A1C and fasting glucose both answer the question "is my blood sugar too high?", but over very different stretches of time. Fasting glucose is a single reading taken after a night without food. A1C is an average of roughly the last two to three months, recorded in the sugar coating on your red blood cells. The American Diabetes Association accepts either for diagnosis, and when they disagree, the reason is usually something about your red cells or your mornings.
What is the difference between A1C and fasting glucose?
Fasting glucose is the concentration of glucose in your blood plasma after at least eight hours without food or caloric drinks. It reflects what your liver and insulin are doing overnight, on that particular morning. A1C (also called HbA1c or glycated hemoglobin) is the percentage of hemoglobin, the oxygen-carrying protein in red blood cells, that has glucose permanently attached. Red cells live about three months, and the higher your average glucose over that time, the more hemoglobin gets coated. 1
The two answer different questions. Fasting glucose asks how your body handles sugar at its quietest point of the day. A1C asks how high your sugar has run on average, after-meal peaks included. Each marker has its own page: hemoglobin A1c and fasting glucose.
Glycated hemoglobin was similarly associated with a risk of diabetes and more strongly associated with risks of cardiovascular disease and death from any cause as compared with fasting glucose.Selvin E, et al. New England Journal of Medicine 2010, ARIC study of 11,092 adults 4
The cutoffs side by side
| Category | A1C | Fasting glucose |
|---|---|---|
| Normal | Below 5.7% | Below 100 mg/dL (5.6 mmol/L) |
| Prediabetes | 5.7 to 6.4% | 100 to 125 mg/dL (5.6 to 6.9 mmol/L) |
| Diabetes | 6.5% or higher | 126 mg/dL (7.0 mmol/L) or higher |
These are the American Diabetes Association criteria. A diagnosis of diabetes needs two abnormal results, either the same test on two occasions or two different tests from the same sample, unless blood sugar is clearly very high with classic symptoms. 1 The prediabetes and type 2 diabetes guides cover what each range means.
Why A1C is the steadier number
A1C averages out daily swings, so it moves less between tests. In a national survey sample retested about two weeks apart, the within-person variation was 5.7% for fasting glucose and 3.6% for A1C. 3 In practice, a fasting glucose of 126 mg/dL or higher on one day fails to repeat about 3 times in 10.
A1C also translates into an estimated average glucose. The ADAG study, which compared A1C with continuous glucose monitoring in 507 people, found an A1C of 7% matches an average glucose of about 154 mg/dL, and 6% about 126 mg/dL. 2 The tool below converts your own number.
What your A1c means in daily glucose
Your HbA1c, translated to an average glucose. Nothing leaves your browser.
What throws off a fasting glucose
Fasting glucose is the more direct measurement, but it is easy to disturb. The ADA criteria assume a true fast of at least eight hours, 1 and a single reading carries whatever happened the night before into the result.
- A short or broken fast: coffee with sugar or milk, juice, or a late snack counts as food.
- Acute illness, surgery or severe stress, which raise glucose through stress hormones.
- Steroid medicines, which raise glucose for as long as they are taken and sometimes after.
- A poor night's sleep and the early-morning rise in hormones that lift glucose before waking.
- Sample handling: glucose keeps falling in a blood tube left unspun, so a delayed sample can read low.
None of these touch an A1C, which is one reason the two numbers can tell different stories about the same week. Equally, nothing about your red cells changes a fasting glucose, which is why it is the fallback when A1C is unreliable.
When A1C and fasting glucose disagree
Split results are common. In US national survey data, A1C criteria identified about one-third as many people with undiagnosed diabetes as glucose criteria did, and about one-tenth as many at high risk (defined in that study as an A1C of 6.0 to 6.4%). 5 The two tests pick up overlapping but different groups of people, and each has blind spots.
A1C high, fasting glucose normal
Sometimes this reflects blood sugar that is normal overnight but rises after meals, which A1C captures and a fasting test misses. Sometimes it reflects the red cells instead. A systematic review found that iron deficiency, with or without anemia, tends to push A1C up without any rise in glucose. 6 Older red cells collect more glucose, and iron deficiency slows the making of new ones. If your ferritin is low, read a borderline A1C with that in mind; iron deficiency covers the iron side.
A1C normal, fasting glucose high
Some people run high in the morning but normal for the rest of the day, often an early sign of insulin resistance in the liver. A lower-than-expected A1C can also come from red cells being replaced faster than usual: after blood loss or a transfusion, with hemolysis, in pregnancy, or with some hemoglobin variants. 1 In a study of 4,620 African American adults, those with sickle cell trait had an A1C about 0.3 percentage points lower than those without it at the same fasting glucose. 7 The fasting insulin page and the HOMA-IR calculator cover the insulin side.
What the guideline does with a split result
When two different tests disagree, the ADA advises repeating the one that is above the diabetes cutoff, and considering whether something is interfering with the A1C. In conditions that change red cell turnover, including sickle cell disease, pregnancy in the second and third trimesters, hemodialysis, recent blood loss or transfusion, and erythropoietin therapy, it recommends diagnosing with plasma glucose alone. 1
Which one to ask about
For routine screening, either test works, and many clinicians use both. A1C is the more convenient choice because it needs no fasting and varies less. Fasting glucose, or a glucose tolerance test, is the better choice if you are anemic or low in iron, have sickle cell trait or another hemoglobin variant, are pregnant, have advanced kidney disease, or have recently lost blood or had a transfusion. If the two have already disagreed, the next step is usually to repeat the abnormal one and look for a reason, rather than average them.
- Do my A1C and fasting glucose agree, and if not, which one is above the line?
- Could anemia, iron deficiency or a hemoglobin variant be affecting my A1C?
- Should the abnormal result be repeated, and when?
- Would a 2-hour glucose tolerance test or fructosamine add anything for me?
- What does my estimated average glucose say about after-meal highs?
For the 2-hour test, see the OGTT; for a shorter-term average that does not depend on red cells, see fructosamine. The A1C calculator converts between A1C and average glucose, and the blood sugar guide puts every number on the panel in order.
Link · American Diabetes Association, Diabetes CareDiagnosis and classification of diabetes: Standards of Care in Diabetes 2025The ADA's diagnostic criteria for diabetes and prediabetes, including when A1C should not be used.pubmed.ncbi.nlm.nih.govThe working rule: A1C is the steadier long-term average, and fasting glucose is the direct reading that does not depend on red blood cells. When they disagree, repeat the one that crossed the line and ask whether anything is changing your red cells.
Questions
Is A1C more accurate than fasting glucose?
A1C is more stable from week to week and in long-term studies tracks heart disease risk more closely than fasting glucose. It is less accurate when something changes red blood cells, such as anemia, iron deficiency, sickle cell trait or pregnancy, and fasting glucose is the better test then.
Can my A1C be high but my fasting glucose normal?
Yes. Your sugar may be normal overnight but rise after meals, which A1C captures and a fasting test misses. Iron deficiency can also push A1C up without any change in glucose. A clinician will usually repeat the A1C and check for a reason.
What A1C matches a fasting glucose of 126?
They are not direct equivalents, because one is an average and the other a single morning. As a rough guide, the ADAG study found an A1C of 6% corresponds to an average glucose of about 126 mg/dL, and 6.5%, the diabetes cutoff, to about 140 mg/dL.
Do I need to fast for an A1C test?
No. A1C can be drawn at any time of day, fasting or not. Fasting glucose needs at least eight hours without food or caloric drinks.
Which test is better for prediabetes?
Both are accepted. A1C 5.7 to 6.4% or fasting glucose 100 to 125 mg/dL each count as prediabetes. They often disagree at this level, so many clinicians look at both and repeat borderline results.
Citations
- American Diabetes Association Professional Practice Committee. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes 2025. Diabetes Care. 2025;48(Suppl 1):S27-S49.
- Nathan DM, Kuenen J, Borg R, et al. Translating the A1C assay into estimated average glucose values. Diabetes Care. 2008;31(8):1473-1478.
- Selvin E, Crainiceanu CM, Brancati FL, Coresh J. Short-term variability in measures of glycemia and implications for the classification of diabetes. Arch Intern Med. 2007;167(14):1545-1551.
- Selvin E, Steffes MW, Zhu H, et al. Glycated hemoglobin, diabetes, and cardiovascular risk in nondiabetic adults. N Engl J Med. 2010;362(9):800-811.
- Cowie CC, Rust KF, Byrd-Holt DD, et al. Prevalence of diabetes and high risk for diabetes using A1C criteria in the U.S. population in 1988-2006. Diabetes Care. 2010;33(3):562-568.
- English E, Idris I, Smith G, et al. The effect of anaemia and abnormalities of erythrocyte indices on HbA1c analysis: a systematic review. Diabetologia. 2015;58(7):1409-1421.
- Lacy ME, Wellenius GA, Sumner AE, et al. Association of sickle cell trait with hemoglobin A1c in African Americans. JAMA. 2017;317(5):507-515.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.