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: 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 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.