Two HbA1c Results That Disagree
Before treating a difference as a change in control, check the two most boring explanations: whether the results came from the same laboratory, and whether both were reported in the same units. The two scales in use produce numbers that bear no resemblance to each other, so a value that seems to have leapt can be the same measurement written another way. After that, a genuine difference has to exceed what the test varies by on its own before it means anything.
The pattern on your report
- HbA1c High Key
- Fasting glucose High-normal Key
- Hemoglobin Normal Key
- Ferritin Low Key
Printed as: Fasting glucose in mmol/Lor mg/dLFerritin in ug/Lor ng/mL— Iron deficiency lengthens red cell survival and raises the HbA1c, so correcting it lowers the number with no change in glucose.Hemoglobin in g/Lor g/dLHbA1c in mmol/molor %— Two scales are in use and their numbers bear no resemblance to one another, which makes mixing them up a frequent source of alarm.
Why the numbers look like this
Every measurement carries two sources of variation. The analyzer contributes a small amount, and modern methods for this test are tightly standardized, so that part is minor. The person contributes more, because glucose control genuinely fluctuates and because the red cells being sampled turn over.
Combining those gives a threshold below which a difference is noise. Small movements in either direction between two tests are expected in someone whose control has not changed at all.
Beyond that, several things shift the value without any change in glucose. Anything that shortens red cell survival lowers it; anything that lengthens survival raises it. Iron deficiency does the second and is common enough to explain a rise that puzzles everyone.
Laboratories run different methods too, and although they are calibrated to a common standard, a hemoglobin variant can interfere with one platform while leaving another alone. That produces two results that are genuinely different because they measured different things.
The timing matters in a way people rarely account for. The test looks back roughly three months and weights recent weeks more heavily, so two samples six weeks apart share much of the same period and should not differ greatly.
Not being flagged is not the same as normal
Two reporting scales are in use, and a report may give one or both. Because the numbers look so different, mixing them up is a frequent source of alarm, and it is worth confirming which scale each result was in before anything follows. Comparisons are also more reliable within one laboratory than across two, so where a trend matters it is worth using the same one consistently rather than combining a hospital result with a private test.
What else on the report can hide this
Check the units and the laboratory first. Between them these explain a large share of apparent disagreements.
The interval matters next. Two tests close together sample overlapping periods and should agree; two a year apart legitimately differ.
Iron studies with a blood count come next, because iron deficiency raises the value and correcting it lowers the value again, both with glucose unchanged. This is the commonest biological explanation for a puzzling rise.
Anything that shortens red cell life pulls in the other direction: recent blood loss, a transfusion, hemolysis, or a hemoglobin variant. Where a variant is suspected, the laboratory can say whether its method is affected.
Where the disagreement persists and matters, glucose measured directly resolves it. Home readings or continuous monitoring answer the underlying question without depending on red cells at all.
What usually causes it
Listed from most to least common — not from most to least serious.
- Very common
Two different reporting scales
The two units look nothing alike for the same result. Confirming which scale each was in resolves a large share of these.
- Very common
Two different laboratories
Methods differ, and comparisons are more reliable within one laboratory. Combining a hospital result with a private test invites this.
- Very common
Ordinary variation
Small movements in either direction are expected in someone whose control has not changed. A difference must exceed that before it means anything.
- Common
Iron deficiency
Lengthens red cell survival and raises the value, and correcting it lowers the value again. The commonest biological reason for a puzzling rise.
- Common
A genuine change in control
Diet, weight, activity, illness or a medication change over the intervening period. This is what the test is for.
- Common
Recent blood loss or transfusion
Changes the age mix of circulating cells and therefore the value, without any change in glucose.
- Uncommon
A hemoglobin variant
Can interfere with one platform and not another, so two laboratories genuinely measure different things. The laboratory can say whether its method is affected.
- Uncommon
Reduced kidney function
Introduces errors in both directions, so results become less reproducible as filtration falls.
- Uncommon
A transcription error
Worth considering when a value is wildly out of keeping with everything around it. Checking the original report is quicker than repeating.
What is usually checked next
- Confirm the units and the laboratory for each result Between them these explain a large share of apparent disagreements, and neither needs a test.
- The interval between the two samples Tests close together sample overlapping periods and should agree; a year apart they legitimately differ.
- Full blood count and iron studies Iron deficiency raises the value and its correction lowers it, with no change in glucose at all.
- A history of blood loss, transfusion or known variant Each changes what the test is measuring rather than what it is measuring about.
- Glucose measured directly Home readings or continuous monitoring answer the underlying question without depending on red cells.
When to seek care sooner
- Emergency Vomiting with abdominal pain and rapid breathing
- Same day Excessive thirst with passing large volumes of urine and weight loss
- Same day Blurred vision that has changed quickly
- Same day A foot ulcer, or a wound that is not healing
- Soon A large unexplained rise between two results
- Soon Sweating, shakiness or confusion relieved by eating
Questions worth bringing to your appointment
- Were both results in the same units?
- Did both come from the same laboratory?
- How far apart were the two samples?
- Has my iron been checked?
- Is the difference large enough to mean anything?