To dose, or not to dose, that is the question.
That’s not exactly what was phrased by Hamlet in the opening soliloquy of the Shakespeare play. But it is a question heard frequently around the diabetes online community when considering our CGM (continuous glucose monitoring) systems. And it is a question that some would argue has life or death implications.
My answer to this question is sometimes, in certain circumstances, and within certain guidelines.
My current CGM is the Dexcom G4 Platinum and it is by far the most accurate system that I have used. The numbers are almost always in the ballpark with my meter readings. Twitter is littered with my photos of identical G4 and Verio meter readings. Earlier this week I downloaded my Dex into Dexcom Studio and my 3 meters into Diasend. Comparing the 2-week reports from each, my average BG differed by only 3 points. The standard deviations were also 3 points apart and my graphs were remarkably similar. So essentially I am getting the same information from these devices.
Like everyone with Type 1, I am awash in data and make decisions knowing that none of the numbers are 100% accurate. My meters are allowed to have a 20% variance. My pump is very precise in its insulin delivery, but my body’s absorption of insulin may vary based on the location and age of the infusion set, my exercise patterns, and various hormonal excursions. There are inherent inaccuracies in carb counting. Food labels are allowed a 20% fudge factor. Fruits may differ in their carb impact based on ripeness or variety. One portion of vegetable soup might have more potatoes than another portion filled with green beans and squash. So the Dex CGM is not perfect, but neither are my other tools.
I start my day with a fasting BG reading on my meter. If a correction is needed, I bolus for it. If my Dex calls for a calibration, I record the number. After this I will rely on my G4 for my breakfast bolus and throughout the morning for alerting me to lows and highs. Late morning I’ll take a fingerstick and make sure that the Dex is still on track. If a correction bolus or snack is needed, I’ll take it. Then when lunch comes, I’ll usually use my Dex as the guide for my bolus along with carb counts. Same for the afternoon, dinner, and throughout the evening. I use the meter for a “grounding” every few hours, but rely on the Dex the rest of the time.
I play the game of mistake management and try to minimize the number of diabetes mistakes that have serious consequences. If I am woken at night by a high alert from my Dex, I will double check the reading with my meter before taking a correction bolus. That is because an overdose of insulin while sleeping is potentially life-threatening. If my Dex wakes me with a low alert, I often pop a couple of glucose tabs from my bedside table without a meter confirmation. The result of a Dex mistake in this case is just an errant high from eating glucose tabs.
Similarly, I take driving with Type 1 Diabetes very seriously. If I’m on the south side of BG 100 or have any symptoms of a low, I will always test with my meter before driving. No way that I want to have a low at 65 miles per hour. On the other hand, if the Dex shows me level at 110 and I feel good, I’ll probably trust the Dex and head out. At this point I’m comfortable I’m in a target range that is safe although I may not know the exact number.
During the daytime, I usually trust the Dex high alert and bolus based on it. I have to admit that I get some false lows from my G4, but I get very few false highs. Sometimes it will alert me with a 180 reading and I think “no way!” But the Dex is usually right.
My trust in my G4 is bolstered by an average of 8 meter readings a day. Some of these are for calibration. Others are before driving or halfway through a round of golf. In general, my meter helps me trust my Dex while the trends on my Dex helps me trust my meter. (As a caveat, I don’t trust the Dex on the first day of sensor insertion and I lose confidence as it approaches Day 14.)
In summary, I confidently bolus throughout the day based on my Dexcom G4 readings. Why can I do this? First, because I also test a lot with my meter. Secondly, because I have the Dexcom G4 to alert me if I make a mistake.
(I will end this post by reminding everyone that Dexcom and our doctors advise us never to bolus based on our CGM numbers.)
Test Guess and Go 
Every time I read a post like this, I second-guesss why I’m still using the CGM made by “the other guys. At a time when I’ve been contemplating whether I should morph into a completely different brand of cyborg, this gives me more to think about. Every bit of information helps. Thanks for this.
I switched to the Ping to get in line for the Vibe. I still miss the Revel pump, but I am totally committed to Dexcom. When I made my pump decision, Tandem would not guarantee an upgrade program to a sensor augmented pump. It’s hard making these decisions when you can’t see all of the pieces to the puzzle.
There’s a line for the Vibe? I figured I could sign up as soon as it’s released — I never considered there could be a backorder and that existing customers could get upgrades first. My current pump is still under warranty anyway (though my insurance is changing, so they likely don’t care when I got my last pump)
No, there is no line. I’m sorry that I was confusing. I’m just enrolled in the $99 Animas upgrade program. The timing of my pump purchase in December was based on the fact that I had 4-1/2 years left until Medicare and I wanted to have one more chance to buy a pump before getting to Medicare. I bought my Ping with so many questions unanswered. How long will it be before the Vibe is released in the USA? Are the Enlite sensors good enough to justify staying with Medtronic? When will Tandem released a Dexcom/Tandem pump and will they have an upgrade program? Pumps are big purchases to make wearing a blindfold, but that’s what I felt like.
Interesting to read your perspective on bolusing based on Dex readings and not testing first during the day, as well as your downloaded reports on the pump and Dex being so close. I download the Dex to make adjustments to basal rates now, but don’t bother with downloading the pump any more.
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I was using a pump for ten years with good a1c’s but frequent and severe hypoglycemia. An accountant told me about the Dexcom Seven Plus CGM. It was easy to use but had a tendency to indicate high sugars for no reason. After one year I switched to the Dexcom G4 platinum. This has been nearly flawless. It is so accurate that I only need to calibrate it twice a day and can make accurate food and insulin decisions based on its readings. Symptomatic hypoglycemia has been almost eliminated. I hope to get the Animas Vibe as soon as it is available. The G4 is so good that my only concern is leaving it at home.