose "Prefer not to say", the app uses the rapid-acting curve, the most common one for people who inject mealtime insulin.

Where the numbers come from

The timings are based on published sources, and on the insulin models used by widely used open-source diabetes software:

• The official US prescribing information for each insulin, which describes when it starts working, peaks and ends: Humulin R, Novolin R, Afrezza, and the Humalog prescribing information (Eli Lilly).
• A clinical study of insulin lispro's activity over time: Howey et al., Diabetes 43(3):396–402 (1994).
• The insulin models of the open-source projects Loop, OpenAPS and AndroidAPS.

Where the sources give a range, Insulin Pal leans toward the longer duration. That means the app tends to show insulin as still on board a little longer, rather than a little shorter.

The "Account for CGM lag" setting

Continuous glucose monitors measure the fluid under the skin, which trails blood by roughly 5 to 15 minutes. If you turn on "Account for CGM lag" in the app's settings, every curve is moved 10 minutes later, so the timing lines up better with what your sensor shows. It changes when things happen on the graph, not how much insulin is counted.

What the estimates leave out

• Only mealtime insulin is counted. Long-acting (basal) insulin, NPH and premixed insulins are not in the app, because one simple curve can't describe them.
• The curves are typical patterns, not measurements of you. How fast insulin works varies from person to person and day to day: with the dose size, the injection site, exercise, temperature and more.
• The app only knows the doses you log. A missing, late or wrong entry changes every number.
• The app doesn't know your glucose, what you eat, or any other medicine you take.
• Afrezza units are not interchangeable with injected units. The app draws the curve for whatever number you log.

Not medical advice

Insulin Pal is for personal record-keeping and general information. It does not diagnose, treat, or recommend doses, and its estimates can be wrong. Never make a dosing decision based only on what the app shows, and make any change to your insulin routine together with your care team. More in our Medical Disclaimer.

Questions

Spotted something that looks off, or want to know more? Write to hello@insulinpal.app.