Disclaimer: I’m not guaranteeing that any of the diabetes math below is accurate in the least because I don’t do any diabetes math by hand anymore. Thank goodness!
Disclaimer 2: Sorry to be all doom and gloom today. Also, long-winded.
So here’s the thing: There are fifteen things keeping you from having perfect blood sugar…many of which you can’t even control. (Number one being that busted pancreas!)
Two of the tools that people with type 1 diabetes use to make most of their decisions are blood glucose meters and counting carbs. But here’s the problem: we are relying on two inaccurate numbers to do our diabetes math. Something we do upwards of ten times a day.
Blood Glucose Meters
Blood glucose meters aren’t accurate. In fact the FDA allows quite a bit of variation, in my opinion, that can make a huge difference for the little bodies of children with type 1 diabetes who are very sensitive to insulin (as opposed to people with type 2 who do not use insulin, but make up the majority of people with diabetes).
From Diabetes Forecast:
The Food and Drug Administration (FDA) has accuracy requirements for all meters marketed in the United States. The FDA’s requirements allow for a range of readings.
For results at or above 75 mg/dl: 95 percent of meter test results must be within plus or minus 20 percent of the actual blood glucose level. Example: An actual blood glucose result of 180 mg/dl could potentially show on a meter as any value from 144 mg/dl to 216 mg/dl and meet the standard.
For results below 75 mg/dl: 95 percent of test results must be within plus or minus 15 points of the actual blood glucose level. Example: An actual blood glucose result of 70 mg/dl could potentially show on a meter as any value from 55 mg/dl to 85 mg/dl and meet the standard.
Those might seem like wide ranges, but Robert Ratner, MD, FACP, FACE, the American Diabetes Association’s chief scientific and medical officer, says those ranges are generally safe and reasonable. “[This level of] accuracy is sufficient for reliable clinical decision making and therapy,” Ratner says. Simply put, if a meter meets the FDA’s current standards, it is likely to give a reading close enough (emphasis my own) to the actual blood glucose level that users would treat any number in the range in a similar way. The range is accurate enough for you to decide if and how you will treat yourself.
Take, for instance, the first example listed above. Whether your blood glucose level is 144, 180, or 216, you are likely to decide that is a little high. For the majority of people with diabetes—who do not use insulin (emphasis my own)—the only immediate way to lower that high is exercise. Otherwise, it’s just one number—but a pattern of highs at the same time of day may indicate the need to make changes in medication, food, and/or exercise.
The two phrases that I emphasized in this quote should definitely give us caution: “close enough” and “who do not use insulin.” When we are determining insulin for small bodies, “close enough” isn’t good enough. And because type 1’s do use insulin, we are using this number for more than just deciding to take a walk or eat a little something. We are using this number to decide how much insulin–a substance that is actually deadly if used incorrectly–to give to our children.
The acceptable level of accuracy in the higher range is pretty wide. This means that the amount of insulin that we use to correct a blood sugar of 300 mg/dL could be way off.
Let’s take the numbers given in the example and figure out the amount of insulin based on a correction factor of 100 mg/dL per unit and a blood sugar target of 120 mg/dL.
144 mg/dL –>> 0.20 units
180 mg/dL –>> 0.60 units
216 mg/dL –>> 0.95 units
Now let’s take a blood sugar of 250 mg/dL.
200 mg/dL –>> 0.80 units
250 mg/dL –>> 1.3 units
300 mg/dL –>> 1.80 units
That is a full unit difference! I don’t know about your child, but if my daughter was 250 mg/dL and the meter read 200 and it gave her 0.5 units less, her blood sugar might not come down very quickly. And if she was 250 and the meter said she was 300 and gave her an extra 0.5 units based on that, she could drop very low, very quickly.
I’m a vigilant carb counter.
I count. I weigh. I measure.
I have taught Q to count and weigh and measure. In fact she counts the carbs as she packs her own lunch each and every morning before school. It’s one way that I have empowered her to eat when she’s away from parental oversight.
The article “On Food Labels, Calorie Miscounts” in the NY Times this week got me thinking about the accuracy of nutrition labels on packaged food.
The method most commonly used to assess the number of calories in foods is flawed, overestimating the energy provided to the body by proteins, nuts and foods high in fiber by as much as 25 percent, some nutrition experts say.
“The amount of calories a person gets from protein and fiber are overstated,” said Geoffrey Livesey, the head of Independent Nutrition Logic, a nutrition consulting company in Britain, and a nutrition consultant to the United Nations. “This is especially misleading for those on a high-protein, high-fiber diet, or for diabetics” who must limit their intake of carbohydrates.
The study Food Label Accuracy of Common Snack Foods “tested label accuracy for energy and macronutrient content of prepackaged energy-dense snack food products.”
Carbohydrate content explained 40% and serving size an additional 55% of the excess calories. Among a convenience sample of energy-dense snack foods, caloric content is higher than stated on the nutrition labels, but overall well within FDA limits. This discrepancy may be explained by inaccurate carbohydrate content and serving size.
We also show that 40% of the excess calories were explained by higher carbohydrate content compared to the label. This observation underscores previous criticism on the accuracy of carbohydrate content measurements (by subtraction, as stated above) and factorial values that determine energy derived from carbohydrates. Therefore, more precise regulations of analytical procedures regarding macronutrient content determination specifically in energy-dense food products may be necessary.
I seem to recall reading somewhere at some point that carbs can be off by 20%. (But don’t quote me on that because I have no idea where I read that.) But based on the NYT article and the study above, it’s safe to say that labels are not accurate. So even if I count the carbs correctly, I am probably giving my daughter an inaccurate amount of insulin for the carbs I think she is consuming.
So let’s do like above and I am going to conservatively say that carb counts can be off by 25%. And we are going to assume an insulin:carb ratio of 1 unit of insulin per 20 grams of carbs.
For a meal with 50 carbs:
37.5 carbs –> 1.9 units
50 carbs –> 2.5 units
62.5 carbs –> 3.1 units
Once again, that’s over 0.5 units of difference in either directions which can make a huge difference on where her blood sugar will go next!
Why It’s Inaccuracy On Top Of Inaccuracy
Now let’s imagine that her blood sugar is 250 and she is eating 62.5 carbs (according to the food label) and the meter inaccurately says her blood sugar is 300 mg/dL and that package of food really has 50 carbs. She should get 3.8 units to safely get her blood sugar down to 120 mg/dL, but instead she gets 4.9 units. That extra unit of insulin could bring her down 100 mg/dL below her target blood sugar of 120.
And now we have a serious, life-threatening problem.
(And now I have a headache from doing diabetes math because I let the insulin pump do all the diabetes math for us these days!)
Maybe the majority of the time the meter isn’t wildly inaccurate at the same exact time that a food label is wildly inaccurate and so this example is an exaggeration. But you can see how easily it might actually happen.
On the other hand, given that people with type 1 diabetes make a dozen decisions every single day based on their blood glucose meter and the packages of the foods they eat, no matter how accurate one thinks they are making decisions, the decisions can never be 100% right.
So it’s no wonder that sometimes high blood sugars are stubborn, or low blood sugars just won’t come up, or an A1c is not exactly where we expected it to be.
(If I made a big diabetes math error, please let me know and I’ll correct it. Thanks!)
You Might Also Like…
If you would like more information about test strip accuracy and a group of people with diabetes (and their caregivers) who are lobbying for better meter accuracy, visit Strip Safely.
“Blood Glucose Meters 2014” from Diabetes Forecast (the large quote on meter accuracy above)
“On Food Labels, Calorie Miscounts” in the NY Times.
Please remember that I never give medical advice. Ask your endocrinologist or pediatrician for advice about your own child. Make your own informed decisions for your own child.