Hemoglobin A1c (HbA1c)
A1c shows your average blood sugar over about the past 2 to 3 months, based on how much glucose is stuck to your hemoglobin.
AI-verified against cited sources. Awaiting expert reviewAlso called HbA1c, A1c, A1CLOINC 4548-4
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What it is
Hemoglobin is the protein in red blood cells that carries oxygen. When there is extra glucose in your blood, some of it sticks to hemoglobin. The A1c test measures what percent of your hemoglobin has glucose attached. Red blood cells live about 120 days, so A1c reflects your average over roughly the past 2 to 3 months, with the most recent month counting the most. It doesn't need fasting.
Why it matters
A1c is one of the standard tests clinicians use to find prediabetes and diabetes and to see how well blood sugar is controlled over time. In people with diabetes, a higher A1c over the years is linked to more problems with the eyes, kidneys, nerves and blood vessels. Because it averages many weeks, it is steadier than a single glucose reading. It can mislead when red blood cells behave unusually, such as with anemia or some inherited hemoglobin types, so clinicians sometimes confirm it with a glucose test.
If it’s low
An A1c below about 4.8% is under the range one large lab prints. A low A1c does not by itself mean blood sugar is very low. It can reflect red blood cells being replaced faster than usual, or a lab-method effect. In people treated for diabetes it can also reflect frequent low blood sugar, so context matters.
Common causes
- Hemolytic anemia or other conditions that shorten red blood cell life
- Recent blood loss or a recent transfusion
- High fetal hemoglobin or certain hemoglobin variants, with some lab methods
- In people who take diabetes medicines, frequent low blood sugar
- Pregnancy, which changes how A1c relates to glucose
What can help
- Ask whether a glucose-based test or another measure of average sugar would be more reliable for you
- Ask whether a blood count or a hemoglobin test would help explain the result
- If you take diabetes medicines, ask how to tell whether lows are contributing
If it’s high
The American Diabetes Association (ADA) calls an A1c of 5.7 to 6.4% (39 to 47 mmol/mol) prediabetes and 6.5% (48 mmol/mol) or higher the diabetes range. The ADA needs two results in the diabetes range to make that call, either from two different tests (such as A1c and fasting glucose, which can be drawn at the same time) or from the same test repeated at a different time. Some things other than blood sugar can nudge A1c up, so a result is read in context.
Common causes
- Prediabetes or diabetes
- Iron-deficiency anemia, which can raise A1c without a change in glucose
- Late pregnancy in people with iron deficiency
- Certain inherited hemoglobin variants, with some lab methods (the effect can go either way)
What can help
- Ask your clinician whether to confirm the result with a fasting glucose or another A1c
- Regular physical activity, a balanced eating pattern and a healthy weight are the main lifestyle levers clinicians discuss for A1c that runs high
- If iron deficiency is a possibility, ask whether your iron or ferritin should be checked
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
For adults: 4.8–5.6 %. Source: Labcorp, American Diabetes Association (Diabetes Care), MedlinePlus, National Library of Medicine, National Glycohemoglobin Standardization Program (NGSP).
- Adults (this one applies)4.8–5.6 %
Sources: Labcorp, American Diabetes Association (Diabetes Care), MedlinePlus, National Library of Medicine, National Glycohemoglobin Standardization Program (NGSP)
See A1c by age and sex, with the U.S. survey numbers where they exist.
Wellness targets
Narrower ranges some clinicians aim for. Being outside one doesn’t mean something is wrong.
up to 6.9 % (18+, diabetes). Evidence: Strong. The ADA Standards of Care (2026) recommend an A1C goal below 7% (53 mmol/mol) for many nonpregnant adults with diabetes who don't have severe or troublesome low blood sugar (grade A). Goals are individual: some people aim lower (below 6.5%) and others higher (up to about 8%).
ADA diagnostic categories (nonpregnant adults)
- Normalbelow 5.7 %
- Prediabetes5.7 to under 6.5 %
- Diabetes range (needs a confirming result)6.5 and above %
If this applies to you
A1c means a little more for some people. Open the one that fits you.
Had weight-loss surgery
For people who had diabetes before surgery, BOMSS suggests keeping track of A1c afterward. It suggests the same for cholesterol in people who had high levels before.
High cholesterol or heart risk
The 2026 U.S. guideline notes that statins slightly raise the chance of type 2 diabetes in people who already have risk factors for it, such as an A1c of 6.0 to 6.4%, and that this is rare with normal blood sugar. The atorvastatin label also lists rises in A1c and fasting glucose.
Fatty liver or liver condition
AASLD calls type 2 diabetes the risk factor with the most impact on fatty liver and on liver scarring getting worse. A1c is part of its first workup, and AASLD suggests that people with fatty liver be checked for diabetes.
Tracking for long-term health
The ADA suggests diabetes screening for all adults starting at age 35, repeated at least every 3 years when results are normal and yearly with prediabetes. A1c can mislead with anemia, recent blood loss or some hemoglobin variants, and the ADA suggests glucose-based criteria in those cases.
Age 65 or older
For older adults, the ADA suggests A1c goals set person by person. Lower goals fit people in good health, less strict goals fit those with frailty or complex health, and avoiding low blood sugar comes first. For people in very poor health, it suggests not relying on A1c.
PCOS
If an OGTT isn't possible, the guideline says A1c or fasting glucose could be used, though they are noticeably less accurate in PCOS. Blood sugar is checked at the start and then every 1 to 3 years, depending on other risk factors.
Prediabetes or diabetes
A1c is an indirect measure of average blood glucose. The ADA suggests checking glucose status at least twice a year, and about every 3 months when goals aren't being met or treatment recently changed. With prediabetes, the ADA suggests testing once a year.
Perimenopause or menopause
USPSTF recommends diabetes screening from age 35 to 70 for people with overweight or obesity, and says every 3 years may be reasonable after a normal result. NICE says the risk of type 2 diabetes doesn't go up with HRT.
Pregnant or postpartum
Before 15 weeks, the ADA suggests testing people with risk factors for diabetes that may have been there before pregnancy, and A1c is one of the tests used. The ADA also lists pregnancy among conditions that change how A1c relates to blood glucose, and suggests glucose-based criteria in those conditions.
Keto or low-carb eating
The ADA says reviews found carbohydrate-restricted eating lowered A1c in the short term, under 6 months, with less difference after a year. In one 12-week trial, a keto pattern didn't clearly improve A1c and raised LDL compared with a low-carb Mediterranean pattern.
On a GLP-1 medicine
For people with diabetes, A1c is the usual way to follow treatment. The ADA suggests checking glucose status about every 3 months after a treatment change or when goals aren't met, and at least twice a year once things are steady.
How common is your level?
NHANES describes the US civilian, non-institutionalized population, not a 'healthy' reference group. People with disease, on medication, fasting or not (except fasting-subsample markers), smokers and so on are all included. A percentile says where a value sits among Americans of that sex and age, not whether it is normal.
Use the left and right arrow keys to read each age. Shift moves five years.
Reference data derived from CDC/NCHS National Health and Nutrition Examination Survey (NHANES), cycles 2015-2016, 2017-March 2020 (pre-pandemic), August 2021-August 2023. Analysis by bloodtesttracker.com; not endorsed by CDC, NCHS or HHS.
Explore A1c by age and sex: every percentile as a table, with a download.
What can affect your result
- Red blood cell lifespan and turnover: anemia, blood loss, red cell breakdown, transfusion
- Iron deficiency, which raises A1c
- Inherited hemoglobin variants and the method your lab uses
- Advanced kidney disease and dialysis
- Pregnancy
- Average A1c differs slightly between groups; the ADA notes Black adults average about 0.3 percentage points higher than non-Hispanic White or Hispanic adults
- Fasting, time of day and recent meals do not affect A1c
- Anything that shortens red blood cell life, such as hemolytic anemia or recovery from blood loss, falsely lowers A1c with every method. A recent transfusion can also make A1c unreliable.
- Iron-deficiency anemia is linked to a higher A1c, and treating it lowers A1c.
- Inherited hemoglobin variants (such as S, C, E and D) and high fetal hemoglobin can distort results on some lab methods but not others. Ask which method your lab uses.
- In advanced kidney disease, altered hemoglobin can interfere with some methods, and A1c may under-read glucose in people on dialysis.
- Pregnancy, G6PD deficiency and HIV change the relationship between A1c and glucose (ADA).
- Fingerstick (point-of-care) A1c devices should not be used for diagnosis unless they are approved for it (ADA).
Any time of day. About 3 months in total, but the most recent month counts the most, so a large change in glucose shows up within weeks.
When to talk to a clinician
If your A1c is 6.5% (48 mmol/mol) or higher and no one has discussed it with you, book a visit to confirm it. See a clinician soon if a high A1c comes with extreme thirst, frequent urination, weight loss or blurred vision. Call 911 or your local emergency number for confusion, fainting, or vomiting with fast breathing. An A1c above 15% is unusual, and the NGSP advises considering a lab interference when a result is that high or at odds with other tests.
Questions you could ask
- What A1c goal makes sense for me, if I have diabetes?
- Which method does my lab use, and should I use the same lab each time?
- Should we confirm this with another test, such as a fasting glucose?
- Could anemia, my iron level or a hemoglobin variant affect how reliable my A1c is?
- How often should I be rechecked?
- Could a red blood cell condition, recent blood loss or a lab-method effect explain a low A1c?
Questions people ask
What is the difference between A1c in % and in mmol/mol?
They are the same measurement in two unit systems. US labs print a percent (such as 5.7%), and many other countries print mmol/mol (such as 39). The conversion is not a straight multiply: mmol/mol = (percent - 2.15) x 10.93. Common pairs are 5.7% = 39, 6.5% = 48 and 7.0% = 53 mmol/mol.
Do I need to fast for an A1c?
No. A1c reflects the past couple of months, not what you ate this morning, so it can be drawn at any time of day.
My A1c is normal but my fasting glucose is high, or the other way around. Why?
Glucose is a snapshot, while A1c is a 2 to 3 month average. A1c can also be shifted by iron deficiency, blood loss, kidney disease, pregnancy or an inherited hemoglobin variant. When the two keep disagreeing, the ADA suggests clinicians check for a problem or interference with one of the tests.
How is A1c related to my average glucose?
The estimated average glucose (eAG) is about 28.7 x A1c - 46.7 mg/dL. In round numbers, 6% is about 126 mg/dL, 7% about 154 mg/dL and 8% about 183 mg/dL. It's an estimate across many people, so your own average can differ.
How often should A1c be checked?
The ADA suggests screening at least every 3 years, starting by age 35. With prediabetes, testing yearly is suggested. With diabetes, it is checked at least twice a year when stable, and about every 3 months if you aren't at goal or treatment has changed. Your clinician may choose a different schedule.
Show the science and sources
Units
- 1 % = 1 %
- 1 % = -12.59 mmol/mol
Is a change real?
A1c naturally varies about 1.2% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 4% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Critical appraisal and meta-analysis of biological variation studies on glycosylated albumin, glucose and HbA1c (Ricos C et al.).
For clinicians and researchers
A1c is the glycated fraction of hemoglobin, reported as a percent of total hemoglobin (NGSP/DCCT units) and as mmol HbA1c per mol hemoglobin (IFCC units). The master equation linking them is NGSP % = 0.09148 x IFCC + 2.152, or IFCC = 10.93 x NGSP - 23.50, so 7.0% is 53 mmol/mol. The ADA advises using a method certified by the NGSP as traceable to the DCCT reference, and restricting point-of-care A1c to devices approved for diagnosis. Methods include ion-exchange HPLC, boronate affinity, immunoassay and enzymatic assays. Hemoglobin variants (S, C, E, D), elevated HbF and carbamylated hemoglobin can interfere, with effects that depend on the method (the NGSP keeps a table). Anything that shortens red cell survival lowers A1c with every method, and iron deficiency raises it. A1c is a weighted average: glucose in the preceding 30 days contributes much more than glucose 90 to 120 days earlier. The ADAG regression relates A1c to estimated average glucose (eAG, mg/dL = 28.7 x A1c - 46.7, r2 = 0.84), so 6% is about 126, 7% about 154 and 8% about 183 mg/dL. A1c does not measure glucose variability, real-time glucose or hypoglycemia. The ADA advises using plasma glucose criteria when conditions alter the A1c-glucose relationship, and checking for interference when A1c and glucose consistently and substantially disagree. Pregnancy, G6PD deficiency and HIV are among the conditions it names. MedlinePlus notes that A1c is not used to diagnose gestational diabetes.
LOINC 4548-4 (primary), 17856-6, 59261-8, 71875-9, 4549-2, 17855-8, 62388-4. Specimen: whole-blood.
Sources
- Hemoglobin A1c (test 001453), Labcorp
- 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes-2026, American Diabetes Association (Diabetes Care) (2026)
- 6. Glycemic Goals, Hypoglycemia, and Hyperglycemic Crises: Standards of Care in Diabetes-2026, American Diabetes Association (Diabetes Care) (2026)
- Hemoglobin A1C (HbA1c) Test, MedlinePlus, National Library of Medicine
- IFCC Standardization of HbA1c: IFCC and NGSP, National Glycohemoglobin Standardization Program (NGSP)
- Factors that Interfere with HbA1c Test Results, National Glycohemoglobin Standardization Program (NGSP) (2026)
- HbA1c and Estimated Average Glucose (eAG), National Glycohemoglobin Standardization Program (NGSP)
- Clinical Use of HbA1c, National Glycohemoglobin Standardization Program (NGSP)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.