She blinded me with science

When Katy commented on my review of Genteel lancing device, she asked a very important question: are blood glucose readings from alternate test sites reliable and accurate? Is there any special magic to finger blood?

I confess: it crossed my mind before but I was avoiding this question. Blood is blood, so it should all be good, right? Besides, I confess that I did not want to find out, after getting really comfortable with alternate sites testing, that it was not as good as finger testing. Alas, Katy’s question sprung me into action. Naturally, first thing I did was consult with Dr. Google. It directed me to several reputable (or reputable-appearing?) diabetes websites that told me in unison that there is, indeed, magic to finger blood because it reaches fingertips quickly. Alternate testing sites can produce blood sugar readings 20-30 minutes old. When BG is relatively stable, it should not be an issue. However, when it is going up or down rapidly, such as during exercise and after eating, and when a low blood sugar is suspected, we should only test on fingers.

I did not like the sound of it at all. How useful is testing on alternate sites if we can’t do it most of the time? How often are a child’s BG levels stable? Almost never, that’s how often. My kids are active, they are constantly in motion, and they are also constantly eating.

Just another day of V's blood sugars. Do you see a lot of stability? Neither do I.

Just another day of V’s blood sugars. Do you see a lot of stability? Neither do I.

There was only one way to find out if the internet was right. Fortunately V was game for it, and in the name of science we began our experiment of comparing BG readings from alternate sites to BG readings from finger pokes. If I were a real scientist, I would meticulously check BG both ways every time I tested. However, I really did not want to subject poor V to poking herself twice as often (she already tests 8-10 times a day), nor do I have an unlimited budget for test strips, nor can I actually remember to poke her twice every time. Instead, we tested a few times for several days, making sure that we do it under different circumstances. Behold our (almost) scientific results!

Day 1

11:45 AM: We’ve been at the amusement park since 10 AM; V says she is feeling low. Finger = 72; Above knee = 95. Hmm, maybe Dr. Google was right? While the difference is not that significant per se, when it comes to suspected lows it is a big difference and suggests a different course of action. We are more likely to treat for 72 and more likely to leave things alone for 95.

7:05 PM: Dexcom alarms indicating a low (set to alarm at 75 and below). This is also within 1 hour of eating, and since we totally guessed carbs for dinner I suspect that I over-bolused her. Finger = 60; Arm = 59. Hey, that’s spot on!

11:00 PM: V had been asleep for about an hour and I am testing her before my bedtime. Finger = 133; Arm = 130. Score!

Day 2

6:00 PM: Testing after V just got out of an hour-long intense swim practice. Finger = 111; Above knee = 109. 

10:15 PM: Dexcom sounds a low alarm. Finger = 80; Above knee = 84. (Gotta love those false alarms! NOT.)

11:45 PM: Testing again because at about 11 PM V had a low for real and I treated her with two glucose tabs. Finger = 111; Above knee = 111. BULLS EYE!

All three readings are very close despite very different circumstances and lack of BG stability in all of them.

Day 3

1:30 PM: Testing V after she spent 2 hrs jumping in a trampoline park. Finger = 67; Arm = 67. Aha!

10:30 PM: Dexcom indicates that BG has been inching up, and it’s not surprising as there was ice-cream for dessert. Finger = 235; Arm = 261. Wait, this does not make sense. If BG from the arm site is delayed, how come it is higher than BG in the finger, given the upwards trend? And anyway, these readings are not terribly far from one another.

Day 4

7:10 PM: Testing before dinner and one hour after administering a correction for high BG. Finger  = 215; Arm = 248. At least this makes more sense if the delayed readings theory is right. Still, the readings are not that terribly far apart and we could easily test twice on fingers and get the same results. In fact, I can test BG twice from the same finger poke and get a difference of about 20 points.

10:45 PM: Testing before my bedtime. Dexcom graph is showing a very slow upward trend. Finger = 168; Above knee = 150. This is pretty close.

Day 5

4:30 PM: Testing within an hour of giving insulin for a snack, with Dexcom indicating an upwards trend. Finger = 295; Arm = 284. Very close.

So what do I make of this data, be it limited and not particularly methodical? In my opinion, there is no special magic to finger blood. I find readings from alternate sites close enough to finger poke readings and now I feel a lot more comfortable about alternate sites testing under any circumstances.

What do you think about our little experiment?


6 responses to “She blinded me with science

  1. That’s a very interesting experiment! Only a D-Mom would pour through every circumstance and read every number. 😉 The difference between the sites is smaller than I thought it would be. Now I wonder what two readings of the same site would read as a contrast. I would not at all recommend it — there are limits to poking! Just a thought.


  2. I just googled “How long does it take for blood to circulate through the entire body?” and the answer was one minute. Of course, I may be totally missing some understanding of blood glucose, but it seems like within a minute, all blood at any testing site should be about the same. Right? Or does it have nothing to do with blood circulation?

    Liked by 1 person

    • You made me dig deeper into science. My inner nerd is happy. So, from what I’m gathering, blood glucose readings are not the same as blood circulation. Taking a paragraph from one article:
      “The observed slower glucose kinetics at the forearm are most likely related to physiologically occurring site-specific differences in dermal circulation. In healthy individuals as well as those with type 1 and type 2 diabetes, dermal blood flow is 5–20 times higher at the fingertip than at the forearm ( 13– 15). This is due to the fact that arteriovenous anastomoses within the dermis are numerous in glabrous (hairless) skin (e.g., fingertip) but nearly absent in nonglabrous (hairy) skin (e.g., forearm) ( 13, 16). Therefore, the total exchange of blood within cutaneous venous plexus, from which blood obtained by skin-pricking is mainly derived ( 17), will take more time at the forearm than at the fingertip. This interpretation is supported by the ameliorating effect of local rubbing of the skin, which increases local blood flow.”

      Full article here:


      Liked by 1 person

  3. So if the difference is hairless vs hair-ed sites, would an upper arm or a lower back be better than a forearm? Could you test an earlobe? The Red Cross punctures there to check iron levels before blood donation as there are fewer nerve endings.
    Kudos to V for her willing participation!

    Liked by 1 person

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