Are You ON The CGM Rollercoaster? Get OFF With These PRO Tips

by Brian Lucido; Editor Scott King

Brian Lucido

It is generally accepted that tight blood sugar control can reduce the risk of microvascular complications (kidney disease, nerve damage, and eye disease) in people with diabetes. Tight control can be dangerous if you dip too low without warning. CGMs (Continuous Glucose Monitors) make tight control more feasible by giving us a layer of safety with automatic warnings when blood sugar is too high or too low. The technology to achieve instantaneous readings with 100% accuracy hasn’t arrived yet, but it’s coming. The Dexcom G5 with a MARD (Mean Absolute Relative Difference between CGM and fingerstick) of 9%[1] is close. Lower is better, and when we reach 0% with instant results, we will have some fantastic tools in our hands. For comparison, the Medtronic Enlite has a MARD of 13.6%[2], and the Abbot Freestyle has a MARD of 11%[3].

In spite of these official MARD results, this technology varies in different hands and bodies. Here we present some strategies to get the best possible results with the Dexcom.


Roller Coaster Glucose Connection

First, we must discuss the limitations of modern CGM technology. Foremost, modern CGMs measure the glucose concentration of the interstitial fluid — NOT blood. This is very important to know because it means that your readings from the CGM are delayed by about 15-20 minutes from your actual blood glucose levels. (The lag time varies greatly depending on many things and whether glucose is going up or going down.)

To conceptualize this, imagine a roller coaster where the front car is your blood glucose (fingerstick), and the back car is your interstitial glucose (CGM reading). As your blood glucose goes up, your CGM reading in the back car lags behind. After the blood glucose crests the top and starts heading downward, the rear car (your CGM) could actually still be going up!


When to NEVER Calibrate

A major rule of calibrating your device is to ensure that you NEVER calibrate when your blood sugar is changing. For example, maybe you injected some fast acting insulin an hour ago — and now it is time to calibrate your CMG. Your blood glucose could be flying down that slope — and the interstitial glucose could be just cresting the hill. If you enter your fingerstick blood glucose number at this point, you’re just going to confuse your CGM. So what should you do if you get the “red blood drop” symbol asking you to calibrate when you know your glucose is changing?

Wait. You aren’t required to immediately “answer the call” of the Dexcom’s request for calibration. Dexcom technical support often asserts that the device does best with fewer calibrations — just two per day. If the device is off by less than 20mg/dL, you shouldn’t correct it unless the device is asking for a calibration.

You want to make sure that you ONLY calibrate on a horizontal arrow / flat line. Take that a step further: Be sure that your blood sugar isn’t changing from food, rapid temperature changes, recent fast-acting insulin or exercise. Also, be sure that nothing is going to change your blood sugar rapidly in the next 15-20 minutes (i.e., don’t calibrate right before eating or showering (explained below)).

When you calibrate your device, you also want to make sure that you are giving it accurate readings. It is therefore essential that you have an accurate fingerstick reading. As they say, “garbage in, garbage out.” Be sure to follow all of the rules of getting good fingerstick readings shown in the sidebar with this article.


Review These for Best Accuracy

1-CGM sensors are not usually very accurate for the first 24 to 36 hours after insertion. One method to work around this is to insert a sensor one day before you take your old one out. Just let the sensor sit there “stewing in your juices.” Then when you plug in your transmitter, it’s already acclimated. You still need to do the 2-hour wait with calibration.

2-Proper hydration is critical to the performance of the device. If you are dehydrated, your body will naturally pull water from your interstitial fluid into the cells. That leaves your sensor high and dry — producing inaccurate readings. If you see the ??? on a Dexcom, one of the first checks should be making sure that you are hydrated.

3-Rapid temperature changes in your skin will also affect the results of your sensor. Users frequently (but not everyone) see “shower spikes” where the CGM reading goes up quickly due to the effect of hot water on the skin and movement of interstitial fluid. In accordance with the guidelines we set above, you want to make sure that you do not calibrate your CGM 20 minutes before or after a shower or other dramatic temperature shifts.


Consider This Example

If you calibrate right before you jump into a shower, and you tell the CGM that your blood glucose is 100 mg/dL, the Dexcom knows it lags about 20 minutes behind your true blood glucose (fingerstick). If you then jump in a shower, and you get a “Shower Spike,” you have just confused your CGM. Think of the analogy of the roller coaster; you want things to be stable for 20 minutes on both ends of when you’re calibrating. The device works amazingly well if you manage to do this!

Another challenge are compression lows. Compression lows occur when there is pressure on the sensor site. This has the effect of pushing the interstitial fluid away from the detector. In doing so, you will get a false low reading. This can be a frustrating time for calibration. A common example is a false low that occurs when you sleep on your sensor. Your CGM may be giving you low alarms of 50mg/dL, but your fingerstick reads a cool 90mg/dL. Resist the temptation to correct it by calibrating it to read 90mg/dL. That will hurt future accuracy because at the site of compression, the glucose concentration really IS low and it takes several minutes of decompression for it to re-equilibrate. If you’re having trouble getting good results, don’t be afraid to try a new sensor location, especially one you might not sleep on. People seem to be reporting good luck with sensors on thighs, behind their triceps, and back (small of the back or under the pant line, but above the actual buttock).


Other Calibration Suggestions

If your Dexcom is giving you wildly inaccurate readings between days 1 and 7, you may need to take additional steps.  If you call Dexcom Technical Support, they will likely tell you about the 3 x 15 calibration procedure (calibrate using three fingersticks 15 minutes apart). The 3x calibration can solve your accuracy woes sometimes by resetting the device – but it is difficult to get the timing right (in a busy life, can you be there to set a timer every 15 minutes and do a calibration)? Also, importantly, if your blood glucose is moving during that time, these 3 calibrations aren’t going to be very useful. So, an off-label solution would be to just shut the sensor down and start it up again. Yes, you lose 2 hours, but once it asks you for those two finger sticks, you’re going to get a better calibration — especially if your BG isn’t on the move. Also, you would have already lost 45 minutes of Dexcom accuracy following the 3 x 15 protocol, so net loss is minimal.

Dexcom tech support will not recommend this (restarting) because the sensor is only FDA approved for 7 days. If you restart it, the clock restarts, so you could accidentally leave the sensor in too long. Of course, many people are already restarting their sensors all the time and getting 2, 3 or more weeks out of a sensor.

Hopefully, these tips will help you get improved performance from your CGM.



  1. Bailey, T.S.; Chang, A.; Christiansen, M. Clinical accuracy of a continuous glucose monitoring system with an advanced algorithm. J. Diabetes Sci. Technol. 2014, 9, 209–214. [link]
  2. Bailey, T.S.; Ahmann, A.; Brazg, R.; Christiansen, M.; Garg, S.; Watkins, E.; Welsh, J.B.; Lee, S.W. Accuracy and acceptability of the 6-day Enlite continuous subcutaneous glucose sensor. Diabetes Technol. Ther. 2014, 16, 277–283. [link]
  3. Bailey, T.; Bode, B.W.; Christiansen, M.P.; Klaff, L.J.; Alva, S. The performance and usability of a factory-calibrated flash glucose monitoring system. Diabetes Technol. Ther. 2015, 17, 787–794. [link]

Brian Lucido manages his type 1 diabetes using MDI (Lantus and Novolog) and a low and slow carb diet (30-50g carb per day). The low carb diet, combined with a strategy of frequent “micro corrections” has enabled him to safely achieve an A1c of 4.8% with a Dexcom 90 day standard deviation of 14. Depending on his level of activity, he will vary his basal and bolus dose — sometimes dramatically. Brian recently raced and won the 2,700 mile Great Divide Mountain Bike Race — a race where only 50% of the entrants finish. He believes his low carb training helped his endurance through an improved ability to burn fat for fuel.



Getting Accurate Fingerstick Readings

by Brian Lucido

1) Wash and dry your hands.

2) Get a good enough flow that you don’t have to squeeze the blood out.

3) Consider using the second drop. To do this, simply wipe away the first drop of blood that comes out and use the 2nd one instead. The 2nd drop will more closely represent the circulating blood glucose.

4) If the fingerstick reading doesn’t match your expectations, do a second one before using the reading as your calibration number.

5) Make sure you buy the most accurate meter you can! This can be covered in a future article.

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