Your meter shows you a moment. Your lab report shows you a season. HbA1c is the number that stretches across about three months, and it answers a different question than your fingerstick does. Here is what it measures, where the familiar ranges come from, and why it never quite matches your daily readings. This is general information from the sources listed at the end, not advice for your situation.
What HbA1c actually measures
Glucose in your blood attaches to hemoglobin, the protein in red blood cells. HbA1c, often written A1C, is the percentage of hemoglobin that has glucose attached, which is why it is also called glycated hemoglobin [1] [2]. Because it counts the glucose that has attached to those cells rather than the glucose in your blood right now, it reflects average blood glucose rather than a single moment, and the result is reported as a percentage [2].
Why three months?
The test measures your average blood sugar over the past 3 months, which ties to red blood cell turnover, roughly every 3 months [3]. Another way it is put: a picture of your average over the past 2 to 3 months [1]. Either way, one result is a snapshot of a longer stretch of time, not of any particular day. A rough way to picture it: the lab result is the season, and your meter readings are the daily weather.
Where the familiar ranges come from
For diagnosis, the published ranges list an A1C below 5.7% as normal, 5.7% to 6.4% as prediabetes, and 6.5% or above as diabetes [2] [3]. A diagnosis generally needs a second measurement to confirm it, unless there are clear symptoms [2]. These ranges are what health professionals use to diagnose; what they mean for you, and any personal target, is a conversation for your care team.
From percent to mg/dL: estimated average glucose
Meters and sensors speak mg/dL or mmol/L, while A1C speaks in percent. To translate, an A1C can be converted into an estimated average glucose, or eAG. The eAG is calculated from your A1C [2], and a published conversion table puts an A1C of 6% at an eAG of 126 mg/dL, 7% at 154 mg/dL and 8% at 183 mg/dL [1].
The eAG number will not match your daily glucose readings, because it is a long-term average [2]. It also may not account for spikes or lows [3].
GMI: the A1C-style number from a sensor
If you wear a continuous glucose monitor (CGM), you may see a number called GMI, the glucose management indicator. It is an A1C-style percentage worked out from your average sensor glucose, using a formula from a 2018 paper: GMI (%) = 3.31 + 0.02392 × mean glucose in mg/dL [4]. As an example, an average sensor glucose of 154 mg/dL works out to a GMI of about 7.0%. The published eAG table uses a different conversion, so the two line up around 7% but not everywhere: 126 mg/dL is an eAG of 6% [1] but a GMI of about 6.3% [4].
The paper recommends using at least 10, and preferably 14 or more, days of CGM data [4]. It also explains that a GMI and a lab A1C can differ, and among the reasons it gives is that each person’s red blood cells may live for a slightly different number of days [4].
Same average, very different days
A meter or CGM shows your level at a given moment. A1C gives the longer-term view that helps you and your care team judge whether a treatment plan is working [1]. The A1C test does not replace home monitoring [3].
Because an A1C is an average, the same result can come from steady readings or from readings that swing between highs and lows. The international consensus on CGM targets makes the same point: A1C does not show how large or how frequent the swings in glucose are within a day or between days [5]. Daily readings show those swings; the A1C does not.
When the result may not tell the whole story
Several things can falsely raise or lower an A1C result, including severe anemia, kidney failure, liver disease, blood disorders such as sickle cell anemia or thalassemia, certain medicines, blood loss or transfusions, and early or late pregnancy [3]. A hemoglobin variant can also make an A1C test unreliable for diagnosing or monitoring diabetes and prediabetes, though not every test is affected [2]. If any of this might apply to you, ask your doctor how to interpret your results.
How often it gets checked
Experts recommend that people with diabetes have an A1C test at least twice a year [2]. How often is right for you depends on your situation, so your care team sets that.
Sources
- American Diabetes Association, A1C.
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), The A1C Test & Diabetes.
- Centers for Disease Control and Prevention, A1C Test for Diabetes and Prediabetes.
- Bergenstal RM, et al. Glucose Management Indicator (GMI): A New Term for Estimating A1C From Continuous Glucose Monitoring. Diabetes Care, 2018;41(11):2275–2280.
- Battelino T, et al. Clinical Targets for Continuous Glucose Monitoring Data Interpretation: Recommendations From the International Consensus on Time in Range. Diabetes Care, 2019;42(8):1593–1603.
We made Steadion, an iPhone tracker for people with diabetes or prediabetes. If you log HbA1c lab results in it, you can watch progress toward the goal you set, next to your glucose readings and time in range. Learn more at steadion.fit.