CGMS, Does Narrowing the High/Low Alarm Range Increase Accuracy?

Question for all CGMS users, any model. I am particularly interested in Dexcom 7 Plus. In our last trial of the Minimed cgms sensor, we were told to set the low alarm at 90 and the high at 180, with a two hour interval between high alarms. Lows would keep going off until you correct. I found the readings on the Minimed sensor to be highly inaccurate (after day 3 usually). She would almost always be 30 points higher than her lows and way, way off on the highs, up to 100 points. If her blood sugars fell within the 100 to 150 range, cgms was very accurate. Well, I hardly need cgms then! Need it for the lows and highs. So my burning question is: DOES NARROWING THE HIGH LOW ALARM RANGE (i.e, Low = 80; High = 130) INCREASE THE ACCURACY OF THE CGMS. And my second question: DOES CORRECTING WITHIN A NARROW RANGE INCREASE THE ACCURACY OF THE CGMS? In other words, if you correct at 120 down to 100 (which is her target), rather than waiting to see if the cgms will trend up or down, and correcting with carbs if 80 up to 100, increase accuracy of cgms and decrease the roller-coaster like swings, since you are catching the trends at a very early level?

No. The alarm settings don’t affect the accuracy any more than your bathroom scale controls your weight.

Those settings only control the alarms and have nothing to do with the connection between the transmitter and receiver or with the sensor. If you set the low alarm to 70 the alarm will sound when the receiver reads 70, even if the receiver is wrong.

I am interested in this discussion. It seems that when I first got the sensor it was absolutely spot on…after using it for several months, but it now seems that it is always inaccurate. I have tried different sensor batches, different insertion places and I am always very sure that I insert the sensor at the right angle…it seems that I have to wait longer than the 3 hours for it to get “wet enough”. If you cannot use the sensor when your BG’s are going up or down…which is MOST of the time, when can you use it? I am using the MM pump and sensor. I have my low set at 90 - because I am finding that it doesn’t alarm quick enough before I am sweating and getting confused. The high alarm is set at 250 and it seems that it is always going off. My last A1c was 7.0%. I have had D for 55 years and there are days when it just gets me perplexed. The one and only time I had a seizure was at night and the alarm didn’t even wake me up but it was beeping away. My husband woke up because I was shaking the bed so much. I just wish we had all the answers…I do like the little arrows going up and down though, it really is very accurate when BG’s are changing rapidly - the actual numbers ? are they just a number?? For having D for 55 years I really have very few complications (thank god) and it is not thanks to good management over the years.

Let me clarify:

Narrowing the range will have no affect on the accuracy of the CGM. It will, however, give you a smaller target to aim for, which may help control.

As for CGM “accuracy”, the number is just a number and is practically meaningless by itself. A 90 on your CGM does not always mean you have a BG of 90. This is why all manufacturers tell you not to make decisions based on the CGM number but to take a fingerstick. The number 90 IS useful in context with the number before and after - it lets you know if you BG is steady or going up or down, and sometimes lets you understand how fast.

It is a mistake, and a difficult concept to let go of, to believe that your CGM and BG meter readings should always match or even be very close. This way leads to frustration and heartache. The CGM is best used to see trends and to catch lows.

If you CGM is not catching the lows, then you have a problem and need to work with the manufacturer, read the manual, or try something else. It could be the placement of the sensor, which should be away from scar tissue and away from your injection or infusion sites. Don’t for instance, place the sensor where you were injecting last week or where you place your infusion sets. Using acetaminophen (Tylenol) can make reading go wacky and become unreliable. The way you lie on your insertion site at night can also affect its readings. Calibrate often, but not when your BG meter shows a high over 350 or so or a low.

In any case, don’t fall into the trap of chasing your meter readings with your CGM and trying to get them to match. The CGM is a tool, but is not an automatic pilot. We still have to pay attention to our bodies.


I’m usually always lower then my alarm I have it set for 80 and when I hit 80 on the sensor I’m usually 60 or less. I drop very quickly though due to my lifestyle (activity level) so for the most part it’s pretty accurate so far. If I try to leave my sensor past 3 days it doesn’t seem to be very accurate. But you’re supposed to change it every 3 days…so…yeah

Edit - accurate other then the dropping lows…my highs are usually right on or within 10 points. I haven’t had any bad highs since using the CGM but my lows have dropped dramatically, I’m a rebounder.

Yes, we had this same problem with the Minimed sensor, though we had it set to 90, she would be almost 30 points below before we got the alarm. I think I have gotten the answer to the question on the Dexcom forum. Although most disagree with the technique of keeping the high/low alarms within a narrow range (mostly because the high alarms would drive them nuts), cgms is most accurate within the 80 to 150 range, I had certainly noticed this myself when DN used the Minilink. Was told Dex 7 Plus was accurate with highs, but still there is that 30 point discrepancy with the lows (sensor reading higher than the actual BG). I will be experimenting with a tight range at night when we manage DN’s blood sugars. When she is not with us, the alarms will be set a lot higher. And if she wakes up from the constant alarms, then I guess we cannot try this either. I am sure cgms will improve her control, even with a higher alarm range.

One thing to know that many people either don’t know or don’t talk about (for reasons unknown to me), is that every one has a “tipping” point in their BGs. I’ll give you my own example. If my BGs are 120 +/- 10 then they will remain very stable for a long time. Now my BGs are around say 90, they will start to go down, sometime rapidly. When they are 135-140 they will continue to go up no matter what exercise I may be doing at the time. I have my MM CGM low alarm set at 80. When this alarm goes off, my finger stick BG is almost always at 60 or a little less. This is because my BGs dropped rapidly even though I didn’t take a bolus or exercise. I was just below my “tipping” point in my BGs. The flipside also hold true for me.

Why this is I have no idea. But I was talking to my endo about this and he said that it’s actually pretty common for diabetics to have this problem.

Are there exceptions to this, absolutely. But have been using the CGM for 15 months virtually non-stop and it has helped me drop my A1c from well over 10 to 6.8 (as of 4 days ago. Yay!) More importantly, its a great tool to use to help cut down the wild BG swings that plague us. You can have an A1c of 7 and have such huge swings that you are still at considerable risk at complications. They are just as dangerous and damaging to your system.

Sorry for the super long post. But I wanted to get that out there. I totally agree with Terry in that narrowing your alarms won’t make the sensor more accurate. But by heeding those alarms you will have tighter control as a result.

no, setting the range will have no effect on the CGM device. It tracks like it does and whatever you set on the pump is separate. . i do find that my cgm is more accurate in my abdomen than my leg. I also found a better place on my abdomen. I tseems liek the further i push it back, it tends ot track better. other folks suggest differnet sites.

rick phillips

I now do get that setting a lower high alarm range itself will have no effect on the CGM device. To quote you: “But by heeding those alarms you will have tighter control as a result.” The problem we had with the Minimed cgms is it was very very accurate within the range of, say 90 to 140 but, in general, read 30 points higher than the lows (NOT a good thing). It was in the higher ranges where all hell broke loose. She is 12 and the hormones are crazy at that age. The time period of 6pm to 3am is murder. By setting the high alarm to a low setting, such as 120, we can then catch the spike before it occurs. She has gone up 100 points in an hour, even 80 points in a half hour without food (on basals tested within the past two days). This is not something we would ask her to do, as I think it would be burdensome for her. And, frankly, she would not cooperate and would refuse to do it. But we still handle the D care when she is at home; she does it when out and about or at school. When the alarm beeps at 120, we can then watch and get a heads up. We will be experimenting; we have our appointment on Monday. It is one thing to read someone’s post and think, “Hmm, that sounds good, and look at the amazing results this man has achieved with his daughter,” and it may be another to experience all those alarms. I wish there were a separate parent monitor so we could see the BS readings from another room. If we can knock down a lot of the blood sugars over 180 to 200, from 7pm through 3am, we will have achieved great control, because from 3am to even 3pm the next day, handling her D is a piece of cake. For some reason, her basals are relatively flat for that time period; which is a shame since that is the time period she eats (she does spike postprandially). But when we test, if she does not eat carbs, she stays relatively flat. Ah, if only the reverse were true and this period of stability occurred at night…

Yes, I have noticed this! Mainly at the lower levels, she will usually keep dropping. I will watch for the tipping point at the 135-140 mark; wasn’t as aware of that one.