What I’ve Learned about my CGM

I’ve been using a Medtronic Minimed continuous glucose monitor (CGM) for a little more than a year and a half now, and here’s what I’ve learned about it.

  • Having CGM is the greatest thing. It’s a decision-support tool, a dashboard, and a security blanket. I’ve learned so much about my diabetes from it, and it’s provided the data I needed to make some important changes to my self-management. I wish everyone had it.
  • The BG values it reports are delayed by 15 minutes. This is a fact of life. The CGM looks at “interstitial glucose” between cells (ISIG) instead of blood glucose, and there’s a lag time between the two. Don’t expect the BG numbers from your meter and your CGM match unless your having very stable BGs.
  • The most important factor in getting believable CGM readings is calibration. Wait until your interstitial glucose (ISIG) is the most stable before calibrating.
  • If my BG is on the move but the system says I need to calibrate it, I’ll let it go past the “Meter BG Now” deadline and look at the “Sensor ISIG” values on the CGM information screen. When this value isn’t changing much, that’s when I’ll calibrate.
  • Lots of things affect ISIG values and therefore CGM accuracy: the rate of change of BGs, the level of hydration, and the wetness of the sensor (both too wet and not wet enough).
  • I find that if I keep my sensor and transmitter covered, I get a longer sensor life and better readings.
  • Tegaderm film works the best for me. Unlike IV-3000, it doesn’t come off when I’m swimming or sweating a lot from exercise; it doesn’t leave behind a lot of residue; and it’s stretchy enough to cling nicely and create a good barrier.
  • Hydration matters a whole lot to accuracy. If you’re dehydrated, the sensor will read incorrectly, or perhaps not at all.
  • Don’t calibrate first thing in the morning. There’s too much going on. Almost everyone is slightly dehydrated when they wake up, and I’ve noticed that the very act of waking up and getting moving causes my CGM sensor to give inaccurate readings for about a half hour.
  • Don’t calibrate right before or after exercise. For me, my blood sugar usually is on the move during the first bit of a workout and immediately after it. Plus, the chance of BG swings is greater while exercising, which is bad news for calibration.
  • I find that sometimes I can count on my CGM values when I exercise, and other times I can’t. The lag is a little less during exercise, but my hydration is also different so it’s less accurate. Sometimes it catches big BG swings, and sometimes it doesn’t.
  • Swimming: I find that I can swim for at least an hour without affecting the sensor or transmitter. But I also find when I swim three or more times in a week, the off-label lifespan of each sensor is noticeably shorter.
  • Sensor life: The longest-lasting sensor I’ve used that produced reliable BG estimates is 12 days, which is not too bad for something that’s only supposed to work for three. (Just tell the CGM that you’re starting a “new” sensor when it says “Sensor End,” even though it’s the same one and not new at all.) Because my transmitter battery requires a quick recharge after 6 days, I have to very carefully peel off the Tegaderm I put over the sensor and transmitter, give the transmitter a charge for 15-30 minutes, and then reattach it and apply more Tegaderm.
  • Occasionally, the sensor values will just drop out after the three days; they might come back to match my BG, and they might not.
  • When the CGM graph gets really noisy, the sensor is dying.
  • Sensor freshness matters. A sensor getting close to its expiration date won’t last as long and seems to give less accurate readings.
  • Treat the number after finding a “Lost Sensor” with some skepticism. I find I often need to recalibrate 30-60 minutes after reacquiring the sensor.
  • To avoid “CAL ERROR” messages, I don’t calibrate if the CGM and BG meter values are very different.
  • The Medtronic CGM does some curve fitting, so it often doesn’t “snap” the CGM values to the calibration value when they’re very different. I hate that. Entering another, new BG value usually gets them closer, though.
  • Post meal, a period of flat BG values followed by a rise 1.5-2 hours later almost always means that the meal bolus was too small. (There are a lot of other trends you can spot, but this one was the most consistent.)

Do you use CGM? What have you noticed?

Update – 23 September 2014: Be sure to read the follow up post about the Enlite sensor.

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23 Responses to What I’ve Learned about my CGM

  1. Sara says:

    That is very interesting information.

    We are waiting for a pump from the local hospital however, I was worried that the sensor for the CGM have to be changed every three days, that is a lot of money.

    I will have to keep these in mind for when we get the pump and CGM (which might come later since my kid is only 6).

  2. Jeff Mather says:

    I typically get 5-6 days. Anything over six is very atypical.

  3. Karmel says:

    Word. This aligns extremely closely to my experience. A few additions:

    • During the life of the sensor, the ISIG will take periodic drops, establishing a new mean. Don’t be fooled and assume you’re 40 all of the sudden, and, when the mean drops below 5, it’s time for a new sensor.
    • It’s okay to lie to the sensor. The pump is essentially using a weighted average of the last 4 calibrations to compute BG from ISIG; if you need to calibrate, but are heading up or down, or know that the sensor is picking up a dampened version of your actual BG, calibrate with a BG that is slightly closer to the currently reported value than the actual value reported by your BG meter. This may lead to greater inaccuracy in the short term, but you’ll be better off an hour out once BGs have stabilized again.
    • Try to clean the transmitter on day 6 when you take it off to charge it. It gets pretty nasty under that Tegaderm…
    • Buy Tegaderm from Amazon. Cheaper and better than IV-3000s.

    Thanks, Jeff, for an excellent collection of tips!

  4. Jeff Mather says:

    Karmel, thanks for the additions! I’ve noticed most of these myself. The first one and the last one are especially true.

    I’ll have to try the “lie to the sensor” trick.

    The rubbing alcohol swabs, which I rarely use anymore before testing my BG, come in handy for cleaning the transmitter. They’re fast drying and antiseptic, and they make it less likely that you’ll get water into the guts of the transmitter when it’s detached from the sensor.

  5. Karmel says:

    Yes! The alcohol pads are exactly what I use. Two and a half years into this sensor, sometimes it doesn’t charge correctly– the little light on the charger just goes steady green. Cleaning the connectors of the sensor and the little glove of the charger seems to help, and alcohol pads are perfect for that too.

  6. Mark says:

    Hi Jeff, what’s the deal with the ISIG value? I realise it’s affected by alot, but what range is a “good” range? Also – are you pumping or on pens? I’m currently on pens and want to use the CGMS as a monitoring tool for a period – do you know if the fact that I’m not bolusing from the pump affect it’s measurements?


  7. Jeff, this post is full of some awesome information. Thank you!

  8. Pingback: MySentry Trial: Wearing the Sensor | Scott's Diabetes

  9. jill says:

    I just received and started using MM cgm today. I’ve used MM pumps for 21 years but this thing is amazing! I am stunned and intrigued by the accuracy of the sensor and meter…moslyt 5pts off..in less than 8 hours I was warned of and avoided a low while shopping and stopped a post meal high. Already noting what basals will most likely be adjusting with a few days more consistency. I think I will go for a record A1c change in the shortest period!
    Thanks for the tips!


  10. Lori Rogers says:

    Jeff and Karmel,

    This is THE most reliable and affirming info I have seen on the nuances of using the CGM. It’s nice to meet you. Let’s keep in touch. The sharing of info like this is invaluable.

  11. Ned says:

    I’ve been using the MM 530G pump with Enlite Sensor for a few weeks. Really like it. One problem, though. The morning after I insert the sensor, the results are often way off: anywhere from 60-110 points. (I’ll calibrate before bed, not having any swings during night.) After a day, it works itself out and usually very accurate from days 3-7. Happened several times–always the morning after insertion. I’m calling MM tomorrow. Just curious if anyone else has experienced this.

  12. Laura says:

    I recently started the MM530G with Enlite Sensor. I have changed it 3 times in only a little over a week due to inaccurate readings and low ISIG less than 10. I have a lot of difficulty with tape due to exercising. Also noticed that my ISIG is low when I am standing vs. sitting. When my ISIG is low then my reading on cgm is low even though my bg is fine (stating less than 40 on cgm and bg on meter stating 118). Feeling very frustrated. Any insight into things would be fantastic and greatly appreciated.

  13. Jessica says:

    Laura, I am going through the same thing. I work nights and it gets frustrating to be ‘alerted’ several times each hour of a low glucose, only to check and find out it is 120. This is the down side to this machine, otherwise I like it. My MM530G seems to do this any time day or night and always wants to suspend my insulin when my glucose is normal.

  14. Tammy says:

    Now that I have read this, I know why my sensor has been so off of my BG, I am drinking wine and I am not hydrated. Even when I don’t have wine I haven’t been in the mood to drink water because it is so cold. So, I am on a mission to see if I can get my isig value between 3 and 5 before I calibrate. Lately its been high teens. Going to look at the standing vs. sitting thing too mentioned here. The 530 g can be very annoying when not working well. Have any of you Enlite folks tried to insert by hand?? I found it much easier than with serter, nerve racking since I pay out of pocket.

  15. Jared says:

    Thanks for the tips. I just started using the 530G Pump and Enlite system (after being on Paradigm pump for 5 years). I just completed my first 6 days on the initial sensor. To be honest, the first day I did have some trouble with alerts going off even though my blood glucose was between 90 and 120. Some of the things you mentioned in your post shed some light on why that might be. After that first day, however, it has been phenomenal and I don’t want to go without it. It has been extremely accurate (more so than I thought it could be) and an awesome tool to see what things I need to work on such as basal rate adjustments – a big one for me right now. Thanks for you input as it made a lot a sense and gave me some of the answers after only being on the CGM for just under a week. Kudos!

  16. Ali says:

    Hi there. Thanks for all the great info. I have finally caved and agreed to try the cgm/enlite sensor. I have not been too keen on sticking something else on my body. Also, I swim with a masters team several days a week and do yoga and am worried how I will manage with it while exercising. Usually then workouts are very intense. I haven’t starting using it yet and will be going to a class tomorrow. Any other insights into dealing with the sensor while working out would be greatly appreciated!

  17. Dan says:

    Great post. I’m digging around trying to find best adhesive tape – I work out a lot and the combination of sweating , swimming, abundant showering takes its toll on taping down the transmitter ….I do find the bottle of ultra diabolical adhesive is good but looking for tape that matches its strength

  18. Renee says:

    I am so glad to read these posts-what a world of information. My trainer for CGM did not explain to me anything about ISIG numbers so I am a little confused. I just removed my first transmitter and will attempt my first insertion alone tomorrow. Thwe first 4 days were very accurate but this mornings values where 200 points apart and off all day so I removed the sensor a day early. When I removed it I could see the cannula was bent…perhaps why the readings where so far off?
    I would like to read an explanation of ISIG and the goal number…
    Thanks for all the great info and thank you in advance for sharing your wisdom with the newbies!

  19. Kat Richter-Sand says:

    I am a longtime MM pump user (about 30 years!) but new to the Enlite system. Struggled a LOT with getting inaccurate CGM readings the first 3 days of CGM use, but it’s “settling in” now. I have trained my amazing dog Abbey as a Diabetes Alert Dog, and she, too, is tuning into the beeps and beginning to connect sounds with my glucose levels!

    Thank you, everyone, for sharing so much helpful information. For now, your input is my guide as I work into learning more about wearing this pump and CGM!

  20. Gail says:

    Jeff, thank you SO much for the information on taping and hydration! I’ve been really frustrated with very low CGM readings compared to finger poke readings and had to take out 2 sensors recently because of CAL ERROR and LOST SENSOR and I don’t have insurance coverage. I will try the Tegaderm film and remember to drink enough on hot days. Thank you again for sharing your valuable information!

  21. Evonne says:

    I have been using the 530G and Enlite sensors since March 2016. I didn’t have much luck with the CGM on my abdomen no matter what tape I used. I found that Skin Prep and Opsite Flexifix works the best for me if I need reinforcement. I am menopausal sweaty, and a little chunky. I had problems with pistoning, inaccuracy, and decreased sensor life when using my abdomen. I heard about people using the sensors on their upper thighs and decided to give it a try. I have had much better luck since switching to my upper thighs. My only problem was that my initial ISIG was very low. It would come up after a day, but that first day was super frustrating with lots of alarms and inaccuracy. I read that some people insert the new sensor at night, but don’t activate it until the morning. That way, they have their old sensor still working overnight for safety. Here is what works for me. I take a shower before bed because it makes it easier to see veins that I want to avoid. I insert the new sensor and apply one piece of overtape. Then, I use a small piece of medical tape to cover the little tab that sticks up that goes over the transmitter and the sensor plug. This is so that I don’t snag it pulling my pants up or down in the middle of the night. Then, I go to bed and the sensor has all night to marinate in my interstitial fluid. In the morning, I turn off the sensor. Then, before my morning shower, I remove my old sensor and charge the transmitter. After charging is complete, I connect the transmitter to my sensor, apply second piece of overtape, and link to the new sensor. Then, I take my shower. I never take a shower before connecting the transmitter because I think getting water in the sensor connector might pose a problem. I know this sounds complicated, but after much trial and error, frustration, and tears, I discovered that this works for me. My ISIG is always great and I rarely have any alarms. I always calibrate when there are no arrows and my blood sugar number divided by my ISIG is between 3 -8.3. My numbers are usually very accurate unless I am going up or down very quickly. My My A1C dropped from 9.4 to 7.4 in the six months since I started with the pump and CGM. So, all of my hard work has really paid off! I just wanted to share this because I have done so much fiddling around with this and I think it might help another newbie out.

  22. Kelly says:

    Thank you so much for posting this, it is so helpful!
    I just came back to the CGM after taking many years off. I really love all the improvements (especially the new inserter and sensor size!).
    When I did the first insert with the trainer it was super accurate for the whole 6 day period but I just did my first solo insert last night and it has been about 100 points under my meter BG all day. I’m hoping it will even out like others said but I’m usually pretty well hydrated. I had to go in and turn off the low alerts on it last night because it kept waking me up every half an hour!
    If it doesn’t even out I guess I’ll have to try a new sensor?

  23. B B says:

    For all who have erroring sensors, call Minimed to troubleshoot them and most of the time they will replace the sensor :)

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