Use HbA1c to check average glucose over months.
HbA1c may help if you're very thirsty, urinate often, have blurred vision, lose weight without trying, or had a high glucose result before. It also helps when you have another diabetes risk. It shows a longer period than one glucose test.
Average
It looks back across recent weeks
HbA1c reflects glucose attached to hemoglobin across the life of red blood cells. Recent weeks influence the result more than the oldest weeks it covers.
Screening
It can screen for sustained high glucose
The result can support diabetes screening or diagnosis when symptoms, an earlier glucose result, age, pregnancy history, family history, PCOS, or other risk factors raise concern.
Timing
It can't show your glucose level at the moment your thinking changed
A single average can't show whether glucose was high, low, or changing quickly during a particular morning, meal, walk, or missed meal.
HbA1c can't show whether glucose caused your thinking problems. Dehydration, missed meals, medicine effects, or something unrelated can feel similar. A normal average doesn't rule out short highs or lows.
Save this test
Keep HbA1c in My Fog.
Saving adds HbA1c to Saved Checks, so you can find the guide before the test, when the result arrives, or before a visit.
My Fog saves the check, not the laboratory report or result value. Keep the original report in your health record.
Screening starts at different ages for different groups. Pregnancy and blood-cell changes can alter the result.
The standard ranges for people who are not pregnant are the same for men and women. Age, pregnancy, red blood cell changes, other health problems, and the reason for testing can change how useful the result is.
Children and teenagers
Routine screening is risk based, not for every child. It may be considered from puberty or age 10, whichever comes first, when weight and another risk factor raise concern. HbA1c is an option, but fasting glucose or glucose tolerance testing may work better in some young people.
Adults younger than 35
People younger than 35 may still need screening. Reasons include symptoms, family history, PCOS, past diabetes during pregnancy, high blood pressure, little activity, or medicines that raise glucose.
Adults 35 and older
For adults not covered by risk-based screening, the American Diabetes Association recommends screening from age 35. HbA1c is one option alongside fasting glucose and the oral glucose tolerance test.
Pregnancy and after birth
Pregnancy changes red blood cells and usually lowers HbA1c. The test may help early in pregnancy or follow known diabetes. It isn't the main test for diabetes during pregnancy at 24 to 28 weeks. After diabetes during pregnancy, you need glucose checks after birth and later in life.
Periods, blood cells, and hemoglobin
Periods matter if heavy bleeding or iron deficiency changes red blood cells. Recent blood loss, transfusion, anemia, kidney failure, or a hemoglobin variant can also make HbA1c look higher or lower than measured glucose.
Older adults
For an older adult who already has diabetes, the safest goal may change with health, function, frailty, medicines, and risk of low blood sugar. A strict generic target can cause harm through low blood sugar or treatment that's too demanding.
You usually don't need to fast, unless another test needs it. Bring your medicine list.
Before the appointment, find out whether HbA1c is the only blood test you're having. HbA1c itself doesn't need fasting, but another test at the same visit might. Follow the instructions for the full set of tests.
Bring or photograph your medicine list, including prescribed medicines and supplements. Keep taking prescribed medicine unless the clinician who ordered the test tells you otherwise.
Tell the person reading the result about pregnancy, recent blood loss or transfusion, anemia, kidney or liver disease, dialysis, or a known hemoglobin variant. Bring any glucose readings that don't seem to match the HbA1c result.
The blood sample
A health professional takes blood from a vein, or sometimes from a fingertip with an approved point-of-care device. A vein draw usually takes less than five minutes.
After the sample
A small bruise or brief soreness can follow a vein draw. You usually won't need recovery time.
What do these HbA1c ranges mean?
The ranges below are ADA cutoffs for screening and diagnosis in people who are not pregnant. If you already have diabetes, use the treatment goal set with your care team.
Below the prediabetes range
Below 5.7% (below 39 mmol/mol)
This is below the standard prediabetes range. It makes sustained high glucose less likely, but blood-cell or hemoglobin factors can still make the result unreliable.
Prediabetes range
5.7% to 6.4% (39 to 47 mmol/mol)
This is the standard prediabetes range. It doesn't mean you have diabetes. It's a reason to confirm the result is reliable and to lower diabetes risk.
Diabetes range
6.5% or higher (48 mmol/mol or higher)
This meets one standard diagnostic threshold for diabetes. Without clearly high blood sugar or a high blood sugar crisis, diagnosis normally needs a second abnormal HbA1c or glucose-based test.
An average can hide short glucose swings.
If meter or continuous glucose monitor (CGM) readings, fasting glucose, symptoms, or a glucose tolerance result don't agree, ask what explains the mismatch.
See research details
These rows separate diagnosis, test quality, ongoing care, and situations where glucose is safer to use.
A first diabetes-range result normally needs a second abnormal HbA1c or glucose-based test to confirm it.
Laboratory HbA1c should use an NGSP-certified method. Point-of-care diagnosis should use an approved device in a qualified setting with trained staff.
A follow-up goal varies by person. It isn't the 6.5% cutoff used to diagnose diabetes.
Diagnosis should use plasma glucose tests when pregnancy, a hemoglobin variant, or another condition changes how HbA1c relates to glucose.
Choose one change you can keep doing.
Daily habits can help if the result is in the prediabetes range or you already have diabetes. Start with one change that fits your food, budget, movement, medicines, and sleep.
Build meals you can repeat
There's no single required diet. Useful basics include vegetables, whole fruit, beans or lentils, whole grains, nuts, seeds, and water or another unsweetened drink. Choose combinations that fit your culture, budget, appetite, and medicines.
Add movement that fits your day
Regular activity can help lower blood sugar. Start slowly with movement you enjoy and can repeat, such as a short walk, seated exercise, or breaking up a long period of sitting. If activity tends to cause lows, use your existing safety plan.
Protect sleep and judge HbA1c over weeks
HbA1c changes over weeks, not after one good day. Keep a steady sleep opportunity where you can, and compare changes over a longer period with the result, symptoms, food, activity, and any glucose readings.
Food, movement, and sleep can help lower diabetes risk and manage diabetes, but can't confirm a diagnosis. If you use insulin or a sulfonylurea, are pregnant, or often have low blood sugar, changes need to fit your existing care plan. There's no clear proof that a specific herb or supplement can manage diabetes, and some can interact with medicines.
Keep the result and the reason for testing.
Keep these together
- HbA1c value and unit, including % or mmol/mol
- The lab's range or flag and collection date
- Whether the test was for screening, diagnosis, or ongoing diabetes care
- Fasting blood sugar, glucose tolerance, meter, or CGM results used for comparison
- Pregnancy, anemia, blood loss or transfusion, kidney disease, or a known hemoglobin condition that may affect reliability
Question for the visit
“Does this HbA1c match my glucose readings, or could pregnancy, anemia, blood loss, kidney disease, or my hemoglobin type make it unreliable?”
Common questions about HbA1c
Is HbA1c the same as the A1C test?
Yes. NIDDK lists HbA1c, A1C, hemoglobin A1C, glycated hemoglobin and glycohemoglobin as names for the same blood test. If your lab report says A1C, every range on this page applies to it.
Sources: NIDDK, MedlinePlus
Can high blood sugar or a high HbA1c cause brain fog?
Yes, over time. CDC says long-lasting high blood sugar damages blood vessels in the brain, which can cause memory and thinking problems. You may not notice at first. In 5,189 adults in England followed for about 8 years, higher HbA1c went with faster decline in memory scores (Zheng, 2018). Low blood sugar, below 70 mg/dL (CDC), can cause confusion (NIDDK).
Sources: CDC, Zheng 2018, NIDDK
What average blood sugar does an HbA1c (A1C) result equal?
CDC gives an estimated average blood sugar: 6% is 126 mg/dL, 7% is 154 mg/dL and 8% is 183 mg/dL. For other results, multiply the % number by 28.7, then subtract 46.7. That formula comes from 507 people who each had about 2,700 glucose readings over 3 months (Nathan, 2008). The estimate may miss spikes and lows (CDC).
Sources: CDC, Nathan 2008
Can stress affect A1C (HbA1c) levels?
Stress can change blood sugar. CDC says stress hormones make it rise or fall, and stress from being sick or injured can raise it. HbA1c is less likely than a fasting glucose test to be affected by short-term changes (NIDDK). In people with diabetes, a review of 61 studies linked depression and diabetes distress to higher HbA1c (Wojujutari Ajele, 2025).
Sources: CDC, NIDDK, Wojujutari 2025
How often should you repeat an HbA1c (A1C) test?
After a result in the prediabetes range, you usually repeat the test every 1 to 2 years (CDC, MedlinePlus). People with diabetes have it at least twice a year, and more often if they are not meeting their treatment goals (NIDDK). After a normal result, your age and risk factors set the timing (CDC).
Sources: CDC, MedlinePlus, NIDDK
What is a dangerous HbA1c (A1C) level?
No single number marks where danger starts. In 3,642 people with type 2 diabetes, each 1 percentage point lower HbA1c went with a 21% lower risk of any diabetes complication, and the study found no cutoff (Stratton, 2000). CDC says higher A1C levels are linked to health complications. Its goal for most people with diabetes is 7% or less.
Sources: Stratton 2000, CDC
What does a low HbA1c (A1C) mean?
Below 5.7% is normal (CDC). In 14,099 US adults without diabetes, people below 4.0% had the highest liver enzymes and about 3 times the risk of death over about 9 years compared with people at 5.0% to 5.4% (Carson, 2010). The study could not say why. Below 4.0%, ask whether your liver tests and blood count were checked.
Sources: Carson 2010, Christman 2011, NIDDK, CDC
Sources for HbA1c
Diagnostic ranges, confirmation, screening ages, test quality, pregnancy, and interference
Why an ongoing diabetes goal is personal and differs from a diagnostic cutoff
Food choices, activity, and lowering diabetes risk in prediabetes
Health, function, frailty, low-glucose risk, and individualized goals
Pregnancy-related HbA1c limits and why glucose monitoring comes first
What the test measures, the blood sample, medicines, pregnancy, and result basics
No-fasting preparation, result ranges, short highs and lows, and accuracy limits
The 3-month average, diagnostic use, precision, interference, and personal goals
Symptoms of high and low glucose and the extra detail from time-specific readings
Food, movement, sleep, low-glucose safety, and supplement limits
Hemoglobin variants, blood-cell turnover, anemia, transfusion, and kidney disease
See each claim's sources
indication
HbA1c estimates average blood glucose over roughly the past 2 to 3 months and is used for diabetes screening, diagnosis, and monitoring.indication
Diabetes screening can be appropriate when symptoms, prior glucose results, age, or established risk factors raise concern.preparation
HbA1c itself does not require fasting and can be measured from a venous sample or an appropriate point-of-care sample, though other co-ordered tests may require preparation.range
For nonpregnant people, standard HbA1c diagnostic ranges are below 5.7%, 5.7% to 6.4%, and 6.5% or higher.interpretation
Without unequivocal hyperglycemia or a hyperglycemic crisis, diagnosis requires confirmatory abnormal testing.procedure
Diagnostic HbA1c should use an NGSP-certified laboratory method or an approved point-of-care device in a qualified setting.limitation
HbA1c is an average and does not show short glucose highs, lows, or day-to-day variability.limitation
A substantial mismatch between HbA1c and glucose should trigger evaluation for assay interference or a condition that changes red blood cell turnover.context
Adult screening generally begins at age 35 when earlier risk-based screening does not apply, while screening in children and teenagers is risk based from puberty or age 10.context
Pregnancy changes HbA1c interpretation, and HbA1c is a secondary measure rather than the main test for gestational diabetes at 24 to 28 weeks.interpretation
The HbA1c threshold used to diagnose diabetes is not a universal treatment goal for a person already living with diabetes.diet
Sustainable food choices, regular physical activity, and enough sleep can support diabetes prevention or management, with plans adapted to medicines and low-glucose risk.safety
There is no clear proof that a specific food, herb, spice, or dietary supplement manages diabetes, and supplements can interact with diabetes medicines.