The A1C blood test measures your average blood sugar level over the past two to three months. Unlike a finger-prick glucose test that captures a single moment, the A1C gives a broader picture of how well your body has been managing blood sugar over time. It’s one of the primary tools used to screen for prediabetes, diagnose type 2 diabetes, and monitor how well diabetes treatment is working.
How the A1C Test Works
Your red blood cells contain a protein called hemoglobin, which carries oxygen through your bloodstream. Glucose naturally sticks to hemoglobin as it circulates, and the more sugar in your blood, the more hemoglobin gets coated. The A1C test measures the percentage of your red blood cells that have this glucose coating.
The reason the test reflects a two-to-three-month window is simple: red blood cells live about 120 days. Once glucose attaches to hemoglobin, it stays there for the life of that cell. So at any given time, your blood contains red blood cells of various ages, and the glucose stuck to them serves as a rolling record of your blood sugar levels. More recent weeks carry slightly more weight in the result than earlier ones, since newer red blood cells make up a larger share of the total.
What the Numbers Mean
A1C results are reported as a percentage. The standard thresholds break down like this:
- Below 5.7%: Normal blood sugar regulation
- 5.7% to 6.4%: Prediabetes, meaning blood sugar is elevated but not yet in the diabetes range
- 6.5% or higher: Diabetes, typically confirmed with a second test
For people already diagnosed with diabetes, the general target is an A1C below 7%, though your target may differ depending on your age, health, and how long you’ve had diabetes.
Translating A1C to Daily Blood Sugar
If you’re used to seeing blood sugar numbers from a glucose meter, A1C percentages can feel abstract. There’s a direct conversion: multiply your A1C by 28.7 and subtract 46.7 to get your estimated average glucose (eAG) in mg/dL. In practical terms, an A1C of 6% corresponds to an average blood sugar of about 126 mg/dL. At 7%, it’s roughly 154 mg/dL. An A1C of 9% means your blood sugar has been averaging around 212 mg/dL. Each full percentage point on the A1C scale represents roughly a 29 mg/dL increase in average blood sugar.
What You Need to Do Before the Test
Nothing. You don’t need to fast before an A1C test, skip meals, or change your medication timing. That’s one of its biggest practical advantages over fasting glucose tests. You can eat normally beforehand and get the test at any time of day. If your doctor orders other bloodwork alongside it, like a cholesterol panel, those tests may still require fasting, so check ahead of time.
How Often You Should Get Tested
The CDC recommends testing every six months if you have diabetes and are meeting your blood sugar and treatment goals. If your treatment has recently changed or you’re struggling to keep blood sugar in range, every three months is more appropriate. For people without a diabetes diagnosis, an A1C test is a standard part of routine screening, especially after age 45 or if you have risk factors like excess weight, a family history of diabetes, or a history of gestational diabetes.
When A1C Results Can Be Misleading
Because the test depends on the normal behavior of red blood cells, anything that changes how long those cells survive or how hemoglobin behaves can throw off the results. This matters more than most people realize, and a misleading A1C can lead to missed diagnoses or unnecessary alarm.
Conditions that shorten the lifespan of red blood cells, like hemolytic anemia or recovery from significant blood loss, cause red blood cells to turn over faster. That means glucose has less time to accumulate on hemoglobin, and the A1C reads falsely low. Iron deficiency anemia has the opposite effect: it’s associated with higher A1C readings, and treating the iron deficiency with supplements tends to bring the A1C back down even when blood sugar itself hasn’t changed.
Genetic hemoglobin variants, including sickle cell trait, can also interfere with the accuracy of certain testing methods. People with chronic kidney failure, particularly those on dialysis, often get A1C results that underestimate their true blood sugar levels. In these cases, alternative markers like glycated albumin may give a more reliable picture. Certain medications, including opioids and some HIV drugs, can also skew results in either direction.
A1C During Pregnancy
The A1C test is not recommended for diagnosing gestational diabetes. Pregnancy changes how quickly red blood cells are produced and destroyed, and it increases blood volume through a process called hemodilution. Both of these shifts cause the A1C to underestimate actual blood sugar, particularly in the second and third trimesters. Women with lower hemoglobin levels during pregnancy are even more likely to get falsely reassuring results.
An oral glucose tolerance test remains the standard for diagnosing gestational diabetes. The A1C does have a role early in pregnancy for screening for pre-existing diabetes, and it can be used alongside daily glucose monitoring to track overall control in women who are already managing diabetes during pregnancy.
What A1C Can and Cannot Tell You
The A1C is excellent at revealing patterns. If your blood sugar has been running high for weeks, the A1C will reflect that even if your glucose happened to be normal on the day of the test. It’s far harder to “game” than a single fasting glucose measurement, which makes it a reliable long-term indicator.
What it cannot do is tell you about day-to-day variability. Two people can have the same A1C of 7% while experiencing very different glucose patterns. One might have steady blood sugar hovering around 154 mg/dL. The other might swing between dangerous lows and highs that happen to average out to the same number. The A1C won’t distinguish between those two situations. It also can’t detect episodes of low blood sugar, which is why people on insulin or certain medications still need to monitor glucose daily in addition to periodic A1C checks.
For most people, the A1C is the single most useful number for understanding blood sugar control over time. But it works best as one piece of a larger picture, interpreted alongside daily glucose readings, symptoms, and any conditions that might affect red blood cell turnover.