Ask D'Mine: A Primer on Insulin Pen Diabetes Math

Happy Saturday, all! Welcome back to Ask D’Mine, our weekly advice column hosted by veteran type 1 and diabetes author Wil Dubois in New Mexico, who happens to have experience as a clinical diabetes specialist. This week, Wil takes a look at that pesky diabetes math that becomes even more confusion when applied to various insulin brands, pen sizes and dosing formats…

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Maureen, type 1 from New Jersey, writes: Well, my question seems pretty simple, but I just can’t figure it out. I take NPH at bedtime, only 12 units. It’s Humulin N in a U-100 KwikPen. The pen has 3ml. How many days should my pen last? It never lasts as long as I think it should. I must be doing something wrong…

Wil@Ask D’Mine answers: Your pen should last exactly 14 days. Here, let me guide you through the math, which isn’t as simple as it seems, as insulin pen mathematics are laced with an ambush or two.

Now, logically starting at the logical start, a 3ml pen of U-100 insulin has 300 units, as there are 100 units per mL, which is where the whole “U-100” thing comes from (3 x 100 = 300). So if you take 12 units per day, the pen should seemingly last 25 days (300 ÷ 12 = 25), right? But the pen, as you’ve found, doesn’t last as long as the math indicates it should.

So WTF?

Is Eli Lilly short-changing you? Not quite topping up those pens to make an extra few bucks at our expense? I’ve got a friend who has four pill boxes, instead of the single weekly box most of us use, and she loads them up for the month all at once when she picks up her meds. She was shocked to discover recently that her pharmacy shorted her two pills on each of her prescriptions. It seemed so unbelievable she figured she made a mistake. But it happened again the next month.

Now, this seems like such a small larceny as to hardly be worth it, until you consider that Americans filled 4,468,929,929 prescriptions in 2016, the most recent year I could find reliable data for. If you multiply that by 30 pills in a typical month, you get a number so big I couldn’t even make sense of it, so I put it into one of those digit-to-words converters. The answer? One hundred thirty-four billion and change. Pills range in price from a penny or less to who-knows what, but you can see that millions could be made “shorting” us.

But that’s not what’s going on here. At least I don’t think that’s what’s going on. I confess, I’ve never drained a KwikPen and measured the volume. That might be a fun science project for my home-schooled kiddo. But possible larceny aside, there’s an ambush lurking inside your insulin pen, and you set yourself up for it either by paying attention to your doc, listening to your diabetes educator, or reading the patient instructions.

And those patient instructions for your pen, in Step 8, tell you to prime the pen before each injection, warning: “If you do not prime before each injection, you may get too much or too little insulin.” The bold typeface is theirs. You know, to make sure you’re paying attention. Now, how skipping the prime could possibly give you too much insulin is beyond me, although skipping the prime can defiantly short-change your dose. But priming before each injection, as you’re instructed to do, takes a bigger bite out of your insulin supply than you might suppose. The instructions say to start with priming two units. If you don’t see insulin at the tip of the needle, you are to repeat the prime.

Up to four times.

In all likelihood this process will eat at least three of the units in the pen’s reservoir each time you prime. That realistically ups the ante of your 12-unit dose to 15 units, and suddenly the pen you thought would last 25 days is dry on day 20. Priming once per day ate five days of insulin over the life of the pen.

Not that you shouldn’t prime. You do need to. And for most folks it doesn’t matter, because very few people have a dose that exactly matches the volume of a pen, and if they did, the doc could easily write the script for a slightly higher volume to compensate for the daily priming. Although this issue becomes much more important with rapid-acting insulins, where you could have 6-8 primes in a day, adding up to as much as 720 units of “wasted” insulin per month—more than two full pens’ worth squirted into the air!

But wait, you say. How on earth did you get to 14 days for my pen? Adjusting for the prime should still give me 20 days. What happened to my other six days?? I want my six days back!

Remember I mentioned an ambush or two? The priming was the first ambush. The second ambush is below Step 17 in the patient instructions, which tells you how to put the cap back on the pen. Actually, it’s a bit below that in the middle of page 8 of the instruction sheet, down below the section on disposing of pens and needles, in the section called “Storing your Pen.” It’s nearly the last thing in the instructions, right between “the used pen may be discarded in your household trash” and “keep your pen and needles out of the reach of children.” That’s where is says: “Throw away the Humulin N Pen you are using after 14 days, even if it still has insulin left in it.”

Yep, old-fashioned N insulin doesn’t even last as long as a quart of milk. So it doesn’t matter how much you use or don’t use, at the 14-day mark, it goes in your household trash. Assuming that your household trash is out of the reach of children, that is.

Isn’t that wasteful? Not necessarily. Your dose is on the light side, as is common with type 1s. A type 2 would easily use that pen up in a week, or maybe less. Heck, your pen is fully capable of delivering 60 units in one push. At doses that high, even without compensating for priming, the pen would be dry in five days. The real waste, of course, is putting something into your body that doesn’t work. The 14-day limit exists for a reason. The very nature of how N insulin is made gives it a short lifespan once the cartridge is pierced by the first needle, and nothing can change that.

Now, N, also called NPH is an old-school insulin that dates back all the way to 1946. By all rights it should be extinct, but it’s made a comeback. Why is that? It’s cheap, that’s why, and we are all poor. In some cases health plans don’t want to cover state of the art meds, in other cases PWDs simply can’t afford the copays on the “good stuff.” So NPH is baaaaaaaack.

Whether you are new to N, or you’ve not used it in decades, here are a couple of quick factoids to help you get the most out of this affordable, but short-lived, juice:

• Keep your stash of unopened pens in the fridge. Don’t let them get pushed to the back, because if they freeze they won’t work. The butter compartment was really designed by PWDs.

• Unless you live in Death Valley with no air conditioning, keep your active pen at room temperature (up to 86°F). Keeping it cold won’t extend its life, and cold insulin stings when you inject it.

• N needs to be mixed before each use. For pens, roll them between your hands ten times, then grab the pen by the base and flip it up and down ten times. Make sure you have a good grip on the pen so you don’t put your eye out. A little bead inside the pen helps mix the insulin. Well mixed N should be white and cloudy. If you have sections of clear and cloudy, keep mixing.

• N doesn’t have a “flat” action profile like modern basals. It has a significant peak action point, usually around 8 hours after injection, which has the potential to trigger a wicked hypo while you are sleeping. The old-fashioned workaround is a high fat snack at bedtime to avoid going low in the night.

So sorry about that. All that confusing math for nothing. But that’s the thing about diabetes: There are no simple questions. Thanks for writing, and keep your questions coming, people!

This is not a medical advice column. We are PWDs freely and openly sharing the wisdom of our collected experiences — our been-there-done-that knowledge from the trenches. Bottom Line: You still need the guidance and care of a licensed medical professional.