A1C is a blood test that measures your average blood sugar over the past two to three months. Unlike a finger stick or glucose monitor that captures a single moment, A1C reflects the bigger picture of how well your blood sugar has been controlled over time. It’s used both to diagnose diabetes and to track how well treatment is working.
How the A1C Test Works
Hemoglobin is a protein inside red blood cells that carries oxygen. When glucose circulates in your bloodstream, some of it naturally attaches to hemoglobin through a chemical reaction. The more glucose in your blood, the more hemoglobin gets coated. The A1C test measures what percentage of your hemoglobin has glucose stuck to it.
Red blood cells live about 120 days before your body replaces them. Because glucose accumulates on hemoglobin throughout that lifespan, the test captures a rolling average of your blood sugar. It’s not a perfect average, though. About half of what the test reflects comes from the most recent 30 days. Another 40% comes from days 31 through 90, and only about 10% from beyond 90 days. So while A1C is often described as a “three-month average,” it’s weighted heavily toward the last month.
One practical advantage: you don’t need to fast before an A1C test. Your doctor may order other bloodwork at the same visit that requires fasting, but the A1C itself isn’t affected by what you ate that morning.
What the Numbers Mean
A1C results are reported as a percentage. The higher the percentage, the higher your average blood sugar has been. Here’s how the ranges break down:
- Below 5.7%: Normal
- 5.7% to 6.4%: Prediabetes
- 6.5% or higher: Diabetes
These numbers can feel abstract, so it helps to convert them to the kind of blood sugar readings you’d see on a glucose meter. The conversion formula is (28.7 × A1C) − 46.7, which gives you an estimated average glucose in mg/dL. Some common translations:
- A1C of 6%: average blood sugar around 126 mg/dL
- A1C of 7%: around 154 mg/dL
- A1C of 8%: around 183 mg/dL
- A1C of 9%: around 212 mg/dL
- A1C of 10%: around 240 mg/dL
Your lab report may include this estimated average glucose alongside your A1C percentage.
A1C Targets for People With Diabetes
For most non-pregnant adults with diabetes, the American Diabetes Association recommends an A1C below 7%. That target balances the benefits of tighter blood sugar control against the risk of episodes where blood sugar drops too low.
But the ADA is clear that targets should be individualized. Some people can safely aim lower, below 6.5%, if they can get there without frequent low blood sugar episodes. Others benefit from a less strict goal. Older adults with multiple chronic conditions, cognitive impairment, or limited life expectancy often do better with higher A1C targets, because aggressive blood sugar lowering can cause dangerous lows without providing meaningful long-term benefit. The key factor is whether tighter control improves quality of life or risks harming it.
How Often You Should Get Tested
The CDC recommends testing every six months if you’re meeting your blood sugar goals and your treatment plan is stable. If your treatment has recently changed or you’re struggling to reach your target, testing every three months gives you and your doctor faster feedback on whether adjustments are working.
When A1C Results Can Be Misleading
Because A1C depends on the normal behavior of red blood cells, anything that changes how long your red blood cells live or how your hemoglobin is structured can skew results.
Conditions that shorten the lifespan of red blood cells, like hemolytic anemia or recovery from significant blood loss, make A1C read falsely low. Your red blood cells haven’t been around long enough to accumulate glucose at the usual rate, so the test underestimates your actual average blood sugar.
Iron deficiency anemia pushes results the other direction, making A1C read falsely high. This is particularly relevant during late pregnancy, when iron deficiency is common. Studies have shown that iron replacement therapy lowers A1C in both diabetic and non-diabetic individuals, confirming the effect is a testing artifact rather than a real change in blood sugar.
Hemoglobin variants, which are genetic differences in the structure of hemoglobin found in people with sickle cell disease or sickle cell trait, can also interfere. The degree and direction of the error depend on the specific variant and the lab method used, making results unreliable without careful interpretation. People on dialysis for kidney failure also tend to get falsely low A1C readings. In these situations, alternative markers of blood sugar control may give a more accurate picture.
Certain medications, including some opioids and HIV drugs, can also affect results. If you take these, let your doctor know so they can interpret your numbers in context.
A1C vs. Daily Glucose Monitoring
A1C and daily glucose checks answer different questions. A glucose meter or continuous glucose monitor tells you what’s happening right now: whether you need to eat, adjust a dose, or hold off on exercise. A1C tells you how the overall strategy is going. Two people could have the same A1C of 7% but very different daily patterns. One might have relatively steady blood sugars hovering around 154 mg/dL. The other might swing between 80 and 250 throughout the day, averaging out to the same number. Both readings are useful, and neither replaces the other.
Because roughly half of the A1C value reflects the most recent month, changes you make today will start showing up in your next result relatively quickly. If you’ve made a significant shift in diet, activity, or medication, a three-month retest will capture most of that change.