Free Style Libre showing big swings

I just started using the Free Style Libre monitor. I have LADA and still have some residual insulin. My A1C is around 5.6-5.9. I thought I had pretty good control but am finding that after eating even small amounts of carbs (10-15 grams) and doing 2-3 units of Humolog insulin, my BS is up around 200 for about 45 minutes and then falls back to normal levels (100-120) or even goes low. I have confirmed that the numbers are correct with finger sticks. I guess this was always happening, but with more limited finger sticks I didn’t know about the swings. Should I be concerned about these short elevations? I am wondering if I need to do my insulin shot 30 minutes before eating in order to have it prevent the big spike but am also worried about going low. Any suggestions?

I would certainly be concerned about those spikes. However, what you’re suggesting that you do to combat them is a good idea. Pre-blousing to match the blood sugar spike from the carbs you’re eating to the exogenous insulin you’re injecting should help, because from what you described it seems the sugar from the carbs are hitting your blood much faster than the injected insulin is being absorbed, creating a disparity.

Rapid-acting insulin such as Humalog starts working in about 15 minutes. but doesn’t peak until about 60 minutes, assuming you aren’t insulin resistant, overweight, etc. I would suggest pre-blousing no more than 15 minutes to start, to avoid risking seriously low blood sugar. Pay close attention to your Libre and once it starts trending down you can eat accordingly and see if that helps.

Serious warning: CGMs are generally 10-30 minutes behind actually blood sugar readings, so be very, very careful when doing this strategy.

I personally wouldn’t worry about those numbers, it is such a short time. Later as you make much less insulin it might get worse though.

However I got rid of most of it by prebolusing. Prebolusing is one of the best tools to control your spikes. Because a lot of the food starts to hit before the insulin is working.

When on MDI I used to take a shot 20-30 minutes before. A pump is even better as I do half 30 minutes before and the other half when I eat.

But if you forget to eat, or don’t eat in time and that can depend on what you are eating also, you can have lows. It is trickier.

If she wants to preserve her remaining pancreatic beta cell function, keeping her blood sugar below 140 as much as possible is the best way to do it. But if that’s not a concern of hers then occasional spikes to ~200 likely won’t cause any long term damage

I do have to add, Peep’s right in that humalog can start working in 15 minutes. So the lower your BG is to start, you then have to be more careful with timing of the prebolus.

My prebolusing timing is much different if I am at 95 versus 130. I used to generally be higher when I was on shots.

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I believe it is critical to be able to plan but equally important is being able to react as your body will throw curve balls at you constantly. Sometimes I will pre-bolus for 30 minutes. Other times I have to pre-bolus for 2 hours. As long as you have a CGM it is easy to do as you simply wait for BG to drop before you eat. I try to eat very few carbs during the meal so that I can eat meat and vegetables without much worry and when BG finally comes down I will eat more carbs (my dessert is usually something like 1/2 cup cottage cheese and pineapple or peaches.) So I might have my dessert at the time of the meal or may have to wait 90 minutes. Once you figure out how your body reacts you cam figure out how to better time your carbs. Using this method I have reduced my ave BG from 160 t0 115, time in range (70-150) to 99% and standard deviation to 15. Takes a lot of experimenting but everyone should experiment on themselves as long as it is done safely.


Since Libby4 says she still has some insulin production, that would explain the occasional hypos. Reactive hypoglycemia happens when her beta cells decide to erratically emit insulin which, on top of the exogenous insulin she has shot would put her into hypo territory. I would suggest more use of extended boluses were she on a pump. Injecting smaller amounts of insulin when prebolusing and then following up with more smaller injections at the end of the meal or even a half hour after the meal could be useful in MDI.

You don’t say what your total daily dose or sensitivity factors are, but if you are LADA with residual insulin, it is likely that 3 units for 15 grams of carbs is too much.

A very super rough guide to your insulin-to-carb ratio is 500/TDD = number of carbs covered by a unit of insulin. For example if your total daily dose would usually be 40 units, then a single unit of insulin should cover 12 g of carbs.

Me, my opinion? Don’t sweat the swings too much. You may be able to learn, that by taking your insulin a half hour before a meal, you are able to limit the swings, and that’s a useful thing to learn from a CGM.

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Prebolusing made all the difference for me. I remember being utterly amazed at how well I was able to control the swings and wondering why the doctor, cde, and nutritionist hadn’t suggested it! When I shared with them what I was doing, they weren’t for it, which is probably why they didn’t suggest it. They worried about lows. I explained that there was almost always something nearby that I could eat (a dinner roll [at restaurants], juice, dex tabs, …) if I was going low. That tempered the converstaion a little then.

Like @rcarli, what I am eating matters … for how long before I eat, that I prebolus. A salad doesn’t require much time, but something like spaghetti or potatoes does. Then you have the pizza effect - which, for me, I would need to prebolus for the fast acting crust, but then continue to monitor/micro bolus, for the slow rise over several hours.

It’s a lot, @Libby4 to take in and learn. It comes with time and experience. And, I’m still learning every day! And, just when you think you’ve put in your time and have experience to back you up, gremlins appear (in your case, endogenous insulin [your own insulin production] is spurted out, or muscle uptake improves for that moment, or absorption isn’t an issue, or …).

I’m with @Tim12 that you may be bolusing too much, but only you can tell that. And, I think prebolusing will help you a lot.

I think basal testing is what is needed right now. Make sure your basal is working as intended and then work on insulin/carb ratios - for different times of the day and different foods.