Why is my A1c high but my fasting sugar normal?
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They measure different things. A fasting glucose is one morning’s number, mostly set by your liver overnight. An A1c is about 3 months of sugar stuck to your red cells, after-meal highs included, and how long your red cells live shifts it too. Repeat both, and check your blood count, since anemia can throw A1c off.
What we’d do
Don’t pick the number you like better. An A1c in the prediabetes range puts you in the prediabetes range, even with a normal fasting number, so start the plan. Ask for a repeat of both, a blood count, and if they still disagree, the 2-hour glucose drink test, which catches after-meal highs directly.
Why it feels like a mistake
Two blood tests about the same sugar should agree, so when one says normal and the other says prediabetes, it’s tempting to believe the friendly one. But the American Diabetes Association counts each test on its own. A1c of 5.7 to 6.4% is the prediabetes range whatever your fasting number says, and two A1c results of 6.5% or more count as diabetes even when fasting glucose is normal.
One photo and a three-month video
A fasting number is a photo of one morning. It mostly shows what your liver did overnight and doesn’t see your meals at all. An A1c is closer to a video of the last three months, day and night, so it includes every after-dinner hill your fasting number missed.
After-meal numbers can be high while the fasting one still looks fine. The DECODE study showed it in 1999. Researchers took 25,364 Europeans who’d had both a fasting test and a 2-hour glucose drink test and followed them for 7.3 years. Within every fasting band, the risk of death rose with the 2-hour number, so fasting glucose alone missed people at higher risk.
When the A1c is the one that’s off
A1c assumes your red cells live a normal life span. In 6 healthy people, the average red cell was anywhere from 38 to 60 days old, and that spread alone was enough to change A1c for the same blood sugar. Someone whose cells live longer carries more sugar on each one, so their A1c comes out higher.
These can make A1c disagree with your blood sugar (American Diabetes Association, 2026, and MedlinePlus):
| Can throw A1c off | |
|---|---|
| Your blood | anemia, iron levels, recent blood loss, a transfusion, spleen removal |
| Hemoglobin | variants such as sickle cell trait, G6PD deficiency |
| Other | pregnancy, kidney failure, liver disease, some HIV medicines, opioids |
On average, Black adults have A1c values about 0.3 points higher, a finding the ADA calls controversial, and the lines are the same for everyone.
What to ask for
A repeat of both tests, ideally from the same draw, and a blood count. If the A1c stays in the prediabetes range and fasting stays normal, a 2-hour glucose drink test settles it, since it measures the after-meal side head on. Either way, walking after meals and losing 5 to 7% of your weight if you carry extra work on both numbers. How do I reverse prediabetes? has the plan, and the A1c converter shows what your A1c means as average blood sugar.
4 sources
- American Diabetes Association, Standards of Care in Diabetes 2026, section 2, Diagnosis and Classification of Diabetespmc.ncbi.nlm.nih.gov
- DECODE Study Group, Glucose tolerance and mortality: comparison of WHO and American Diabetes Association diagnostic criteria, Lancet, 1999pubmed.ncbi.nlm.nih.gov
- Cohen RM and others, Red cell life span heterogeneity in hematologically normal people is sufficient to alter HbA1c, Blood, 2008pmc.ncbi.nlm.nih.gov
- MedlinePlus, Hemoglobin A1C (HbA1c) test, US National Library of Medicinemedlineplus.gov
