An A1C test measures your average blood sugar over roughly the past two to three months, based on the typical red blood cell lifespan of about 106 days. The result gives your doctor a single percentage that reflects how well your blood sugar has been controlled over that entire window, rather than a snapshot of one moment in time.
Why It Covers Two to Three Months
The A1C test works because of a simple chemical reaction happening inside your red blood cells. Glucose in your bloodstream naturally sticks to hemoglobin, the protein in red blood cells that carries oxygen. This process, called glycation, isn’t controlled by enzymes. It happens passively, at a rate that depends on how much glucose is floating around. The higher your blood sugar, the more glucose attaches to hemoglobin.
Once glucose binds to hemoglobin, it initially forms an unstable connection. Over hours to days, that bond rearranges into a permanent attachment. The glucose is locked on for the rest of that red blood cell’s life. Since red blood cells live about 106 days before your body recycles them and makes new ones, the A1C test captures a rolling average of blood sugar exposure across that entire lifespan. At any given time, your blood contains a mix of younger and older red blood cells, so the test naturally reflects a weighted average rather than a flat number from a single point in time.
Recent Weeks Matter More
Not all weeks within that two-to-three-month window count equally. Newer red blood cells make up a larger share of what’s circulating at the time of your blood draw, so your blood sugar levels from the past 30 days have a bigger influence on the result than levels from 90 days ago. Some estimates suggest the most recent month accounts for roughly half of the A1C value. This means a sudden improvement or worsening in blood sugar control will start showing up in your A1C relatively quickly, but won’t fully register until a couple of months have passed.
What A1C Numbers Mean
The American Diabetes Association uses these thresholds:
- Below 5.7%: Normal
- 5.7% to 6.4%: Prediabetes
- 6.5% or higher: Diabetes
These percentages can feel abstract, so it helps to convert them to an estimated average glucose (eAG), the number you’d see on a home glucose meter. The conversion formula is: (28.7 × A1C) − 46.7 = eAG in mg/dL. For quick reference, an A1C of 6% translates to an average blood sugar of about 126 mg/dL, a 7% corresponds to roughly 154 mg/dL, and an 8% means an average around 183 mg/dL. Each one-point increase in A1C adds approximately 29 mg/dL to the estimated average.
How Often You’ll Be Tested
Because the test captures a two-to-three-month window, testing more frequently than that doesn’t add much information. The American Diabetes Association recommends testing at least twice a year if your blood sugar is stable and you’re meeting your targets. If your treatment plan recently changed, or you’re not yet at your goal, testing every three months (quarterly) gives your doctor enough data to adjust your approach without overlapping windows.
When the Test Can Be Misleading
Since A1C depends entirely on red blood cell lifespan, anything that changes how long your red blood cells survive will throw off the result, even if your actual blood sugar hasn’t changed.
Conditions that shorten red blood cell lifespan, like hemolytic anemia, significant blood loss, or chronic kidney disease requiring dialysis, cause A1C to read artificially low. Your body is replacing red blood cells faster than normal, so they spend less time accumulating glucose. The percentage drops, but your blood sugar hasn’t actually improved.
Iron deficiency anemia pushes A1C in the opposite direction, making it read higher than your true average. This is particularly relevant during late pregnancy, when iron deficiency is common and can inflate A1C results even in people without diabetes. Starting iron supplements tends to bring A1C back down to where it should be.
Sickle cell disease and other hemoglobin variants also complicate the picture. The abnormal hemoglobin, combined with higher red blood cell turnover and potential transfusion needs, makes A1C unreliable as a long-term marker. In these cases, doctors often turn to alternative tests like fructosamine or glycated albumin, which measure protein glycation over a shorter, two-to-three-week window and aren’t affected by red blood cell turnover.
If you have any of these conditions, your A1C number still provides some information, but it needs to be interpreted alongside other measures rather than taken at face value.