Sweet red blood
This is a deciliter (about 3.3 fluid ounces) of “blood”. Where I live, we measure blood glucose concentrations based on how much glucose is present in this amount of blood.
So, if I wanted to find out how much glucose was in the above sample of “blood”, I’d simply extract the glucose, put it on a scale, and weigh it.
(Your nation may vary, and if it does, I apologize in advance for not doing the conversion for you. But mmols involve counting molecules, not weighing them, and mmol folks base it on a full liter – or litre if you prefer – which is an order of magnitude larger)
104 milligrams of glucose in that deciliter of blood? Perfect. 104 mg/dl.
Except nobody in their right mind would want to draw THAT much blood – perhaps ten times a day – to keep tabs on things. So what do we do? We sample – we take a smaller amount and extrapolate the results.
For instance, instead of a full deciliter, we could take one one-hundredth of that amount of blood, extract and weigh the glucose, and take the result … times 100.
Of course, if our measurement is off by a little bit, that error gets magnified by a lot when you do the math.
But it’s still a lot of blood. So why not take one one-hundredth of THAT amount?
To get your blood glucose level, simply draw this amount of blood and multiply the weight by 10,000. If your weight is off by just 0.01 milligrams, your final result is off by 100 mg/dl. Unacceptable.
But even that blood sample is unacceptable.
According to this recent Diabetes Forecast comparison (pdf), some of the most popular meters use samples in the neighborhood of 0.3 microliters (Abbott Freestyle) to 1.0 microliters (OneTouch Ultra)
If I’ve lost you with the math, let me summarize: there are one hundred-thousand microliters in a deciliter.
This is one microliter, the amount of blood needed for a OneTouch Ultra meter. It’s mighty tiny – you might be able to detect it in the second photo.
You get the idea. Weigh the glucose, multiply by one-hundred thousand. A tiny error can make a big difference.
Now, of course, this isn’t how we really get our blood sugar readings. Not at home anyway (Maybe at a lab, I really have no idea). Modern meters mix the blood with a chemical, zap it with electricity, crunch it through a mathematical machine, and out comes mg/dl (accurate to 20 percent or so). To be honest, considering the size of the sample, they do it remarkably well.
Yet we’re still not happy, and justifiably so. Well enough is not good enough.
But is it realistic? Can we really get accurate blood sugar readings from such a small sample-size?
“But blood is not fixed. It’s liquid. It’s dynamic. […] Your heart pumps it both upstream and downstream. […] For our entire diabetic careers, we have been indoctrinated, trained, brainwashed, and bamboozled by our only available technology, the fingerstick meter, to view an inherently fluid part of our body as something static and fixed.”
-Wil Dubois, Beyond Fingersticks, Chapter 5
In other words, the glucose concentration at your fingertip is not the same as it is in your carotid artery, or your superior vena cava, or your forearm. (But you knew about the fingertip/forearm discrepancy. It’s in every standard disclaimer about alternate site testing).
I suppose it could even vary from one drop to the next. One microliter to the next.
The smaller the sample, the more room for variation.
But in the back of my mind, I still wonder — at what cost do we demand it?
Will accuracy-driven regulations mean we need to go back to a sample size of:
* * *
PS – That insulin pump I used above is an old, long-out-of-warranty pump that hasn’t felt the charge of a battery in years. The reservoir and tubing are the byproducts of a set-change a couple of days ago. And the syringe is about a decade old, purchased before I started using insulin pens and before I started pumping. I’ve got a ton of ’em lying around. My own diabetes care (however unbalanced it has been as of late, though I understand there’s a book for that, which I fully intend to get my hands on) was completely unaffected in the writing of this post.
PPS – The wine was from a surprise 40th birthday party my wife threw for me last weekend (I’m really old). My own diabetes care was affected by that.
PPPS- Speaking of books, that edited paragraph I excerpted from Beyond Fingersticks: The Art of Continuous Glucose Monitoring came from a paragraph about expectations, and it totally changed the way I look at not only CGM, but blood sugars in general. If you haven’t read it, I highly recommend it. Brought to you by the author of Ask D’Mine.
PPPPS – Someone once said to be careful what you wish for, ’cause you just might get it. So when I make my demands known (do it here, do it now!), I’m not only asking for better accuracy, but no increase in sample size.