Lab draw bg test question

I’ve been T1 for 35 years so you might think I wouldn’t have any questions anymore. You’d be wrong!

I go to the lab for “fasting blood draw” every 3 months. I’m T1 so I need at least basal insulin in me at the time.

When they take my blood out it has a certain concentration of glucose or bg reading. That sample also has some not-yet-used insulin floating around in it too.

The insulin already in my bloodstream - and in the drawn sample - does that cause the bg in the blood sample already drawn to continue to drop for a number of hours even after they’ve taken it out?

I wouldn’t think so (but I’m not a scientist). Insulin doesn’t eat up available glucose. My elementary understanding is that it attaches to cells and signals them to absorb the sugar from the bloodstream. One analogy often used is that insulin is the key that opens the cell door to let the sugar in. So in a vacutainer, in the absence of any human tissue cells, the insulin is going to be ineffective and have no influence on the blood glucose level.


Just based on experience of testing my bs on my meter at the same time as the blood draw, I would say no. They are normally relatively close to each other.

1 Like

The same would be true for non diabetics. They have insulin in their blood from their pancreas. I think beacher is correct, and there would no change the the BG number.

We ignore the “request” for fasting.

Surprisingly enough, the BG usually comes back quite elevated (from breakfast). You might almost suspect a case of T1 !!!

Seriously - we are not interested in the BG value. Not even a little. But there is a ton of other information we really do want to see. The only time we will actually (almost) fast is if lipids are being tested and then it is only skipping anything with fats.

The bottom line is that “fasting” impacts a very small number of tests components. It is up to you and your doctor to decide if you are actually interested in those components. If so - then fast. If not - then eat. My take.

I find comparing the lab BG value to fingersticks I do concurrent with the lag draw as a way to verify that my meter is reasonably accurate.


Totally makes sense. Just a question if we wanted to fast for that aspect. But I do agree - that would be a point well worth considering in regards to whether to fast or not. And IF fasting for the blood draw, it certainly would be a missed opportunity NOT to do that.

I also don’t usually fast for any of my lab work and never for a1c. Once in awhile there is something being tested for that they ask for a fasting but wow, it’s been a long time since I’ve had to do that.

Fasting is not required for a lab BG value.

Ok - I follow. Didn’t even think of that. Whether it is 90 or 190 - all that matters is if the meter and lab are the same.
(If both are drawn within minutes of each other).

Makes sense - I just wasn’t thinking that way.

I did take BG before and after my three monthly blood tests with both the Accu-Chek Performa and the FreeStyle Optium to try and confirm which meter was the more accurate.
The only correlation between all three was when BGL were in normal range (4 to 6 mmol/L - 72 to 108 mg/dL) and both meters were giving the same readings before and after the blood test.
The before and after tests are about five minutes apart and the greater the discrepancy between the meter reading and the before and after results, showed poor correlation between meter readings and blood tests.
The most extreme results between meter readings and blood tests occurred when I had to change a flat tyre, run 400 meters and a three hour delay prior to the samples been taken.
All three occasions showed Blood Test BGL were much lower then meter readings.
In my cases the reason was that the Blood Samples were not deproteinized and the samples were not centrifugal immediately. The reason given by my GP was that I was active prior to the tests and my red blood cells were actively taking up glucose.
In my situation I know that there may not be any correlation between blood tests and meter readings if I have been physical active just prior to tests, have insulin on board, have high glycogen reserves and/or there is discrepancies between meters.
The blood sample has to be deproteinized to be sure that the BGL is correct.

Can you elaborate on the impact of the timing of the centrifuge in regards to the lab results? This is a new concept to me (timing of).

The test that made me question my blood test results was when I had to wait
for three hours for blood to be taken. My BGL were high that morning (13.2
mmol/L) and after normal basal insulin and an increased bolus then
breakfast, I worked on the tractor.

This was not a fasting Blood Test but my appointment was delayed by 3
hours. When in town my BGL fell to below 5.0, so by law, I could not drive.
Walking would lower my BLG and sitting caused my BGL to rise. The Doctors
Practice was busy so I had to walk to the Pathology Lab for blood to be
taken, a 500meter walk.

These are the tests:

8.36am Performa 5.4mmol/L

11.38am Performa 4.1mmol/L

11.46am Performa 4.0mmol/L Optium 3.5mmol/L

11.59am Performa 3.9 mmol/L Optium 3.0mmol/L (Test Prior to Blood
been taken)

12.03pm Performa 4.2mmol/L Optium 3.4mmol/L (Test After Blood was

While I was at Pathology I injected 6 units of NovoRapid at 12.04pm for a
chicken pie which I began to eat at 12.15pm. I showed no signs nor felt any
symptoms of a Hypo and my readings at 12.55pm were Performa 5.9mmol/L
Optium 5.4mmol/L.

The Blood Test came back at 2.9mmol/L and I was in trouble with my Doctor
until we both worked out what had had gone wrong. Because the Blood Glucose
test came from the same vial for the Bulirubin (Gilbert’s Syndrome), the
blood sample was not “deproteinized”. It takes 10 to15 minutes for the
blood in the vial to coagulate before it can be centrifuged. This allowed
sufficient time for red blood cells to take up glucose because there was
IOB (Insulin on Board) and I had been physical activity prior to bloods
been taken.

We all know as Type 1 diabetics how fast our BGL can fall when Hypoing but
under normal circumstances my BGL will start rising 5 to 7 minutes after

I will make the point that that time is not the important factor as red
blood cells will take up glucose if there is insulin present and the cells
are requiring energy and this happens in under 10 minutes.

The time delay did cause the discrepancy with my readings. To be sure that Blood Glucose results are correct the blood sample has to be deproteinized.
The point that I am making is that the correlation between meters and blood tests may not occur and did not occur for me because the sample was not deproteinized. There can not be any correlation between the time blood is taken and when it is centrifuged.

I fast once a year for my full blood panel. the rest of the time, I don’t bother.

One time, one of the lab technicians asked me why I didn’t fast for my A1C test.

“You DO know what the A1C measures, don’t you?”

1 Like

@Timbeak48 - That one is just too good. Priceless. I am sure you were very kind in your response. lol

In terms of what @newbeach states - this is not testing related info I was aware of previously. I did a quick google search ('cause google is the best way to get medical advice - lol) but actually looks like a very relevant article came back which would appear to completely support what @newbeach said in their post. The article quickly goes over my head - maybe somebody with a strong BioMed (or whatever) background could look and see if the article is legit? Does this mean as stated in the post that the blood draw sample will have continual BG decrease until it is centrifuged?

To me, this seems like a lot of freaking out over a very small thing.

I’m a Bass Trombonist. Subtlety is REALLY not a thing for us–unless we’re playing bassoon cues!

No freaking but I think it is a valid point if the BG degrades. And trying to compare a meter accuracy using the method that Terry described then one would naturally be assuming the value as reported by the lab is golden.

But if the phlebotomist gets backed up and the samples don’t end up being spun until 4 hours later then are your BG values not exactly gold? Bronze maybe? Or if this is accurate information would it be as simple as asking the Phlebotomist to be sure to spin them right away as you want to match up the values?

Plus just plain old interesting.

I submitted a question to the American Clinical Laboratory Association about my practice of comparing fingersticks tests to the glucose number derived from a professional blood draw. I explained how I use a home blood glucose meter to make treatment decisions and I hoped my comparison with a lab glucose value was a reasonable technique to increase my confidence in my blood glucose meter. I’ll report their answer, if/when they reply.

Tim, I was a Sugar Chemist from a former life, graduating way back in 1975. I have only had Diabetes since October 2008 but I have Brittle Type 1 and been advised by my Endocrinologist that a pump would not help me until it is understood why I have so much trouble stabilizing BGL.

Today I received my latest Blood Test results, HbA1c 6.8%and BG 6.2 (Fasting). Before @ 8.24am Performa 5.7 Optium 5.3; After @ 8.39am Performa 6.2: Intravenous @ 8.40am Performa 6.0 mmol/L.

Tim, you have missed the point, the Blood Sample has to be deproteinized to prevent the blood cell from taking up glucose. Even if the blood cells do not take up glucose because they don’t require it or can not because of the lack of insulin my Performa and Optium may very significantly. This is because Glucose comes in two forms called anomers; α- and β-glucose which mutarotate through the open chain aldehyde form. (Read my post 18th March; Different Meters Different Readings.)

I, myself assumed that all meters would give similar readings from the same drop of blood and Blood Tests glucose would be the best way to make sure that my meter was reading correctly until I saw large discrepancies with readings.
I know from information that I have read that when blood has to be stored for testing the accuracy of different meters that perchloric acid has been used but I don’t know what is used for the special test for Glucose to deproteinize the sample. I know Dentists use TCA (Tri Chloric Acid) to denature protein before filling teeth.

I asked my doctor this morning what would be involved in having a Blood Glucose. It would require more costs and a separate special vial. I know from my past data that the only time I will obtain close correlation between the three readings is when the two meters read the same before and after the blood sample is taken. I can do without the costs of extra blood tests.

1 Like