Diabetes Math

After getting basal rates that I believe, I’m up to the part of my on-going “Let’s finally figure out the right pump settings!” project where I check my bolus amounts. I have long suspected that I am over-insulinized in the morning, since I frequently need a snack between 10AM and noon to prevent a hypo. Now that I know my basal rates are correct—I can plank pretty well if I don’t eat—the fiery finger of blame points at my bolus ratios.

Last week, after my awesome endocrinologist “ooh-ed and ahh-ed” over my much improved A1c, I asked her my burning question about active insulin (a.k.a., insulin on board or IOB). “When I’m bolus stacking, when do I consider active insulin and when don’t I?” I frequently eat multiple times during any given five hour period, which means that I have multiple doses of insulin working on their respective meals. I’ve noticed that sometimes I have to take that insulin into account, since its “long tail” will continue to lower my blood glucose for a bit longer. Other times, if I subtract out that IOB, I see high blood sugars later because that insulin was still needed for the previous meal/snack. What to do?

Her advice to me (and this supposes that my pump’s “active insulin” setting is correct) was to subtract out the active insulin if the bolus was three or more hours ago, since digestion should have finished by that point. This is when I’m already near my BG target and have IOB.

(Since I’m often back into the lower part of the “happy” range 2-3 hours after a bolus and/or I didn’t see much of a rise at all, this is probably another indicator that I’ve been taking in too much insulin. That and the fact that bolusing frequently comprised more than 70% of my total daily dose, which is higher than the 40-60% that most recommend. As a result I was eating a lot of “defensive” carbs to prevent lows, which seems to be leading to highs later in the day.)

I had been trying less aggressive bolus ratios over the weekend with some success, but acting on hunches and suspicions is bad form, so today I decided to be a bit more rigorous. Armed with my endo’s advice, a measuring cup, notebook, and pen, I set out to figure out what my bolus ratios need to be. Here is the data through lunch:

05:03 - 164 -       - Wake up... yawn.
05:45 -     -       - Swim 1000 yards in 20 minutes. Disconnected for 35 minutes.
06:21 - 121 - 2.9 u - Granola bar (24g) = 2.6u + 0.3u correction (using 1:9 ratio)
07:34 - 149 - 6.5 u - Cereal, milk, eggs, sausage (64g) = 7.1u (but I know this is too much)
10:18 - 104 -       - There was a bit of walking around in the previous hour
11:15 - 104 - 1.0 u - 1/2 donut from the box next to the printer (19g) = 2.3u - 1.3u IOB
12:00 - 120 - 8.3 u - Sandwich (35g), Yogurt (18g), Apple (~20g) = 75g = 8.3 (using a 1:9)

This morning looks pretty successful. Let’s try to reverse engineer a possibly correct bolus ratio. I ate 24+64=88 grams of carbohydrates for breakfast, and delivered 9.4 units of insulin. That yields, let’s see here, 1 unit for 9.4 grams, or about 1:9. But we know that’s too much from past experience (including this morning).

The 9.4 units of total insulin includes whatever insulin was needed to make up for the 35 minutes that I was disconnected from my pump while swimming and a correction of 0.3 units, which may or may not be the same. Let’s run the numbers again a few different ways.

Not taking correction into account: (24 + 64 g) / (2.9 + 6.5 units) = 9.4 g/unit

Just using the correction: (24 + 64 g) / (2.9 + 6.5 – 0.3 units) = 9.7 g/unit

Using the time off pump: (24 + 64 g) / (2.9 + 6.5 – 0.9*(35/60) units) = 9.9 g/unit — 0.9 units/hour is my basal rate at that time in the morning.

What’s the lesson here? What ratio should I use? 9.4? 9.7? 9.9? Since my pump only works in whole units for the ratio, it’s a choice between 1:9, 1:10, or carrying a calculator/MATLAB everywhere with me. (The pump version one generation after mine does allow fractional ratios.) The 1:9 ratio is clearly too aggressive given what’s been happening to me, so I’m going for 1:10, which has a certain easiness to it, too.

Let’s see what tomorrow brings.

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4 Responses to Diabetes Math

  1. scully says:

    You lost me somewhere around the middle. I was keeping up as much as I could but fell off the intelligent wagon and got lost in the muck.
    How the hell am I supposed to figure this shit out for myself if I can’t even follow a post about yours?
    i feel extremely lacking in knowledge and abilities to calculate this… ergo I continue on in a half-whacked way of living via the way of the vial. :(

  2. Jeff Mather says:


    First, just let me say that no one should have to figure this out. It’s completely not fair that we have to. And—just in case anyone suspected otherwise—I don’t expect perfection or scientific/mathematical accuracy with my diabetes, despite the decimal points here. Nevertheless, I do think that because I’m heading low and then high every day around the same times, there’s room for some general improvements.

    Also, this kind of calculation can probably be summed up this way, supposing it’s correct:

    (1) Enter accurate BG and carb info (if available) into the bolus wizard.
    (2) See if there’s IOB and a correction bolus needed. If it’s been less than three hours, use what the bolus wizard suggests for a correction (i.e., correction – IOB). If it’s been more than three hours and no correction is needed (or IOB is greater than correction), then . . . um . . . subtract out the IOB?
    (3) Hope it works out.

    All the rest of the post is just me trying to figure out what my bolus ratio probably is based on how much insulin I delivered and what I think I ate. I kinda went up a bunch a couple hours after I ate lunch, which in my experience means that I needed a little bit of extra insulin. So that’s what I need to work on next. :-/

    It’s crazy-making, which is probably why it’s been more than ten years that I’ve been on the pump and haven’t yet figured this out.

    p.s. – “Way of the Vial” = brilliant! It’s going to be the name of my punk band.

  3. Céline says:

    Fascinating! Too bad the fascinating info comes at the cost of playing with our bodies but it is pretty interesting on a purely numerical level. I did get a bit lost at the end but will read it a few more times to see if I can figure out what you’re talking about enough to try it out myself.

  4. Dude, my head hurts. I think I’ll go sit with Scully in the mucky tracks of the intelligence wagon. :-)

    But seriously, good on ya for experimenting and working so hard to improve. I love your comment above about it not being fair.

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