Comparing your meter readings with your HbA1c

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Cliff

Cliff

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Relationship to Diabetes
Type 2
I've seen a few threads recently where forum members have been trying to compare their HbA1c with their meter readings and wondering why they don't match up that well, or why the HbA1c usually works out a little lower than the average meter reading. I thought I'd put up a separate post on the subject which some might find helpful.

The confusion arises because the HbA1c and your meter are measuring completely different things. The numbers are only close because of the mathematical way their units are expressed.

The HbA1c measures the amount of glycosylated haemoglobin in the A1c fraction of your blood. That result is then expressed as a percentage of total A1c fraction haemoglobin.

Your meter is measuring the amount of glucose in your blood at a particular point in time (most meters then convert that automatically to the amount in your blood plasma so it correlates with the way hospitals labs do it which is to measure glucose in the plasma rather than the blood). Glucose is expressed in units of millimoles per litre (mmol/l) which is completely different from the HbA1c percentage.

There are ways of matching the two up and estimating your HbA1c from your meter average but it's not an exact science because the formulae (and there are many of them) are all empirically derived. That means that they are not devised from rigid scientific laws but rather from looking at thousands of results from real patients and working out a formula that's a best fit with the results.

For those of you that are comfortable with figures, the formula I use is BG = (1.583xHbA1c)-2.52 which if you rearrange gives HbA1c = (BG+2.52)/1.583. It predicted my HbA1c to within 0.1% the last time. It's at it's most accurate when you are averaging BG readings from your meter over the last 90 days with a good spread of fasting and pre- and post-meal readings.

Hope this helps - sorry for the technical stuff.
 
Cliff that is a fantastic explanation well done that has just explained things to me i had no idea about

Thank you very much .
 
Im usless at maths would you be able to work mine out for me?
 
Cliff said:
Loubie - thanks for your PM - you have one in reply.
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Cliff, you'll be docked points if you didn't show your working! (Assuming that the PM was to do with approximating the HbA1c from the average 90 day bg readings?).:D
 
Thanks Cliff - I had no idea what the difference was. However, I am useless at maths so might find it difficult to work out from the average reading on the meter!😱🙂Bev
 
Andy HB said:
Cliff, you'll be docked points if you didn't show your working!
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It's been over 25 years since I've had to show my workings in an examination. Thank goodness I must add. Spreadsheets rule!
 
Cliff said:
I've seen a few threads recently where forum members have been trying to compare their HbA1c with their meter readings and wondering why they don't match up that well, or why the HbA1c usually works out a little lower than the average meter reading. I thought I'd put up a separate post on the subject which some might find helpful.
Click to expand...

Hi there,
the difference is becoming less important now because the World has voted to change the way A1cs are measured and expressed. They are switching from % to mmol.
since June 2009 results have been given in %s with the mmol figure in brackets e.g. 6% ( 42mmol). From 2012, or is it 2011 ?, the % will be dropped and we will just get the mmol reading. All those in the 6% club will have to sign up for the Under 40s club.
It is becasue of the difficulty in calibrating the different assay methods and has the benefit of knocking down the widespread belief that the HbA1c is an average of bgs.
 
Very interesting Cliff - so we should average our BS levels and apply the formula? I assume the more BS tests you do the more accurate the picture but wouldn't you have to have equal numbers of highs and lows for it to be accurate - so perhaps take a fasting, post brekkie, pre and post lunch and pre and post dinner? I amT2 btw.

I am a bit forgetful with taking my levels but I am now checking 6 times a day as described once a week to see what I get when I change a food type. Last week my average BS on the day I checked was 6.5 which if I apply the formula gets me 5.7? That would please me very much!

Whatever the readings, thanks very much for the explanation - my understanding is begin refined every day - shame I didn't know all this when first diagnosed!
 
thedame said:
so we should average our BS levels and apply the formula? I assume the more BS tests you do the more accurate the picture but wouldn't you have to have equal numbers of highs and lows for it to be accurate - so perhaps take a fasting, post brekkie, pre and post lunch and pre and post dinner?
Click to expand...

What you do is average your BG reading over 90 days and then apply the formula. I can't vouch for the accuracy - all I can say is that it got me pretty close last time round on an almost equal mixture of fasting and pre- and post-dinner levels. I also counted in my pre- and post-lunch readings but there were far less of those to put in the mix.

I understand that it doesn't work too well at the low end of the scale, as you approach the top end of non-diabetic levels (I won't open a whole new can of worms by discussing what the top end of non-diabetic is or isn't).
 
Peter C said:
Hi there,
the difference is becoming less important now because the World has voted to change the way A1cs are measured and expressed. They are switching from % to mmol.
since June 2009 results have been given in %s with the mmol figure in brackets e.g. 6% ( 42mmol). From 2012, or is it 2011 ?, the % will be dropped and we will just get the mmol reading. All those in the 6% club will have to sign up for the Under 40s club.
It is becasue of the difficulty in calibrating the different assay methods and has the benefit of knocking down the widespread belief that the HbA1c is an average of bgs.
Click to expand...
You know I read that they were going to give %s as well as the HbA1C but so far I have never been given the percentage. I have a feeling that there will be lots of confused people when they change over.
 
margie said:
You know I read that they were going to give %s as well as the HbA1C but so far I have never been given the percentage. I have a feeling that there will be lots of confused people when they change over.
Click to expand...

I got my latest A1c yesterday and it had both figures on it, but still haven't got my head around the new numbers yet.
 
margie said:
You know I read that they were going to give %s as well as the HbA1C but so far I have never been given the percentage. I have a feeling that there will be lots of confused people when they change over.
Click to expand...

My understanding is that up until 2011, labs will dual report, giving results in % (as they do now) and mmol/l (the new units). After that, we will be exclusively in mmol/l.

To confuse things a little more, in the US they have been considering reporting HbA1c as an Estimated Average Glucose (EAG) so that patients can compare their HbA1c result directly with their meter results, essentially applying the formula I've given above (or something very like it) in reverse.
 
You guys are far too clever for me. I have, however noticed that my average bg from my meter over 90 days is usually about 5 - 10% higher than the HbA1c given by the doctor at my appointment.
By the way Cliff, you did very well reducing your colesterol. What's the secret?🙂
 
Great post, i was about to ask you if you would do my two's and then i saw the 'calculator' and given my daughters A1c of 9.6 at the clinic on Friday (shes was only dx 3 weeks ago) the calculator made it 9.

It was relative of my sons 'meter' reading and his HBA1c was calculated at 7. (he was dx in November, his 2nd clinic due in April)

xxx
 
how do i work out my averages from my meter readings? and which figures do i use, the fasting ones or pre or post meal ones?
 
Cliff said:
What you do is average your BG reading over 90 days and then apply the formula. I can't vouch for the accuracy - all I can say is that it got me pretty close last time round on an almost equal mixture of fasting and pre- and post-dinner levels. I also counted in my pre- and post-lunch readings but there were far less of those to put in the mix.

I understand that it doesn't work too well at the low end of the scale, as you approach the top end of non-diabetic levels (I won't open a whole new can of worms by discussing what the top end of non-diabetic is or isn't).
Click to expand...

Thanks Cliff🙂 - I am going to discuss this with my DSN next visit - she is very open to "new" ideas. I find I sometimes have to draw info from my team- I think they don't want to overburden patients with detail!
 
rawtalent said:
Cliff, you did very well reducing your colesterol. What's the secret?🙂
Click to expand...

No secret really. I got my BG levels under control through a relatively low carb diet and the cholesterol and triglycerides fell nicely into place. I do not avoid saturated fats nor do I go out of my way to eat them either - I just don't worry about them.

carina62 said:
how do i work out my averages from my meter readings? and which figures do i use, the fasting ones or pre or post meal ones?
Click to expand...

Carina - if you have a good spread of fasting and pre- and post- meal readings over the last 90 days, add up all the readings and then divide by the number of readings - that figure is then your average reading.

A word of caution here - please don't rely on this calculation as anything more than a rough and ready guide. I tried it as a bit of fun because I had an HbA1c coming up. I had extensive testing logs and I was interested to see if it would work. It seems to work for me but might not for you.
 
I second Cliff's last post. Don't rely on this average BG to HbA1c calculation, it is just a bit of 'fun'.

Just another thing to muddy the waters even more, I don't think that HbA1c is a true average of the glycated haemoglobin over 3-4 months anyway. It is weighted to the more recent months (e.g. if the HbA1c was taken at the end of April, then April would have contributed 52% to the result, March 27%, February 14.5% and January 6.5%). No doubt, in reality, it is even more complicated than that. :D

The bottom line is, look after your spot BG results and leave the HbA1c to look after itself! 🙂
 
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