# Comparing your meter readings with your HbA1c



## Cliff (Mar 10, 2010)

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.


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## Steff (Mar 10, 2010)

Cliff that is a fantastic explanation well done that has just explained things to me i had no idea about 

Thank you very much .


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## smile4loubie (Mar 10, 2010)

Im usless at maths would you be able to work mine out for me?


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## Cliff (Mar 10, 2010)

smile4loubie said:


> Im usless at maths would you be able to work mine out for me?



Loubie - thanks for your PM - you have one in reply.


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## Andy HB (Mar 10, 2010)

Cliff said:


> Loubie - thanks for your PM - you have one in reply.



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?).


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## bev (Mar 10, 2010)

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


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## Cliff (Mar 10, 2010)

Andy HB said:


> Cliff, you'll be docked points if you didn't show your working!



It's been over 25 years since I've had to show my workings in an examination.  Thank goodness I must add.  Spreadsheets rule!


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## Peter C (Mar 10, 2010)

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.



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.


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## thedame (Mar 10, 2010)

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!


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## Cliff (Mar 10, 2010)

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?



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).


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## margie (Mar 10, 2010)

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.


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.


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## sofaraway (Mar 10, 2010)

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.



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.


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## Cliff (Mar 10, 2010)

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.



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.


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## squidge63 (Mar 10, 2010)

I know this is from another diabetic site but I stumbled across it yesterday and it has an HbA1c to mmol/L converter

http://www.diabetes.co.uk/hba1c-to-blood-sugar-level-converter.html

made more sense when I put my last a1c in and converted it...


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## rawtalent (Mar 10, 2010)

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?


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## jimmysmum (Mar 10, 2010)

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


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## Carina1962 (Mar 10, 2010)

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?


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## thedame (Mar 11, 2010)

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).



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!


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## Cliff (Mar 11, 2010)

rawtalent said:


> Cliff, you did very well reducing your colesterol. What's the secret?



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?



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.


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## Andy HB (Mar 11, 2010)

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. 

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


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## Cliff (Mar 11, 2010)

Andy HB said:


> 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.



Andy - You're absolutely right here - there seems a broad agreement that HbA1c is weighted towards the most recent results although there's some disagreement about the exact extent.  However, as the various formulae are derived empirically, it should take that weighting into account.


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## Peter C (Mar 11, 2010)

Andy HB said:


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



The hba1c is reckoned to be weighted towards the last ten days. So some dmers try cheat the thing by going low carb in the run  up to the test. they are only fooling themselves of course.
it is complicated, the blood cells involved last up to 90 days ( some of them) so the a1c will reflect values that far back. But glycosulated haemoglobin gets "unglycosulated" and "re-glycosulated" in the course of the 90 days, so its a merry dance anyway.
The HbA1c is a comparatively recent tool ( early 1980s in general practice). The great UKPDS used it as the yardstick in the 1990s research, finding that higher A1cs were definitely associated with more complications. So the emphasis in Britain came to focus on management ( of T2s in particluar) through management of the A1c. This has deflected attention away from SMBG ( particularly for t2s again). The medical establishemnt can't seem to grasp that effective SMBg is the best route to good HbA1cs.


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## Carina1962 (Mar 11, 2010)

Thanks Cliff for your reply.  My partner keeps telling me to wait until June when i go for my 2nd HbAiC but i'm getting curious and want to have a rough idea of what it will be.  For my first HbA1C i bought a home test kit which you send off and my result from them came at 6.4% but my actual doctor's one came at 6.7% so a little difference.  I will try what you suggest and see but i will not take it as 'gospel' - thanks very much, very interesting thread by the way


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## Cliff (Mar 11, 2010)

Peter C said:


> But glycosulated haemoglobin gets "unglycosulated" and "re-glycosulated" in the course of the 90 days, so its a merry dance anyway.



Peter - that's a really interesting point.  The scientific literature tends to describe haemoglobin glycosylation as an irreversible or nearly irreversible reaction but I have found one paper that suggests that HbA1c can reduce very rapidly in insulin therapy stabilisation of newly diagnosed T1 children.  I need to do more reading!


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## falcon123 (Mar 11, 2010)

To muddy the waters some places are using a new measure. My most recent result was bad but 75 (mmol/mol) is more scary than 9.0 when first heard! Is 75 in old money even possible?


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## squidge63 (Mar 11, 2010)

Talking of A1c results my current diabetic clinic I have to go and get blood taken about 2 weeks before my dr's appointment.. at my last diabetic clinic they did a finger prick test which they put in a machine and hey presto my A1c result..


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## Peter C (Mar 11, 2010)

carina62 said:


> For my first HbA1C i bought a home test kit which you send off and my result from them came at 6.4% but my actual doctor's one came at 6.7% so a little difference.  )



hi there. despite the fact that all A1cs are supposed to be calibrated to the standard set by the DCCT in the USA, different labs
 use different assay methods and there can be differences in results. About seven years ago I scored 6.9 and was disappointed, the Doc at the Diabetes Centre reassured me - I had gone to a drop in centre for the test instead of the usual hospital clinic. The Doc said their lab ranges were different, the 6.9 on the drop in centre would have been something like 6.4 on the hosp[tal method. Moral of the story - stick to ONE place for a1c tests for consistency.
The difficulties in calibrating different assay methods and different lab ranges is one of the reasons why they are switching to reporting in mmol/mol after June 2011.
if you still have the self testing kit - what was the lab range on that ? and how does that compare to your hDoc's lab range ?


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## C*5_Dodger (Mar 11, 2010)

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.



Dear Peter,

Just a minor point - they are switching from % to mmol/mol

Warmest Regards   dodger


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## lyndasw (Mar 11, 2010)

carina62 said:


> Thanks Cliff for your reply.  My partner keeps telling me to wait until June when i go for my 2nd HbAiC but i'm getting curious and want to have a rough idea of what it will be.  For my first HbA1C i bought a home test kit which you send off and my result from them came at 6.4% but my actual doctor's one came at 6.7% so a little difference.  I will try what you suggest and see but i will not take it as 'gospel' - thanks very much, very interesting thread by the way



Carina - can I ask where you bought your home test kit from and how much they are?


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## Carina1962 (Mar 14, 2010)

hello Lyndasw, sorry only just saw your post.  I will go and get the box and let you know


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## Carina1962 (Mar 14, 2010)

Lyndasw: the company is called GlucoMen Aic and it cost around ?24.  I ordered it over the phone and you have to send a blood sample and they post out the results to you.  I'll be buying it again at beg of April to give me an indication for when i go for my proper one in June.  The website is www.glucomenA1c.com

Let me know how you get on if you decide to have a go


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## lyndasw (Mar 15, 2010)

carina62 said:


> Lyndasw: the company is called GlucoMen Aic and it cost around ?24.  I ordered it over the phone and you have to send a blood sample and they post out the results to you.  I'll be buying it again at beg of April to give me an indication for when i go for my proper one in June.  The website is www.glucomenA1c.com
> 
> Let me know how you get on if you decide to have a go



Thanks Carina.  I have had a mix up with my appointments so I may try this if I have to wait too long for a further test.  At least I will have an idea of what is going on.


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