ISIG/Calib. factor - LONG
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ISIG/Calib. factor - LONG
Aug 12 2008, 01:51 PM
Joined: 7-February 08
Member No.: 857
My Pump: mm 522
I think I am finally nailing down how the sensors work. Most of this information has been posted here, and some of it I got other places and personal experience. I've never seen it compiled in one place, so I think I'll give it a shot. Felix over on IP gave me the idea for this. Please note that this is not intended for brand-new users.
Sensor glucose (SG) is calculated by multiplying the ISIG by the calibration factor. The calibration factor is calculated by dividing the blood glucose you enter to calibrate with by the ISIG. The software actually uses the average of the last 4 calibration factors it calculates, I believe, to determine the SG it reports.
Thus, understanding the ISIG is fairly important in making the CGM work right. As Felix explained, "I" is the international symbol for electrical current. The 'SIG' either stands for 'signal' or is an acronym for 'signal [for] interstitial glucose'. Either way, I think it is important – not to beginning users, but those that have used it for a few weeks and are starting to get a feel for it, or even experienced users who have had problems.
There are a few things that affect the ISIG values. When the reagents (enzymes?) in the sensor react with the glucose in the interstitial fluid, a current is created due to voltage being sent by the transmitter. That current signal is measured in nanoamps. You can see this on the Carelink data table report.
Thus, the voltage that the transmitter sends to the sensor must be consistent, and strong. So, it is important to make sure the transmitter battery is charged enough. I haven't heard of a transmitter battery lasting more than about 18 months, but I'm taking a couple of (hopefully) proactive steps to increase my transmitter battery's life (the transmitter battery cannot be replaced without getting a new transmitter – about $600).
First, I am changing the battery on the transmitter charger every three months. I remember panicking when I got the flashing red light on my transmitter when my charger battery died - after six months. I won't go that long again. I think I probably wore the transmitter battery down a little by charging it with a AAA battery in the charger that was wearing out.
Next, I am letting my transmitter charge overnight at least once a month. I am fairly certain that a Medtronic electrical engineer would tell me I am wasting my time – that when those lights quit flashing, the transmitter battery is charged as much as it will ever be. Still, I figure it can't hurt, and I just have an intuitive feeling about this.
The next important factor for the ISIG is the amount of interstitial fluid in the sensor cannula. This has been discussed at length here. There are a number of successful users who insert their sensors the night before attaching the transmitter the next morning to get them completely wet. I don't have that problem, and I'm glad, because every few Sundays, I get a morning when I have neither infusion set nor sensor poked in me. Gosh, I love those showers.
I think this is the problem Dave (Spike) had with his sensors (where's he been, anyway?). Given his history of cannula kinking, I think his sensor cannulas just did not get enough interstitial fluid. Thus, his numerous SG low alarms at night - which were, of course, bogus. This theory is supported by the fact that his first sensor worked great for him – he just got lucky and found a site that didn't kink the cannula.
Finally, the more sugar in the fluid, the more the reagents get used up. That creates a higher current (and thus the ISIG), and that causes the sensors to get used up more quickly. This has also been discussed here. So, the longer the sensors are used, the greater the likelihood of them not giving a good ISIG (and thus a rotten SG).
Also, when the BG's remain high for a while, the same is true. If your BG's remain over 200 for a while, your sensors won't last as long. I've always changed my sensor every week, but I started leaving them in for 2 weeks. I've somehow been under extremely good glycemic control lately, so I've not had a problem with this. I'm fairly sure I can get 3-4 weeks out of a sensor, but I can't stand the scars after just 2 weeks.
So, anyway, those ISIG values are closely tied in with the calibration factors. I've found that if your calibration factors are pretty stable, you'll have really good luck with the sensors. If, on the other hand, you have one calibration factor at 5 and another at 12, you're almost doomed to not get good SG readings. I've looked at this fairly closely, and determined that my ideal calibration factor is somewhere between 6-6.5. Of course, everyone is going to be different.
It shouldn't matter if the BG is 60 or 200. If you calibrate when the calibration factor is close to the last three you did, you should do well. I sure wish MM would document the calibration factor and its importance. MM put a chapter in the back of their 'Sensor Features User Guide' that discusses sensor performance and calibration stability that I just now re-read. I still don't completely understand it. It mainly discusses the accuracy while hypoglycemic, and the importance of NOT calibrating too much. I still don't get that.
If you're still reading this, you're either extremely interested in this subject, or you're REALLY bored. Either way, I appreciate your taking the time to make it this far. If I've said anything wrong, please post it here immediately. I've spoken fairly authoritatively here, and I'd hate for someone to rely on something I've said that's just plain wrong. Final note: I am neither a health care professional nor anywhere close to this business: I'm just a successful user who thinks he has finally started to figure this out. I love this forum.
LADA - dx'd 1985 at age 31 - treated as type 2
Insulin therapy in 1987
mm 522 since Aug. 2007, cgms since Nov. 2007
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