What the HbA1c test shows
HbA1c, or glycated hemoglobin, measures the percentage of your hemoglobin — the oxygen-carrying protein in red blood cells — that has glucose permanently attached to it. The more sugar in your blood, the more sticks to hemoglobin, so the result mirrors your average blood glucose over the past two to three months. MedlinePlus calls it a picture of your typical blood sugar over three months.
Two things set it apart from its neighbours. A glucose test is a single snapshot that rises and falls with meals, stress and time of day; HbA1c is the running average and needs no fasting — you can eat and drink beforehand, as NIDDK notes. Fructosamine uses the same glycation principle but tracks shorter-lived albumin, so it reflects only the past two to three weeks.
Because red blood cells live about 120 days, HbA1c is weighted toward the most recent month, per StatPearls; an HbA1c of 7% averages near 154 mg/dL (8.6 mmol/L).
HbA1c normal range
HbA1c is reported two ways: the older percentage (NGSP/DCCT units) still standard in the US, and mmol/mol (IFCC units) common in Europe. The diagnostic bands are the same worldwide:
| Category | % (NGSP) | mmol/mol (IFCC) |
|---|---|---|
| Normal | below 5.7 | below 39 |
| Prediabetes | 5.7–6.4 | 39–47 |
| Diabetes | 6.5 or higher | 48 or higher |
| Common target once diagnosed | below 7.0 | below 53 |
The 6.5% (48 mmol/mol) diabetes cutoff was set by the WHO in 2011. A single result is rarely enough — diagnosis needs a second confirming test unless glucose is clearly high with symptoms. The thresholds are identical for men and women, but HbA1c drifts up slightly with age and can vary with ancestry and inherited hemoglobin variants. Reference ranges depend on the lab and assay, so read your result against your own report.
Why HbA1c is high
A high HbA1c means average glucose has been running high, most often because of:
- Type 2 diabetes and prediabetes (by far the commonest). Insulin resistance lets glucose build up, and most raised results fall here — often in people who feel well.
- Poorly controlled known diabetes — missed medication, illness, weight gain or a course of steroids pushes the average up.
- Type 1 diabetes — autoimmune loss of insulin, more often in children and young adults, which can appear with a very high value plus thirst, frequent urination and weight loss.
- Other causes of high glucose: corticosteroids, Cushing’s syndrome and pancreatic disease.
- Falsely high readings — iron deficiency lengthens red-cell survival and can nudge HbA1c up without more sugar in the blood, which is why ferritin and hemoglobin are read alongside it.
When is it urgent? An HbA1c above about 10% (86 mmol/mol), or any raised value with heavy thirst, blurred vision, weight loss or drowsiness, needs prompt review. Very high glucose can tip into diabetic ketoacidosis or a hyperosmolar state — a same-day emergency.
Why HbA1c is low
A low HbA1c is less common and splits into two very different situations.
- Genuinely low average glucose: usually over-treatment of diabetes with insulin or sulfonylureas, causing repeated hypoglycemia; rarely an insulin-secreting tumor (insulinoma).
- Falsely low readings: anything that shortens red-cell life or floods the blood with young cells — hemolytic anemia, recent bleeding or a transfusion, later pregnancy, advanced liver or kidney disease, and erythropoietin or iron treatment. Some hemoglobin variants also read low on certain assays.
When an HbA1c looks low but does not fit your home glucose readings, a red-cell cause is likely, and fructosamine or a direct glucose test is used instead. The acute danger is symptomatic hypoglycemia — shakiness, sweating, confusion or fainting — judged on a glucose reading and needing fast-acting sugar and medical advice.
What to test alongside
HbA1c is read with the rest of the glucose picture and the risks that travel with it:
- Fasting glucose — the snapshot paired with HbA1c to confirm diabetes.
- Oral glucose tolerance test — catches cases HbA1c misses, including in pregnancy.
- Fructosamine — a two-to-three-week average when HbA1c is unreliable.
- Insulin and HOMA-IR — gauge insulin resistance.
- C-peptide — how much insulin the pancreas still makes.
- Hemoglobin and ferritin — anemia and iron status distort the result.
- LDL cholesterol and total cholesterol — cardiovascular risk beside high glucose.
- Creatinine — kidney function; diabetes harms the kidneys.
- ALT — fatty liver that tracks insulin resistance.
What to do about an abnormal result
- Don’t self-diagnose from one value. A diabetes diagnosis needs a repeat HbA1c or a glucose test on another day, unless your glucose is clearly high with symptoms.
- Confirm on a certified assay. Ask that the lab uses an NGSP-standardised method, and mention any anemia, recent blood loss, pregnancy or transfusion.
- If it shows prediabetes, lifestyle change comes first: losing about 7% of body weight, moving 150 minutes a week and cutting refined carbohydrates can pull HbA1c down and lower the odds of progressing — see whether prediabetes is reversible in three to six months. Recheck in 6–12 months.
- If it shows diabetes, your primary-care doctor confirms it, checks eyes, kidneys, feet and lipids, and starts treatment; type 1, pregnancy or hard-to-control diabetes is referred to an endocrinologist.
- See your GP or primary-care physician first — they coordinate confirmation and next steps, not immediate medication.
Mini-FAQ
What is a normal HbA1c?
Below 5.7% (39 mmol/mol) is normal, 5.7–6.4% (39–47 mmol/mol) is prediabetes, and 6.5% (48 mmol/mol) or higher on two separate tests means diabetes. Most people already treated for diabetes aim for below 7%.
What does HbA1c actually measure?
It measures the share of your hemoglobin coated with glucose, which reflects your average blood sugar over the past two to three months. Unlike a fasting glucose test it needs no fasting and is not thrown off by a single meal.
How is HbA1c different from a glucose test?
A glucose test is a snapshot of one moment and swings with food and stress, while HbA1c is the running average of the last few months. Diagnosis often uses the two together.
Can an HbA1c result be misleading?
Yes. Anything that changes red blood cell lifespan skews it: iron deficiency can raise it, while recent bleeding, hemolysis, pregnancy or a transfusion can lower it. When it does not match your home readings, fructosamine or a glucose test is used instead.
Can I lower a high HbA1c?
Often yes, especially with prediabetes. Losing around 7% of body weight, moving about 150 minutes a week and cutting refined carbohydrates can drop it measurably over three to six months; diagnosed diabetes may also need medication. Never change prescribed treatment on your own.
How often should HbA1c be checked?
Screening from age 35 every 3 years is reasonable for adults with normal results, and sooner or more often with extra risk factors. People living with diabetes are usually retested every 3–6 months.
