Hi Jeff. found some really great tips from other MM users and going to post them here!
With the Minimed system you will have to remove & recharge the transmitter after 7 days as it has a built-in timer. Many people will use a sensor for only 6 days and some will restart it a third time to get that extra day. Note that the time begins as soon as you attach the transmitter to the sensor, not when you do the first calibration.
I try to get as much time as possible from each sensor so every 7 days I carefully peel off the old dressing to recharge the transmitter. You do not want to pull the sensor out of your skin at all, so taping is very important. Using tricks that I've learned here & on other forums and adapting them to my needs, this is what I do.
1. Insert a new sensor and place a strip of tape over the plastic hub, making sure the two little prongs that connect it to the transmitter are not covered. Many people recommend using medical paper tape or similar but I find that a strip of Hypafix works best for me. I have Hypafix rolls in various widths and a strip of the 2" wide is perfect. Anything that will hold the sensor firmly in place will work.
2. I find that getting the dressing off of the transmitter is the hardest part. Some people will place a piece of tissue on the transmitter or smear a little Vaseline on top to make the dressing easier to remove. I have found the easiest way for me is to use some double sided *removable* Scotch tape. When I connect the transmitter to the sensor I put some of the removable tape on top, then tape it all down. Since both sides are sticky everything is held firmly in place but when I peel off the dressing a week later, it comes off easily. The sides/edges of the transmitter are still stuck firmly to the dressing but once I get one edge loose it all comes up.
3. I keep a finger pressed on the sensor while removing everything because you do NOT want that to pull out of your skin even a little bit. The strip of tape helps keep it in.
4. I use an alcohol wipe on my skin before inserting a sensor. Let it dry thoroughly before inserting. If I'm putting the sensor in an area where I know the transmitter has caused irritation in the past I'll slip a small square of gauze under the transmitter after it's connected and before I tape it down. I make sure the gauze is not too thick because I don't want it pushing up on my transmitter.
5. When I remove the transmitter to recharge it I might clean the area up if it looks like it needs it. The only problems I've experienced is irritation from the edges of the transmitter. Using gauze or tissue underneath and avoiding areas that get irritated easily (my abdomen) solves that.
6. The longest I've used a sensor was 34 days, and it was in my arm. My arms do not get irritated by the transmitter and the site looked perfect when I finally took the sensor out. It actually got pulled out by accident when I was trying to recharge the transmitter, otherwise I would have kept going since it was working perfectly. That sensor tracked extremely well and was rarely off from my meter readings. The site looked better than my infusion sets sites which are only in for 3 days.
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.
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:
When there is an inaccurate BG reading (such as when fingers are
contaminated with sugar)
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.
When BG is changing rapidly. This causes the BG you enter to
correspond to an ISIG that
is delayed in time.
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
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.
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
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
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
Here's an additional resource with even more complete