Tandem T-Slim Bolus Settings

Hey guys,

Long time since I've been on here but a lot has changed since then. I was doing 4 shots a day (3 Apidra/NovoLog and 1 Lantus), but now I have been fortunate enough to go on the pump, more specifically the T-Slim by Tandem. It's still a learning process, that is for sure, but I do really like it so far. One of my main questions though, is about the bolus settings, in the personal profiles.

In the bolus settings, there is a setting for insulin duration. I have mine currently at 4 hours, and I'm thinking that 4 hours is too long. Since I usually test 2 hours after eating, it doesn't make sense to me why I would have the insulin take 4 hours to go into my system. Now, I could totally be wrong about how it all works, but that is just my concern right now. Is 4 hours too long and should I lower it to something else?

Thanks guys!

Hi Augustus! I’m a little new myself, and it is my understanding that 4 hrs is the standard, but it can vary (and I think is often slightly longer than 4 hours). However, it means the length of time the insulin is active in your system. For example, if you have bolused 4 units, it takes 4 hrs for your system to process or use that insulin. There will be a peak time, but the dose is active for approximately 4 hrs. Have you read Pumping Insulin by Walsh? It has been a valuable resource for me. It has lots of charts to help you figure out all kinds of settings, and if I remember correctly, it also has an explanation of how to test to ascertain your personal insulin duration. Hope this helps a little, and I’m sure more experienced pumpers will weigh in!



Thanks for your input! I believe I also heard about 4 hrs being the standard, but to me it feels a bit too long. I'm going to talk to my endocrinologist tomorrow and hopefully see if I can change it, as it has only been a few days since I started wearing the pump. I have not read Pumping Insulin, but I might have to pick that up if it is a good resource.


It is fabulous Josh, and available digitally!

Duration of insulin action is an important and often under-appreciated pump setting. If you have it set much shorter than the actual insulin duration then it will invite adding more insulin when it is not needed and cause hypos. Where I came to appreciate it was when I had some very bad lows between four and five hours after eating. To draw this conclusion, however, one must be satisfied that the basal profile is correct or close enough.

I've used various settings on my pumps over the years and have ranged from three hours to four and one half hours, my current setting. This is a personal and customized setting and I think it varies some within the same person from day to day. When I had my pump set to only three hours, I experienced more hypos due to stacking one dose on top of another. Too short of a duration will make you want to over-correct highs, send you hypo and then it's off to the roller-coaster.

I've used four and one half hours for my Apidra insulin for the last several years. It's been a pretty good setting for me. I can watch the effects of an insulin correction on my CGM trace when I'm not eating or exercising during the correction time. What I look for is when the slow downward trace levels out or even turns up some. Night-time corrections around 1:00 a.m. using a CGM facilitates this analysis since I'm not eating or exercising during this time.

Before doing this you must be sure that the insulin sensitivity factor is dialed-in. This is the blood glucose drop that one unit of insulin produces. Delivering insulin successfully depends on many interconnected factors and is as much an art as a science.

Wil Dubois writes for Diabetes Mine and he addressed this issue in a column last year. It may help you to read about why this setting is important and how some of the pump gurus advise testing for this parameter.

Be aware that the size of an insulin bolus will have an effect on its duration. Studies have shown that larger boluses will, all other things held equal, tend to last longer.


That does make sense with what you are saying, I just didn't see it like that. I want to avoid hypos, but also the highs as well. Finding the balance is going to be the key guess.

At this point I'm not seeing the greatest numbers, although I've only been on the pump for 3-4 days, so I'm assuming it takes some time and adjusting to get my numbers to my desired 70-130 range more often.

How long do you think it will take to get fully adjusted to the pump? Am I overthinking it with wanting to change stuff up so quickly after only being on the pump for a few days?


I think people that do best with the pump are people that want to do personal experimentation and learn from the data. I like to do retrospective analysis. That's just a fancy name for looking at the recent past graphed out over a day at a time. There you can start to see the interplay between food consumed, mainly carbs, insulin dosed, exercise, and the BGs that result. Mapped out over the time of a day can help to educate you about how theses factors influence each other.

Some people despise this level of record-keeping and want nothing to do with it. I've found it valuable.

It might seem complicated at first but there aren't too many variables in play so that you can start to see what to adjust to get the outcome you want. There can be a fair amount of trial and error but you have to live with this for the rest of your life, so why not be good at it?

I think with a reasonable amount of effort you can become proficient at pumping within in a few months. I've been pumping for 28 years now and I'm still learning things. On top of that, your body's needs change and the formula that worked yesterday may not work today. The most critical skill I've identified is the ability to react within a reasonable time frame to a change in your insulin needs. This is a dynamic game we play. We all need to learn to roll with the punches and change our responses appropriately.

The pump is an incredible tool, but in the end, it is just a dumb tool. The person wearing it must bring the intelligence to make it work. Doctors and diabetes educators have limited utility in this area. We live with diabetes 24/7/365. If you pay a little bit of attention and give it some thoughtful observance and persistence you can make your pump deliver on its potential.

Very insightful, all of this. Just need to put things into perspective and be ready to be dynamic and change, as things are always changing. As long as I can keep learning and making the right changes to see what works best for me.

I personally think my issues at this moment, have to do with my actually infusion site, and not my pump settings. Still I'm going to bring up the bolus settings and the basal rate, and see what my endocrinologist has to say about that.


If I were you, I would focus first on getting your basal rates set well. Gary Scheiner has a good workable protocol for doing this. If you decide to do this, be patient with yourself. It can take a few weeks to get this done.

It pays very useful dividends, however. Being able to confidently go to bed and sleep through the night with stable in-range BGs is an immense reward. More importantly, once you do this successfully, then you can redo the basal tests to make further adjustments whenever you want.

Good luck to you! Becoming proficient with a pump is not simple but it can reward you with a flexible lifestyle.

Are your pump sites giving you trouble?

At the risk of being a broken record, get Pumping Insulin and, more importantly, read it. You'll quickly realize why what you posted originally doesn't make much sense. Perhaps your duration of insulin action really is unusually short, but it's just as likely to be unusually long, and most likely it's somewhere in the normal range. Your hunch above, based on confusion between peak insulin action and the end of *any* insulin action, isn't a meaningful datapoint, in fact it's not a datapoint at all. Pumping Insulin, plus actual data on how insulin (and carbs) work in your body, will help you pick the right settings, for this but also everything else.

I will have to check out what Gary Scheiner has to say. As of right now my basal rate is 0.5, so very low (I believe the lowest the t-slim will go to).

I just changed my pump site today for the second time, as the first time was at my endo's office. I think I might have messed something up because my sugars were fine after I changed it, but since I hate they have been high and only getting worse. Had to take a quick shot of NovoLog that I still had. Just frustrating honestly, not knowing what is going on.

I'm running at 0.375 right now on my t:slim, so definitely not the lowest it'll go. The t:slim occlusion alarm is very sensitive, at least while bolusing, so if you've successfully given a bolus, it's unlikely to be occluded. Perhaps your insulin is damaged (by heat or freezing)? Or maybe your basal rates or recent boluses are off? Or maybe something totally unrelated like a cold or stress? There are tons of reasons your sugars could be off, and the pump isn't some black box--it's either able to deliver insulin or not, and it'll let you know if it's not.

I started on a tslim recently too and my understanding of this setting is that it simply represents how long you think the insulin is active in your body. It's nothing to do with how the pump is delivering it. The reason for this is insulin stacking - if you go to give yourself a bolus due to a snack and still have insulin active from lunch, then you might end up taking too much. Personally I think 4 hours is right, for me at least.
Good luck!

Two books that will become you D bibles, Think like a Pancreas by Gary Scheiner and Pumping Insulin by John Walsh. Both will walk you through the process of getting your pump settings dialed in.
Start with basal testing, across the full 24 hour day, is usually done in sections so you don't have to fast through a full day. It is vital to do this first, because if it off it will mess with the rest of your settings.
DIA is fairly easy to test for- is done when high with no food or bolus working- can be combined with insulin sensitivity testing. It involves taking a correction and then following until your drop of plateaus and you go flat. Does not matter if you get to target or not. You can see what 1 unit drops you by dividing the amount of change by the number of unit take. The DIA is how long from correction to when you stop selling and go flat.
If you have a CGM is fairly easy to do, if you do not then will nee to do multiple FS to see drop, when it starts to slow would be doing FS every 15 min or so to watch for the tail.
Both books will show you how to do this. It will make your control better if you get thing dialed in, but a large part of control is also how you prebolus vs meals and finding what carbs/portion sizes do or do not work for you.
What works for me will/may be very different from what will work for you. Depending on rime of day I have different I:C ratios, I can stop a spike by waking steps for 5- 1110feet minutes. It isn't just the pump - lots of other useful strategies help.

Infusion sets make a big difference to settings as well. I found that with Cleo90 s I needed 30% more insulin to achieve the same results I get with a comfort short. Manual vs auto sets can make a difference to. If I uses the auto version of my manual set I need 10% more insulin than the manual version.
It will take a bit of trial and error with sets, both from e user error/learning the tricks of placement/finding style and cannula length that works. I use different sets with different length depending on body part of placement. It all takes time to work out.

Here's basically the Pumping Insulin discussion of duration of insulin action:


I actually don't find the method of determining duration of insulin action suggested here workable, at least for me. It requires finding a time when you blood sugar is above 250, and stable, and when you haven't eaten carbs for three hours or injected bolus insulin for five hours. Then you inject the recommended correction bolus and measure your glucose until it flatlines, without going low, and you do this three times to get a consistent duration of insulin action (and correction factor).

But there are literally no times when my blood sugar is stable at 250+ and I haven't eaten carbs in three hours. I still make a little residual insulin, so while my phase 1 post-prandial endogenous insulin response is very poor and I can easily spike to 250 after eating a fairly modest portion of carbs, my phase 2 insulin response is much more robust and will bring me back down eventually, albeit slowly.

So I think for me, I'm instead going to have to find a time when I'm stable at a far lower BG, like 120, and then take a very small correction, and use the resulting data to calculate both my correction factor and duration of insulin action. I'm new to a pump, and currently finding that a DIA of 4:30 works, but I'm sure it can be fine-tuned further.

I use 2 hours, but then I use an Omnipod and it has a very crude model of how insulin is absorbed. I also avoid high glycaemic index food and the result with the new Omnipod (not the old) is that I need a lower duration.

The problem is that the pump models how high your blood sugar should be based on the carbs you entered and the insulin action duration. It then takes its idea of your expected BG and compares it to a later BG measurement to do a correction. Since it doesn't know how fast or slow the food is absorbed (I don't think any pump asks for the glycaemic index of a meal) it's necessary to adjust the insulin action duration to compensate, as best you can, for false assumptions in the pump.

John Bowler

Which of the rapid-acting insulins is out of your system in two hours? That seems implausibly short, even with the most rapid-acting of the insulins, Apidra. I wish we had insulins whose subcutaneous duration of action was two hours!

You don't understand what I am saying. The "insulin action duration" given to an insulin pump is a clue to its rate of adsorption which the pump has to combine with your meal's rate of adsorption to predict your blood glucose levels.

The figure you enter into a pump isn't the time the insulin takes to get "out of your system"; that figure could well be years, depending on how much left you count of as none. It's like radioactive decay, it never ends; look at the graphs, e.g. Figure 2 (serum insulin levels) in the Humalog insert.

If you have the Humalog insert (if not just use the Novolog one) take a look at figure 1 (blood glucose levels); this is the BG level that corresponds to the insulin level in figure 2. Notice how the figure 1 graph wraps negative at 2 hours (bg lower that target) then increases again. It's the food that is doing that, but since we don't and can't tell our pumps how long our food takes to adsorb we have to mess with the insulin adsorption algorithm.

The longer you take to adsorb your food the earlier the trough in the figure 1 graph happens, so the lower your insulin action duration needs to be to avoid a hypo. It doesn't depend on the insulin, it doesn't depend on you, to some extent it depends on the pump but most of all it depends on what you eat. Such is diabetes.

John Bowler

If a 2-hour duration of insulin action setting works for you, and doesn't risk stacking boluses or otherwise misgauging active insulin, great. You may want to check out John Walsh on this issue at the link above; for example, apropos our discussion:

"One study conducted by Novolog found about a 25% variation in action between different users. If we assume an average duration of 6 hours including the tail, the variation between different pumpers would be 5 hours and 15 minutes to 6 hours and 45 minutes, rather than 1.5 to 8 hours. Although DIA times vary from person to person, the differences are usually not that great."

I liked "Pumping Insulin" and "Think Like a Pancreas" when I was the Jon Snow of pumping but, once I started playing with the pump, I have found it easier to look at chunks of time and, if I'm running low fasting, I blame it on basal and if it's after a meal or other bolus, I blame it on the bolus ratio and turn the pump down a shade. I've found that the smallest increments on my Medtronic (.1G of carb/ U hour and .025U/ hour, although I've also cheated by "ponging" back and forth between two settings, .8U/ hour and .775/ hour to "fake" a .7875U/ hour...) work pretty well and will achieve a perceptible difference when I make an adjustment. A lot of times, I'll sort of work around "issues" by eating extra, or correcting lows every day or 200%ing basals after a particular meal for a while and then go "hmmm, maybe if I just change a setting it will smooth out" and it often smooths out not only the "target" timeframe but several "downstream" ones to boot! I use 3 hours for my insulin duration and, even if it's not totally accurate, it seems to work well.