Estimated A1C vs lab A1C: why they differ

The short version

A1C is a lab test that reflects your average blood sugar over the past 2 to 3 months.
An estimated A1C is worked out from your meter or CGM readings with a formula. It's a useful preview, not a lab result.
The two often differ by a few tenths of a percent, sometimes more. Both are normal.
For many adults with diabetes, the A1C goal is under 7%. Your doctor may set a different one.

What A1C actually measures

Glucose in your blood sticks to haemoglobin, the protein inside red blood cells. The more glucose, the more sticks. Red blood cells live about 3 months, so the share of "sugar-coated" haemoglobin, your A1C, reflects your average blood sugar over that time, with the most recent weeks counting the most.

It's measured in a lab or with a finger-prick test at your clinic, usually every 3 to 6 months.

What an estimated A1C is

If you have weeks of readings from a meter or CGM, you can work out your average blood sugar and turn it into an A1C-style number. The most common formula comes from the ADAG study, published in 2008:

Estimated A1C (%) = (average blood sugar in mg/dL + 46.7) ÷ 28.7For mmol/L, multiply your average by 18 first.

Another version, called GMI (glucose management indicator), is used in CGM reports. It gives similar numbers in the middle of the range and slightly different ones at the ends.

Average blood sugar and A1C, side by side

A1C	Average blood sugar (mg/dL)	Average blood sugar (mmol/L)
5%	97	5.4
6%	126	7.0
7%	154	8.6
8%	183	10.2
9%	212	11.8
10%	240	13.4

From the ADAG formula. Real people with the same average can have lab A1Cs a few tenths apart.

Why your estimate and your lab A1C don't match

Not enough readings. Three finger-pricks a day before meals miss the after-meal highs and the overnight lows. The average looks better or worse than it is.
Different time windows. An estimate from the last 2 weeks won't match a lab test that reflects 3 months.
Your red blood cells. Anaemia, iron deficiency, recent blood loss or transfusion, pregnancy, kidney disease and some haemoglobin variants can make the lab A1C read higher or lower than your real average.
People simply differ. How fast glucose sticks to haemoglobin varies a little from person to person.

If your estimate and your lab A1C are always far apart, mention it to your doctor. It can be useful information in itself.

What A1C can't tell you

A1C is an average, and averages hide swings. Someone bouncing between lows and highs can have the same A1C as someone steady all day. That's why many doctors now look at time in range alongside A1C.

Common goals

Many adults with diabetes: under 7% (53 mmol/mol).
Some people, if it's safe without lots of lows: lower, such as under 6.5%.
Older adults or people with frequent lows: often a looser goal, such as under 8%.
Diagnosis: 6.5% or above on a lab test is one of the criteria for diabetes; 5.7% to 6.4% is the prediabetes range.

Your goal is something to set with your care team.

Using an estimate well

Treat the estimate as a trend line. If it's moving down over a few weeks, what you're doing is probably working, and you'll likely see it at your next lab test. If it's creeping up, look at your after-meal readings and your daily pattern for where the highs come from.

In T1dApp, Insights shows an estimated A1C from your readings using the ADAG formula above, next to your time in range and average. It's an estimate to follow between appointments, not a replacement for your lab test.

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Sources

This article is general information, not medical advice. Your targets, doses and diet should be set with your doctor or diabetes care team.