A Raised HbA1c with a Normal Fasting Glucose
These two tests are not measuring the same thing, so their disagreeing is not a contradiction. Fasting glucose is one moment; HbA1c is roughly the last three months, weighted toward the most recent few weeks. The usual reason for this combination is glucose running high after meals and coming back down by morning, which is what early insulin resistance looks like well before a fasting number moves.
The pattern on your report
- HbA1c High · mild Key
- Fasting glucose Normal Key
- Hemoglobin Normal Key
Printed as: Fasting glucose in mmol/Lor mg/dL— An eighteenfold difference. A fasting glucose of 5.5 mmol/L is 99 mg/dL, which is why the two sets of reference ranges look unrelated.Hemoglobin in g/Lor g/dLHbA1c in mmol/molor %— This is the one pairing on a routine panel that is not a simple multiplication. The conversion is (mmol/mol divided by 10.929) plus 2.15, so 48 mmol/mol is 6.5% and 42 mmol/mol is 6.0%. UK reports usually print mmol/mol, US reports print the percentage, and some print both.
Why the numbers look like this
Glucose attaches to hemoglobin slowly and permanently, so the proportion of your hemoglobin carrying an attached glucose molecule reflects the average glucose that cell has been exposed to over its life. Red cells live around three months, which is where the window comes from. Every cell in circulation was exposed to the last few weeks, while only the oldest were around three months ago, which is why recent weeks weigh more heavily than distant ones.
Both consequences of that get used constantly in practice. Anything that changes how long red cells survive changes the HbA1c without glucose changing at all. A rise driven entirely by post-meal peaks, meanwhile, registers here while the morning fasting number stays exactly where it has always been, because by morning the glucose has come back down.
Most people who find this combination are at that early stage. It is the point where the numbers still come back down, and where what you change makes the most difference to where they go next.
Not being flagged is not the same as normal
The thresholds are not the same everywhere, and this is one of the few tests where that changes the label attached to you. US practice puts the prediabetes band at 5.7% to 6.4%, which is 39 to 47 mmol/mol. UK practice uses 6.0% to 6.4%, or 42 to 47 mmol/mol, for the same category. A result of 5.9% is therefore prediabetes on one side of an ocean and normal on the other, from the same blood. Before reading anything into where you sit, it is worth knowing which set of thresholds your report was interpreted against.
What else on the report can hide this
The hemoglobin and MCV printed above it matter here. Iron deficiency raises HbA1c, because red cells survive longer and glycate more, so an untreated iron deficiency can lift the result by a meaningful fraction of a percentage point with glucose entirely unchanged, and treating the iron brings it back down. That is a real and common way to be labeled prediabetic on a technicality.
The opposite happens whenever red cells turn over faster. Hemolysis, recent blood loss, late pregnancy, chronic kidney or liver disease and a recent transfusion all pull HbA1c down, and there the risk runs the other way: a result that looks fine while glucose is not. Hemoglobin variants are a third category. HbS, HbC and HbE trait interfere with some assay methods outright, so if your ancestry makes a variant likely and the HbA1c does not match the glucose, the assay is a fair suspect and the laboratory can say which method it used.
What usually causes it
Listed from most to least common — not from most to least serious.
- Very common
Glucose rising after meals while fasting control holds
The typical early picture of insulin resistance. Look for a raised waist measurement, raised triglycerides, a low HDL, fat in the liver on a scan, or a family history. The fasting number is usually the last one to go.
- Common
Iron deficiency
A low ferritin, often with small red cells. Raises HbA1c independently of glucose, so the result is worth rechecking once the iron has been replaced rather than acted on beforehand.
- Common
Ordinary analytical and biological variation
A value within a few tenths of the threshold that falls back below it on a repeat. HbA1c has real measurement variation, and a borderline result is a reason to test again rather than a diagnosis.
- Common
Corticosteroids
Steroids raise post-meal glucose disproportionately, which is exactly the shape that lifts HbA1c while leaving fasting glucose alone. Recent courses count, not only current ones.
- Common
Older age
HbA1c drifts up with age by roughly 0.1 percentage point a decade, independent of glucose, so a borderline result at 70 means slightly less than the same number at 40.
- Uncommon
A hemoglobin variant interfering with the assay — in people of African, Mediterranean, Middle Eastern or Southeast Asian ancestry
HbS, HbC, HbD or HbE trait. The effect depends entirely on which assay method the laboratory runs, so the laboratory is the place to ask.
- Rare
Anything that lengthens red cell survival
Splenectomy, and aplastic anemia. Cells live longer, accumulate more glucose, and the HbA1c overstates the average.
- Rare
Autoimmune diabetes developing in an adult
A lean adult with none of the metabolic risk factors, whose numbers climb faster than insulin resistance would explain. GAD antibodies and a C-peptide are the tests, and it is the reason the ketoacidosis symptoms are listed among the red flags.
What is usually checked next
- Oral glucose tolerance test, or a repeat HbA1c after three months The tolerance test is the one that catches post-meal rises the fasting number cannot see, which is the hypothesis behind this pattern.
- Ferritin with a full blood count Establishes whether iron deficiency is lifting the HbA1c before anyone acts on the number.
- Liver panel, lipids and a waist measurement Fills in the rest of the metabolic picture. Fat in the liver frequently shows up before any glucose abnormality does.
- Two weeks of glucose readings taken one to two hours after meals Shows the peaks directly if you have access to a meter, and it turns an abstract number into something you can see happening.
- Fructosamine or glycated albumin A two-to-three-week average that does not depend on red cells, for the cases where the HbA1c itself cannot be trusted.
When to seek care sooner
- Emergency Deep rapid breathing, vomiting, abdominal pain, or a sweet smell on the breath
- Same day Heavy thirst, passing large volumes of urine, blurred vision, or unexplained weight loss
- Soon An HbA1c at or above 6.5% (48 mmol/mol) confirmed on a repeat
- Soon Pregnancy with a raised HbA1c: the thresholds and the urgency are both different
- Soon New numbness or tingling in the feet, or a foot wound that is not healing
Questions worth bringing to your appointment
- Which thresholds was this result interpreted against, and where does it sit against them?
- Has my ferritin been checked, and could iron deficiency be raising the HbA1c?
- Would a glucose tolerance test tell us more than repeating the HbA1c?
- Is there anything about my red cells or my ancestry that would make this test less reliable for me?
- If this is early insulin resistance, what would you want to see change before the next test?