Understand

A1C and blood sugar

A1C is the average of your blood sugar over roughly the past three months, expressed as a percentage. Below 5.7 % is normal, 5.7–6.4 % is the prediabetes range, and 6.5 % or above meets the ADA's diabetes threshold. It answers a different question than a meter does — and when the two disagree, the disagreement itself is information.

What A1C actually measures

Glucose in the blood sticks to hemoglobin, the oxygen-carrying protein in red blood cells. The more glucose circulating, the more hemoglobin ends up glycated — sugar-coated. A1C is simply the percentage of your hemoglobin in that state. Because a red blood cell lives about 120 days, the test reflects the average glucose over roughly the past three months.

The window is not evenly weighted, though. Younger red blood cells outnumber older ones, so recent weeks count more: roughly half of the A1C value comes from the last 30 days. A good month right before the test genuinely moves the number; a good month three months ago has mostly faded out of it.

The bands

• Below 5.7 % — normal.
• 5.7–6.4 % — prediabetes.
• 6.5 % and above — diabetes range. Like every diagnostic result, this needs confirmation with a second test before it becomes a diagnosis.

For people already managing diabetes, the common treatment target is below 7 %, but that is individual — targets are set looser or tighter depending on age, hypoglycemia risk and other conditions.

eAG: the same number in meter units

Because "6.8 %" is hard to compare with a meter display, labs often also report the estimated average glucose (eAG) in mg/dl. The two are linked by a fixed formula: A1C % = (eAG + 46.7) / 28.7, or the other way around, eAG = 28.7 × A1C − 46.7. Some anchor points:

• A1C 5.7 % ≈ eAG 117 mg/dl
• A1C 6.5 % ≈ eAG 140 mg/dl
• A1C 7.0 % ≈ eAG 154 mg/dl
• A1C 8.0 % ≈ eAG 183 mg/dl

Note what eAG is: an average across day and night, meals and fasting. It is not comparable to any single reading — an eAG of 154 is compatible with mornings at 110 and post-meal peaks at 220. (For mmol/L, divide the mg/dl figure by 18.)

Same A1C, very different days

Two people can both have an A1C of 7.0 %. One runs steadily between 120 and 180 all day. The other swings between 60 and 260 — lows, spikes, and a lot of turbulence that averages out to the same 154 mg/dl. The A1C cannot tell them apart, yet their situations differ: the second person faces hypoglycemia risk and hard post-meal peaks the first does not.

That is the structural blind spot of an average. Daily readings — or a continuous glucose monitor, which reports "time in range" for exactly this reason — show the variability that A1C flattens. The two measurements are complements, not competitors.

What can skew an A1C

A1C assumes red blood cells with a normal 120-day lifespan and normal hemoglobin. When that assumption breaks, the number bends:

• Anemia and blood loss. Iron-deficiency anemia tends to push A1C falsely high; hemolytic anemia, recent bleeding or a transfusion — anything that makes the red cell population younger — pulls it falsely low.
• Kidney disease can distort the result in either direction, and people on dialysis or erythropoietin therapy often read falsely low.
• Hemoglobin variants such as sickle cell trait can interfere with some lab methods. Labs can test for this and switch methods; alternatives such as fructosamine exist for people whose A1C is unreliable.
• Pregnancy changes red cell turnover, which is one reason gestational diabetes is screened with glucose tests rather than A1C.

A1C or the meter: which to trust when they disagree

First rule out the boring explanations. Home meters carry a ±15 % allowed error, technique adds more (how to test without the errors), and timing matters most of all: someone who only tests fasting will see numbers far below an eAG that includes every post-meal peak and every night.

If the gap survives that check — say, consistent fasting readings around 100 next to an A1C of 7.5 % — tell your doctor about it explicitly. Either the readings are missing the high parts of the day, which more testing at varied times will reveal, or one of the conditions above is skewing the A1C, which a lab can investigate. Structured logging settles most of these puzzles: a week of readings before meals, two hours after, and at bedtime, kept with notes (the journal here is built for that), gives your doctor the daily texture the A1C cannot show — and together the two numbers say far more than either one alone.