WARNING: SEVERELY OUTDATED! I’m seeing this get some traffic again. I just want to be clear. This post is from 2007! Medtronic sensors are a whole lot better than this now.
I decided to create a new post for this so that my initial reaction to continuous metering would remain untouched and available for review. If you would like to see my first reactions to this device, please refer to my original post on the same topic.
I am now at the point where I am satisfied with the sensor glucose readings about 75% of the time. This has taken me a number of months, despite the fact that I consider myself quite technical and quite good at caring for my diabetes. Thus far, the sensor has helped my go from an A1C of 5.8 to 5.3, an improvement of about 9%. I do not know yet whether my next A1C will be as good. My doctor has cautioned be about going low too often. Being even more careful about lows than I had been may raise this slightly.
So, the number one topic you need to know about in order to get results is also the hardest to understand and the hardest to get straight information about.
ISIGs: ISIG is short for Interstititial SIGnal. This has nothing at all whatsoever to do with the strength of the transmission signal between the transmitter and the insulin pump. (Or presumably, the CGM device if you are not using one of the new pumps that serves both functions.) The sensor inserted beneath your skin has enzymes on it that react with your interstitial fluids to produce a “signal”. Personally, I would call this the raw glucose reading. However, a representative from Medtronic just assured me that this is not the case. She even assured me that it is possible for the ISIGs to be rising or falling while the reading is doing the opposite. I’m remaining skeptical about this point at this time. I have not seen that happen and do not understand how the simple measuring of the rate of reaction of the enzyme can produce two numbers.
She also assured me that valid values for the ISIGs while attached to a sensor in your body are 2 – 200. In my experience, 12 – 25 or so is far more normal. When my ISIGs drop too low, especially below 10, it’s a really good bet that the pump is telling me that my glucose is low, sometimes very low, and is another good bet that the value is completely unrelated to my actual blood sugar, which can even be high at such times.
I have requested any internal documentation they have on the meaning of ISIGs. If I get a more detailed answer, I will update this post.
RULE #1: No matter what else is happening, no matter how insistent the device is being about wanting a blood glucose calibration, DO NOT GIVE IT ONE WHILE THE ISIGS ARE LOW!!! Doing so is the number one way to get wildly inaccurate results. I’ve seen 200-300% discrepancy in sensor and blood glucose readings after calibrating during low ISIG times. It is far better to acknowledge the message, move around, massage the sensor area, or just plain wait an hour or two until it comes back to normal before calibrating. At worst, that will leave you with an hour or two of no sensor readings.
Rule #2: Wear your electronics (cell, crackberry, whatever) on the opposite side of your body from your pump. The transmitter is using the frequency that provides for maximum interference with all other electronic devices. Wear your pump as close to your sensor/transmitter as possible. If you put a cell next to your pump, you will frequently get weak signals, often followed by lost sensors. This is a royal pain in the butt. If you work in a modern office with WiFi, cell phones, blackberries, cordless headsets, microwave ovens, and all the rest, keep an eye out for weak signals. By the time it warns you, you will have only about 5 minutes to run outside for a while until the connection is reestablished. This happens to me somewhat regularly. I am seriously hoping for a wire connection option at some time in the future. I’m already tethered to the pump. Having two tethers would be better than all of these weak signals.
Rule #3: If you hope for the pump to warn you about highs and lows, set the range of values much tighter than you expect to keep it. There is a 15 minute lag (at least) between a change in your blood sugar and a change in the interstitial glucose value returned by your sensor. Setting tighter limits will warn you earlier. If you respond to these alerts appropriately (I always recommend checking blood glucose before taking action on the alert), what will happen is that your sensor will show lower amplitude swings in your glucose than your actual blood sugar values. This is because you are circumventing the peaks and valleys before they get to the interstitial fluid. So as an example, your graph may show a range of 68 – 140 when your readings show 50 – 180.
Here is the routine that has been mostly working for me to get reasonably accurate data from the sensor most of the time, beginning with sensor insertion day.
- Remove prior sensor the night before and charge the transmitter. This doesn’t take long, so can be done that morning if you wake even earlier. If you do this the night before, you’ll want to disable the sensor feature on your pump for the night. Your settings and existing graph will not be lost.
- Wake up at least 2 hours and 15 minutes or so before breakfast, 2 and a half hours may be safer.
- Remove the sensor from the refrigerator.
- Clean the area for the sensor with soap and water and dry thoroughly. This makes your skin less oily so the tape will stick better.
- Insert the sensor according to the instructions.
- Place a strip of tape over the flat portion of the sensor. It does not have enough tape of its own. Where it does have enough is on the wrong side. 3M makes a gentle paper tape that holds well and doesn’t irritate my skin when left on for 6 days. The tape Medtronic gives doesn’t hold well and doesn’t breath.
- Wait 5 minutes to ensure that the site does not bleed.
- Reenable your pump’s sensor feature, if disabled from the prior night. Tell pump to begin a new sensor.
- Connect transmitter to sensor and watch for green blinking light on transmitter.
- Check that the pump is receiving the signal from the transmitter.
- Wait a few minutes and check that the pump is receiving ISIGs from the sensor (Press ESC four times to get to that screen). The ISIG values may vary tremendously during this time. As long as they are non-zero, it should be fine.
- Attempt to keep your blood sugar flat during this time. You may wish to check your blood sugar now. I usually do.
- Wait for the pump to request a reading, check blood sugar and tell the pump the value. This wait period is when I usually go to the gym, making this a good schedule for me.
- Now you can take insulin and eat breakfast, even though you will not be getting sensor readings for another 15 minutes.
- Check your blood sugar an hour or two after breakfast and attempt to stabilize it. The 6 hour reading that it will want, just before lunch is a good time, is probably the biggest source of my poor calibrations since I am sometimes not as stable as early morning.
- You do not need to wait the full 6 hours for the next reading. Whenever you are stable, usually before lunch, is a good time to enter a reading.
- Throughout the rest of the sensor life, whenever I do a reading, if I am stable and the ISIGs are good, I enter the reading. This way, I am usually not forced to provide a reading when I am unstable or my ISIGs are bad.
- On the third morning, before the pump registers that the sensor is too old and kills it, tell the pump you are using a new sensor. Immediately after this, tell the pump to Find Lost Sensor. The pump will ask you for a new calibration now and another in 6 hours. But, you will not need to wait 2 hours before giving the pump the first calibration. It’s a useful little hack to work around the need to lie to the pump to get 3 more days from your sensor. Note: Sometimes I find that the 6th day doesn’t give accurate readings. Lately it has been. Some of my sensors may not have been adequately refrigerated for their entire life, making them last a shorter time. I’m not sure.
- Note: If you feel that your sensor is really badly calibrated, following step 18 above may be a good way to recalibrate from scratch without any CAL ERRORs. I’m not positive about this at the moment. Let me know if it works for you. I’ll update this if I verify that it works for me every time. I’ve had far fewer cases of poor calibration lately, so have not needed this much.
Settings that have been working well for me:
- High warning 130, snooze 2 hours.
- Low warning 70, snooze 1 hour.
- Alarm reminder 45 min.
- Cal reminder 2 hours, gives enough time to stabilize.
- Missed data: 40 min.
- I also tend to change to vibrate for the day and long beep for the night. (Main menu: Utilities->Alarm->Alert Type)
Let me know if this helps you at all. Please also feel free to share your own experiences in the replies. If you prefer to stay anonymous about your medical condition, feel free to use an alias and bogus email.
FYI: It appears that this post has been replicated at this diabetes site. If you like this post, perhaps you’ll go there and rate it. Of course, if you don’t like this post, A) Why did you read this far and B) Please don’t rate this post on that other site ( just kidding, mostly 😉 )
UPDATE 1/11/2008: (Data below is empirically derived, not from Medtronic/MiniMed)
SENSOR ERROR: I’ve gotten this error a few times now. I usually get it during the startup period of a brand new sensor. It’s tempting when seeing this to believe that there is a real problem with the sensor. There isn’t!! Or, at least there may not be. Hit ESC four times to get to the screen with next calibration time and ISIGs. If the next calibration time has simply moved an hour or so forward, your sensor is still good. The ISIGs merely haven’t stabilized yet. Do nothing. Wait for it to ask for a reading. The sensor will likely still be fine for days. I think I wasted a sensor the first time I saw this, $30 down the drain.
CAL ERROR: I tend to get these whenever I’m having trouble with ISIGS. The problem is that the pump will immediately ask for a new reading. DO NOT DO A READING IMMEDIATELY!! You’re very likely to get another CAL ERROR or two and lose your sensor if you do. Try moving around and massaging the sensor site. Or, just wait an hour or two until you see normal ISIGs again, over 10 is usually OK if your reading is below a hundred or so, over 15 is better. When you have good ISIGs, then give the pump a calibration reading again. This will leave you with no readings during the time you are waiting. But, you lived without continuous glucose monitoring before. It’s not life sustaining, just very helpful. You’ll do better calibrating when your ISIGs are good and just clearing all requests to METER BG until then.