A1C test results - calculating your RBC lifespan

A person’s A1C can be influenced by how long their red blood cells (RBC) last. A person whose RBC’s live longer will have a comparatively higher A1C, and a person whose RBC’s live shorter will have a comparatively lower A1C.

The life-span of the RBC is difficult to estimate clinically, but here is a way to estimate how long yours last compared to the average range of 100–120 days.

A reticulocyte is an immature red blood cell. They are produced in the bone marrow and released into circulation as reticulocytes, and in a few days they transform into fully mature red blood cells.

A reticulocyte count is a test that measures the level of reticulocytes in your blood.

You can use a reticulocyte count to estimate how long your red blood cells last.

A normal blood panel already gives you hematocrit (the ratio of the volume of red blood cells to the total volume of blood). So for this estimation, the only extra test you need is a reticulocyte count. (https://labtestsonline.org/tests/reticulocytes)

This is just an estimation, and blood chemistry calculations aren’t perfect. But it may give you some insight into your personal red blood cell lifespan and you can consider if it may be affecting your A1C.


Here is the formula for estimating how long your RBC’s are surviving:

RBC survival in days = 100 / (reticulocyte count (in percent) / reticulocyte life span)

You can get the reticulocyte life span from a corrected reticulocyte count table, such as shown here:


Example 1:
If your hematocrit is 45, the table referenced above gives a reticulocyte life span of 1.0. If your reticulocyte count is 0.7%, the formula gives:
RBC survival = 100 / (0.7 / 1.0) = 142.8 days

Example 2:
If your hematocrit is 28, the table referenced above gives a reticulocyte life span of 1.5. If your reticulocyte count is 1.8%, the formula gives:
RBC survival = 100 / (1.8 / 1.5) = 83.3 days

Since RBC’s are generally calculated to live 100-120 days, in example 1 the person might have an A1C that is slightly higher than what would be expected for their level of control. And in example 2, the person might have a lower A1C for their level of control.

Hope this isn’t too confusing.

(There are other ways of estimating RBC lifespan. One method involves measuring the amount of carbon monoxide in exhaled air. Another method analyzes the decay rate of Thiazole Orange stain (REticulocyte-Based-Estimation of Lifespan - REBEL), but these methods involve much more than most people would have time or ability to perform without a serious need for it. Just mentioning them for completeness.)


This is really fascinating – I’m wondering if our pediatric endo will bite though – it’s just an unecessary test in her opinion. Although it would really help us understand if that’s the reason our kiddo’s A1C is about 1 unit higher than his Dexcom data suggests – or if Dexcom exaggerating the severity of lows is the real culprit.

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I don’t think Dexcom exaggerating the lows would affect it. A low does not directly affect A1C, only a high or the absence of a high.

I think the more likely cause of a Dexcom estimate being lower would be if it didn’t pick the high up as quickly, or it never gets up to the full high reading.

For example, if my BG is spiking, I try to knock it down quickly. Almost always, my BG is on its way down before the Dexcom ever picks up the highest number my BG reached before correcting. So my Dexcom estimates would always be lower than my actual A1C.

Kind of like this:

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well but if Dexcom says that my son is LOW for 2 hours and we test him and he’s never below 60, that goes into the average blood sugar… so why would that not artificially lower the estimated average blood sugar – which is what I’m assuming they’re using to calculate the A1C? Also we have tested him when he’s high, and most of the time he’s actually lower than Dexcom suggests. It’s possible there are a bunch of highs that just never show up at all, but we don’t typically see that.

I guess what may not be clear is that Dexcom can sometimes read very low for us hours, especially at night, even if he’s not low at all. I am not sure if those are compression lows or what, but having 2 hours at 40 mg/DL is likely to lower the average. I do get that mathematically, a high can be MUCH higher than the average (assume, say 140 mg/dL), so even if you’re, say 30 minutes at 300 or above, that’s numerically a much bigger impact on BG average than spending 2 hours at 40 or below…it’s just we really don’t see the highs being that much higher when we test. Maybe we really are missing some phantom peaks because we are always testing too late though…something to consider for sure.

That’s not how A1C is determined. A1C does not measure average BG, but glycated hemoglobin. The A1C is only a measure of highs or the absence of highs, not lows.

Or are you not taking about how the A1C is calculated, but rather how Dexcom estimates it?

@Chris posted a good description of A1C calculations a while back. I couldn’t find it, and am too lazy to write it up since Chris did a great job last time. Maybe Chris can locate it for us?

I have made a few attempts at it. Here is one. Is this the one you were thinking about?

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Yes! Wonderful! No need to re-write it, yours was a great explanation.

yes I’m talking about how Dexcom estimates it.

Yeah, I don’t know how they do theirs. It is always estimated lower than mine too.