HbA1c measures the percentage of your hemoglobin, the oxygen-carrying protein in red blood cells, that has glucose permanently attached to it. Unlike a finger prick or fasting blood test that captures your blood sugar at a single moment, HbA1c reflects your average blood sugar over the past two to three months. That longer window makes it one of the most useful tools for diagnosing diabetes, identifying prediabetes, and tracking how well blood sugar is being managed over time.
How Glucose Attaches to Hemoglobin
Glucose in your bloodstream naturally sticks to hemoglobin through a two-step chemical reaction. First, glucose latches onto the end of hemoglobin’s beta chain, forming an unstable bond. That bond then rearranges into a permanent, stable connection. This process is slow, non-enzymatic (meaning your body doesn’t actively control it), and irreversible. Once glucose is locked onto a hemoglobin molecule, it stays there for the life of that red blood cell.
The higher your blood sugar runs, the more hemoglobin gets coated with glucose. That’s a straightforward relationship: more sugar circulating means a higher percentage of hemoglobin ends up glycated. Because the reaction follows predictable patterns tied to glucose concentration, the final HbA1c number serves as a reliable proxy for what your blood sugar has been doing over weeks, not just hours.
Why It Reflects Two to Three Months
Red blood cells don’t last forever. They circulate for roughly 70 to 120 days before your body breaks them down and replaces them. At any given time, your blood contains a mix of brand-new red blood cells (with very little glucose attached) and older ones (with more glucose accumulated). Your HbA1c result is essentially a weighted average across all of those cells, which is why the test captures a window of about two to three months.
Research on patients with very high starting HbA1c values found that it takes approximately 70 days for HbA1c to drop by 90% after blood sugar improves. That timeline aligns with the red blood cell turnover cycle and explains why you won’t see a dramatic change in your HbA1c overnight, even if you make significant dietary or medication changes. The number moves gradually as old, heavily glycated cells are replaced by newer ones exposed to lower sugar levels.
What the Numbers Mean
In the United States, HbA1c is reported as a percentage. The diagnostic cutoffs, based on American Diabetes Association standards, break down like this:
- Below 5.7%: Normal blood sugar regulation
- 5.7% to 6.4%: Prediabetes, meaning blood sugar is higher than normal but not yet in the diabetes range
- 6.5% or higher: Diabetes, when confirmed by a second abnormal test result
A single HbA1c of 6.5% or above doesn’t automatically lock in a diabetes diagnosis. In the absence of obvious symptoms like extreme thirst, frequent urination, or unexplained weight loss, guidelines call for two abnormal results. These can come from two separate HbA1c tests or from an HbA1c paired with a different test, such as fasting blood glucose.
Translating HbA1c to Daily Blood Sugar
If you check your blood sugar at home with a glucose meter, HbA1c percentages can feel abstract. The estimated average glucose (eAG) conversion bridges that gap using the formula: (28.7 × HbA1c) − 46.7 = eAG in mg/dL. Here are some common reference points:
- HbA1c 6%: average blood sugar of about 126 mg/dL
- HbA1c 7%: about 154 mg/dL
- HbA1c 8%: about 183 mg/dL
- HbA1c 9%: about 212 mg/dL
- HbA1c 10%: about 240 mg/dL
These are averages, so they don’t tell you about highs and lows throughout the day. Someone with an HbA1c of 7% might have steady blood sugar hovering around 154 mg/dL, or they might swing between 80 and 250 mg/dL and land at the same average. That’s one limitation of the test: it can’t distinguish between stable control and wild fluctuations.
How Often to Get Tested
Testing frequency depends on how stable your blood sugar is. If you have diabetes and your levels are well controlled and meeting treatment goals, testing every six months is the standard recommendation. If your treatment plan is changing, if a new medication has been added, or if your blood sugar isn’t well controlled, testing every three months is appropriate. That three-month interval aligns neatly with the red blood cell lifespan, giving enough time for meaningful changes to show up in the result.
For people without diabetes, HbA1c is typically part of routine screening, especially after age 35 or 45, or earlier if other risk factors are present.
No Fasting Required
One practical advantage of HbA1c over fasting glucose tests is that you don’t need to skip meals beforehand. Because the test measures glucose that has accumulated on hemoglobin over months, what you ate for breakfast that morning doesn’t change the result. That said, if your doctor orders other bloodwork at the same time, like a cholesterol panel, you may still be asked to fast for those additional tests.
When Results Can Be Misleading
HbA1c depends on two things: your blood sugar levels and how long your red blood cells survive. Anything that changes red blood cell lifespan can skew the number independently of actual blood sugar control.
Conditions that shorten red blood cell life, such as sickle cell disease (HbSS), hemoglobin C disease, certain anemias, significant kidney disease, and liver failure, can produce falsely low HbA1c results. When red blood cells are destroyed faster than normal, they have less time to accumulate glucose, so the percentage drops even if blood sugar is high. Conversely, conditions that extend red blood cell survival, like iron-deficiency anemia before treatment, can push HbA1c artificially higher.
Hemoglobin variants also matter. There are hundreds of known variants, but the most common are hemoglobin S, E, C, and D. Depending on the lab method used, these variants can cause falsely high or low readings. Some testing methods have been updated to handle common variants accurately, but not all labs use the same approach. If you carry a hemoglobin variant, your doctor may choose alternative tests like fructosamine or continuous glucose monitoring to get a clearer picture.
Certain medications can also interfere. Opioids and some HIV medications have been identified as potential sources of falsely increased or decreased results.
Percentage vs. mmol/mol
Outside the United States, you may see HbA1c reported in mmol/mol instead of a percentage. The percentage system (called NGSP) is standard in the U.S. and ties directly to the landmark clinical trials that established diabetes treatment targets. The mmol/mol system (called IFCC) is used in many other countries and is considered more analytically precise. The two scales are related by a conversion formula, so a result of 6.5% equals 48 mmol/mol, and 7% equals 53 mmol/mol. International guidelines recommend reporting both, but in practice each country tends to use whichever system its clinicians are accustomed to.