advertisement

Page 1 of 4 123 ... LastLast
Results 1 to 10 of 40

Thread: CGMS ISIG and Calibration Guidelines

  1. #1
    Join Date
    May 2008
    Location
    Philadelphia
    Posts
    4,313

    Default CGMS ISIG and Calibration Guidelines

    This post is to explain how to calibrate a CGM effectively to achieve overall good accuracy.
    Also included is a description of the CGM's ISIG indicator and how to use it to guage confidence
    factor for each calibration.

    Calibration
    When you calibrate a CGM, you are telling it the current BG. The CGM compares the BG you enter
    with the sensor's electrical output (known as the sensor's ISIG). Let's say that you enter a BG of 120,
    and the sensor's ISIG at that moment is 12. The CGM then knows that the current ratio of BG to ISIG
    is 120/12 = 10, so from that point forward, all future sensor ISIG values are multiplied by 10 to create
    the on-screen SG (sensor glucose) display.

    For example, if 10 minutes later the ISIG has increased from 12 to 13, the CGM's SG reading will
    increase from 120 to 130.

    Bad calibrations happen primarily for these reasons:
    1. When there is an inaccurate BG reading (such as when fingers are contaminated with sugar)
    2. When BG values are very high or very low. The sensor's ISIG at extreme BG values can not
      be accurately extrapolated to the normal BG range.
    3. When BG is changing rapidly. This causes the BG you enter to correspond to an ISIG that
      is delayed in time.
    4. Sensors near the end of life. Near the end of life, the sensor's sensitivity declines by the hour,
      so the BG-to-ISIG ratio is not stable. A calibration with an end-of-life sensor is good only for
      a short time period, if at all.
    Therefore, the following practices help to insure a good calibration:
    • Make sure hands are completely clean and dry before the BG. This is always important, and
      especially important when relying on the data to calibrate your CGM.
    • Avoid using BG's under 70, or over 140 for cal's.
    • Only use a BG for a CGM cal if the CGM shows that BG has been relatively "flat" for the past half-hour
    • Never cal right after you eat. BG is already rising 15 minutes after you eat.
    • Never eat right after you cal. The CGM is counting on your BG remaining stable for 15 minutes.
      (In other words, avoid eating both 15 minutes before and after you cal, if possible).
    If you have no choice but to cal under poor conditions in order to keep the sensor from timing out,
    be sure to do another BG test and cal as soon as BG stabilizes again.

    ISIG
    The ISIG (short for Insterstitial Signal) is an electrical reading that is proportional to BG. In theory,
    the ISIG is linearly propoortional, but in practice it is linearly proportional over a limited BG range,
    which is why you always should cal when BG is within a normal range such as 70-140. Cal's at 50
    or 300 might not linearly extrapolate into an accurate reading when BG is in the normal range.

    The ISIG provides an additional tool to gauge confidence for each calibration. On the Minimed Guardian,
    ISIG can be read by pressing the ESC button twice. Most other meters should have a similar option
    to view the ISIG.

    To make use of the ISIG to improve calibration confidence:
    • Each time you cal, look at the ISIG value at the time of the cal, and determine the ratio of BG/ISIG.
      For example, you may find that a typical ratio is 15:1, or 8:1.
    • For the lifetime of your sensor, the BG/ISIG ratio will remain relatively consistent, but it will change
      somewhat from cal to cal (which is why you have to do cals). However, if your sensor starts at a ratio
      of 12:1, it usually will remain in that general vicinity during its useful life.
    • If you do a cal and find that the BG/ISIG ratio is substantially different from prior cals, it is an indication
      that something might be wrong with the sensor. For example - Let's say you usually have a ratio of 12:1,
      and then one cal has a ratio of 5:1. This is a suspicious cal. Check your sensor to see if it has loosened,
      or if maybe it has been subject to physical pressure such as sleeping on it, or if it has been in use for it's
      typical expected lifetime.
    • Another possibility when you see a suspicious BG/ISIG ratio is that BG just started to change rapidly
      around the time that you did the test. Watch the CGM reading over the next 20 minutes. If you do see
      a rapid change, cal again as soon as the BG stabilizes.
    Finally, if you get a BG reading that differs dramatically from the CGM, don't jump to conclusion that the CGM
    is wrong. It might be a contaminated BG reading. Always re-check the BG and don't re-cal the CGM until you
    are certain the the BG is correct, or you may turn a good cal into a bad one.

    There is always a possibility that the CGM will be wrong and you'll have a low or high bad enough to be symptomatic.
    Not nearly as often though as relying on BG checks alone. The key is to follow good calibration procedure, and
    to use your judgement at all times in interpreting the CGM data.

    The incidence of false CGM readings can be greatly reduced using the methods above.

    Here's an additional resource with even more complete information: http://www.myparadigm.eu/

    Edited 8-10-2009 - A tip to get good initial calibrations with the MM CGM - install the sensor at least
    2 hours before attaching the transmitter. We do this by installing the new sensor at night, then switching
    over the transmitter in the morning. This has helped get more consistetly good initial calibrations.
    Last edited by Darryl; 09-11-2009 at 08:20 PM.
    My daughter Leah was dx at age 8. Has used the Omnipod since 2007, Guardian 2007-2013, Dexcom 2007-present and sang the National Anthem at the 2013 JDRF Walk!

    DCCT: The Study That Forever Changed Treatment of Type 1 Diabetes - Improved Glycemic Control in T1 children Using Real-Time CGMS

  2. #2

    Default

    Wow. That's some great information, Darryl. Thank you so much for posting that. I'm printing it off and keeping it in our binder for future reference.

    I have a question for you - due to an unfortunate snack choice by Grandma, we couldn't calibrate before supper. Our calibration was due by 7:30, and so we haven't had readings until I checked him about a half an hour ago. His sugar was 182. I wasn't thrilled with that, but I used it as a calibration not only because I want readings during the night, but for the following reason. Let me know if this makes sense:

    Our Medtronic rep told us that there is a "sweet spot" with respect to the relationship between ISIG and the BG numbers. The actual algorithm is patented, of course, but she told us that if you divide BG by ISIG, the closer that number is to 8, the "better" the calibration. That doesn't mean that if that number is 6 or 12, it's a bad calibration, but she did tell us not to use a particular BG if that number was above 20, and again, the closer to 8, the better.

    So...when I divided his BG of 182 by the ISIG, I got a result of 7.8889879 or something like that. Because it was quite close to 8 (and because I was impatient to get readings again), I used it as a cal, even though it was higher than I liked. Does all of this jive with your understanding of calibration and ISIG? Would you have still waited for sugar to come down more, or would you do as I did? (I know you're in a lot tighter control than we are, so you probably don't have this problem very often, but your opinion would mean a lot to me.)

    Thanks a lot for your advice, and thanks again for all the time you took to post this. Very informative!
    Mom to J., age 13
    Dx 2007 @ age 3
    Medtronic pump and CGM (4/2008-6/2013)
    Tandem t:slim and Dexcom G5 CGM (current)

  3. #3
    Join Date
    May 2008
    Location
    Philadelphia
    Posts
    4,313

    Default

    Quote Originally Posted by hawkeyegirl View Post
    Our Medtronic rep told us that there is a "sweet spot" with respect to the relationship between ISIG and the BG numbers. The actual algorithm is patented, of course, but she told us that if you divide BG by ISIG, the closer that number is to 8, the "better" the calibration.
    Well, unfortuntately I think that is not exactly correct.

    First, we've used 70+ sensors, and rarely have seen an BG/ISIG
    outside the range of 9 to 15.

    Second, you have no control over the BG/ISIG.

    I think what the rep meant to say is that if the ISIG is way above
    or below 8, it is telling you that the BG is out of the normal range.
    Although again, in our case, a ISIG of 8 would typically correspond
    to a BG between 50 and 80.

    Another possible valid interpretation of the rep's advice would be
    when BG is changing fast. In that case, the BG/ISIG ratio would
    be different from normal due to the time delay. That is essentially
    similar to what I described in part of my post.

    In any case, don't get hung up on expecting a ratio of 8. You will
    have ratios different from that and it does not mean that they are
    bad (nor does 8 it mean it is a good cal - if your ratio is 8:1 at a BG
    of 180, it will still be approximately 8:1 at a BG of 250, which is
    clearly not a great BG to cal at). The most important thing is to
    know the approximate ratio for each sensor, and to look for
    departures in any given cal from the ratio you expect.

    Maybe you could run this by your rep and see if they can clarify?
    Last edited by Darryl; 07-29-2008 at 10:50 PM.
    My daughter Leah was dx at age 8. Has used the Omnipod since 2007, Guardian 2007-2013, Dexcom 2007-present and sang the National Anthem at the 2013 JDRF Walk!

    DCCT: The Study That Forever Changed Treatment of Type 1 Diabetes - Improved Glycemic Control in T1 children Using Real-Time CGMS

  4. #4

    Default

    This website has some extremely interesting information about BG Values and ISIG with the MM CGM. In some ways, the technology has a ways to go, based on this information. But, the cgms is extremely useful and is a godsend. With a larger sample size of users, the technology will be perfected.

    http://www.myparadigm.eu/

    Thank you Darryl for the tips!

    EDIT:Especially Look at Sensor Instrumental Delay and Behavior of the Newly Inserted Sensor
    Last edited by moco89; 07-29-2008 at 10:52 PM.
    Young adult with type 1 diabetes, autoimmune autonomic neuropathy, and chronic inflammatory demyelinating polyneuropathy (CIDP)

  5. #5
    Join Date
    May 2008
    Location
    Philadelphia
    Posts
    4,313

    Default

    Quote Originally Posted by moco89 View Post
    This website has some extremely interesting information about BG Values and ISIG with the MM CGM. In some ways, the technology has a ways to go, based on this information. But, the cgms is extremely useful and is a godsend. With a larger sample size of users, the technology will be perfected.

    http://www.myparadigm.eu/

    Thank you Darryl for the tips!
    Thanks! I think you sent me that link before and it has a lot of great info and data.
    I will edit my original post to include the link.
    My daughter Leah was dx at age 8. Has used the Omnipod since 2007, Guardian 2007-2013, Dexcom 2007-present and sang the National Anthem at the 2013 JDRF Walk!

    DCCT: The Study That Forever Changed Treatment of Type 1 Diabetes - Improved Glycemic Control in T1 children Using Real-Time CGMS

  6. #6
    Join Date
    Nov 2007
    Location
    Hastings, MN
    Posts
    3,803

    Default

    Thanks Darryl for the great tips. Thanks also for putting links to your posts in your signature. I will definitely use them for reference once Jacob has a CGM.

    So how about the importance of using the most accurate BG meter possible for calibrations? And how can one be confident in the accuracy of their meter?
    Dave (non D)
    Single father of Jacob (Type 1, dx'd 11/2005, born 12/11/2000)
    Medtronic Revel 722 since 1/08/2012; Apidra
    AccuCheck Aviva meter, Sure T infusion set
    Guardian CGM since 1/9/2012
    Former Animas and Dexcom user
    Father of Andrea, 15 and Logan, 18 (all non D)


    "Consistency is the last refuge of the unimaginative." - Oscar Wilde
    "Progress is made by lazy men looking for easier ways to do things." - Robert A. Heinlein

  7. #7
    Join Date
    May 2008
    Location
    Philadelphia
    Posts
    4,313

    Default

    Quote Originally Posted by Jacob'sDad View Post
    So how about the importance of using the most accurate BG meter possible for calibrations? And how can one be confident in the accuracy of their meter?
    That's a good point... when we researched meters, the one touch and freestyle
    seemed to have similar and very good correlation to BG in the plots.

    The Omnipod PDM has a built-in freestyle meter, so we use that.

    I'm pretty sure all the current home BG meters are good enough to be relied
    upon as long as the test site is clean.
    My daughter Leah was dx at age 8. Has used the Omnipod since 2007, Guardian 2007-2013, Dexcom 2007-present and sang the National Anthem at the 2013 JDRF Walk!

    DCCT: The Study That Forever Changed Treatment of Type 1 Diabetes - Improved Glycemic Control in T1 children Using Real-Time CGMS

  8. #8

    Default

    It does matter what meter you use while calibrating. The more accurate the reading from the fingerstick, the more reliable the cgms will be. I think the aviva is the most accurate meter. The most accurate meter will most likely give the most consistent readings. Also, if doing multiple blood tests on a certain meter at the same time, the least amount of variance between tests helps determine this factor.

    Watch this dexcom user on youtube. He is using the same sensor, with two receivers calibrated to different meters. The receivers use an algorithm based on the Fingerstick BG readings to correlate to actual BG/Interstitial Fluid Glucose reading. MM has the algorithm patented, in fact. I could provide the papers, if I had to. There is a HUGE difference between readings on the receivers. He talks about all of this stuff in about the last minute of the video. http://youtube.com/watch?v=gGgfhcxD5FE
    Last edited by moco89; 07-29-2008 at 11:57 PM.
    Young adult with type 1 diabetes, autoimmune autonomic neuropathy, and chronic inflammatory demyelinating polyneuropathy (CIDP)

  9. #9
    Join Date
    May 2008
    Location
    Philadelphia
    Posts
    4,313

    Default

    Quote Originally Posted by moco89 View Post
    Watch this dexcom user on youtube. He is using the same sensor, with two receivers calibrated to different meters. The receivers use an algorithm based on the Fingerstick BG readings to correlate to actual BG/Interstitial Fluid Glucose reading. MM has the algorithm patented, in fact. I could provide the papers, if I had to. There is a HUGE difference between readings on the receivers.
    That is a very interesting video.. I didn't even know you could use more than one received with the same sensor but I guess there's not reason why not!

    I was wondering though, the two BG tests he did came out 176 and 179 (then he forgot the 2nd result and programmed a 175 into the 2nd meter). He showed that the meters were off by 15 points (= 0.5% A1C, which is indeed significant), but yet the two BG's in the video were off by only 3 points.

    If the difference is really 15 points, though, it certainly could matter to A1C.

    Monica, do you know a source for any current BG test meter accuracy studies?
    My daughter Leah was dx at age 8. Has used the Omnipod since 2007, Guardian 2007-2013, Dexcom 2007-present and sang the National Anthem at the 2013 JDRF Walk!

    DCCT: The Study That Forever Changed Treatment of Type 1 Diabetes - Improved Glycemic Control in T1 children Using Real-Time CGMS

  10. #10

    Default

    Quote Originally Posted by Darryl View Post
    Monica, do you know a source for any current BG test meter accuracy studies?
    Unfortunately, no. I could probably look it up tomorrow, since I have school and access to journals there.

    But remember, the cgms relies a lot on the meter calibrations. I bet the maker of the video (Bernard) is probably right. He knows what he is talking about.

    Also remember that the transmitter is just sending a signal. It is not "smart", and if the receiver has the serial number for the transmitter, it will receive the transmitter's signal...even if there are two receivers receiving the same signal.
    Last edited by moco89; 07-30-2008 at 12:31 AM.
    Young adult with type 1 diabetes, autoimmune autonomic neuropathy, and chronic inflammatory demyelinating polyneuropathy (CIDP)

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
advertisement