The A1C test is the number your doctor leans on most to judge your blood sugar, and it’s a good one. But it’s an average, not a snapshot, and in some people it reads too high or too low for reasons that have nothing to do with actual glucose. Here’s what the number really captures, what it misses, and when you shouldn’t trust it.
What is A1C actually measuring?
A1C is the fraction of your hemoglobin, the protein inside red blood cells, that has sugar stuck to it. The more glucose floating in your blood over time, the more gets stuck, so the number reflects your average blood sugar over the past two to three months, weighted toward the most recent few weeks. There’s a rough conversion: an A1C of 6.0 percent is an average glucose around 126 mg/dL, 6.5 percent around 140, and 7.0 percent around 154, roughly 29 mg/dL per percentage point (from the ADAG study, Diabetes Care, 2008).1 The diagnostic lines are reported to one decimal place: under 5.7 percent is normal, 5.7 to 6.4 percent is “prediabetes,” and 6.5 percent or higher enters the diabetes range if confirmed on a second test.2
What does it miss?
The big blind spot is everything averaging erases. Two people can both run an A1C of 7.0 percent while one sits steadily around 154 and the other swings from 50 to 280 every day. The number can’t tell them apart. It says nothing about post-meal spikes or low blood sugars. That matters if your glucose is volatile, and it’s why the post-meal spike everyone worries about doesn’t show up here. The conversion to “average glucose” is also looser than it looks: for an A1C of 7 percent, the true average could plausibly fall anywhere from about 123 to 185.1 Treat A1C as a reliable trend line, not a precise readout of any single day.
When does A1C read falsely high or low?
Because A1C depends on red blood cells living their normal three to four months, anything that changes that lifespan skews the number regardless of your real glucose. It reads falsely high when red cells live longer than usual: iron deficiency (the most common culprit, worth about 0.3 to 0.4 percent)3, B12 or folate deficiency, and kidney disease.5 It reads falsely low when cells turn over fast or are being lost: recent blood loss or transfusion, hemolytic anemias, the second half of pregnancy, and starting iron or erythropoietin for anemia.4 Sickle cell trait and other hemoglobin variants can throw it off in either direction depending on the lab method, and someone with two copies of a variant, such as sickle cell disease, can’t get a meaningful A1C at all and needs a different test like fructosamine.4
Decision table: when A1C needs backup
| Situation | How I read it | Better backup test |
|---|---|---|
| A1C under 5.7 percent | Usually reassuring | None, unless symptoms or glucose readings disagree |
| A1C 5.7 to 6.4 percent | A risk flag, not a disease label | Repeat A1C or direct glucose if the result feels surprising |
| A1C 6.5 percent or higher | Diabetes range if confirmed | Confirm with repeat A1C, fasting glucose, or oral glucose tolerance test |
| Iron deficiency, B12 or folate deficiency, kidney disease | Can read falsely high | Fasting glucose, oral glucose tolerance test, or fructosamine |
| Blood loss, transfusion, hemolysis, late pregnancy, some hemoglobin variants | Can read falsely low or uninterpretable | Direct glucose testing or fructosamine |
Does ancestry change how to read it?
It does, and it’s underappreciated. At the exact same measured average glucose, A1C runs about 0.3 percent higher on average in Black individuals than in white individuals, shown with continuous glucose monitors in the GRADE trial6 and a large Kaiser study (2023 to 2024).7 The cause isn’t fully understood, and the differences between individuals dwarf the average gap between groups, so this is not a reason to read anyone’s A1C by their race. But it does mean a single 6.5 percent cutoff can tip some people into a diabetes label they wouldn’t get from a direct glucose test. The 2026 ADA guidance is to not race-adjust the thresholds, and instead to investigate whenever the A1C and the glucose numbers disagree, in anyone.2
Where A1C can mislead
“Normal” on the report is a population threshold, not a guarantee. The prediabetes band (5.7 to 6.4 percent) is a risk state, not a disease: many people in it never progress, and a meaningful share drift back to normal.8 The average-glucose conversion is a population formula with real scatter around it. And if your A1C and your actual glucose readings ever disagree, the A1C is the one more likely to be off. A mismatch is a cue to find out why, not to trust the number by default.
When to ask for backup tests instead
Use A1C for what it’s good at: tracking your blood sugar trend over months. If you land in the prediabetes range, don’t panic. It’s a flag, not a diagnosis, and the highest-yield response is the proven one. Losing around 7 percent of your body weight and moving 150 minutes a week cut progression to diabetes by 58 percent in the landmark prevention trial, more than metformin did.9 Recheck roughly yearly, and if lifestyle isn’t enough to bring a high number down, medications from metformin to GLP-1 drugs are the next step. But if you have iron deficiency, kidney disease, or sickle cell trait, if you’re pregnant, or if your A1C simply doesn’t square with how your glucose actually runs, ask for a fasting glucose or an oral glucose tolerance test instead. The number is a useful average. It’s not the last word.