The hemoglobin A1c test, usually referred to simply as the "A1c," can be accurately translated into our average blood glucose (BG) levels for the past three months or so, right?
On average, you can produce formulas that estimate the average BG levels for the average patient, but these formulas may not work for everyone.
The A1c test assumes a lot of thing. For one thing, it assumes we have normal types of hemoglobin. For another, it assumes our red blood cells (RBCs) live an average of 120 days.
People with unusual hemoglobin types, for example those with sickle cell anemia or thalassemia, or those who keep producing fetal hemoglobin even as adults, may have A1c results that don't jibe with their meter readings. This is because these less common hemoglobin variants, as well as other rarer variants, although they work to transport oxygen in the blood, produce slightly different varieties of glycated hemoglobin that react differently in the A1c tests.
People whose RBCs live longer or for shorter times than average may also have A1cs that don't agree with their meter readings. One study showed that the variation in RBC lifetime in normal people was sufficient to cause "clinically important" differences in A1c levels.
Anything that reduces RBC lifetime, like losing blood, will artificially decrease your A1c level. Hence people with hemolytic anemias (anemias that destroy RBCs) will have lower A1cs than they expect. Anything that increases RBC lifetime, like losing your spleen (the spleen is where most of the elderly RBCs are destroyed), will result in higher A1cs than expected.
One surprising factor that affects the A1c is another type of anemia, iron-deficent anemia. For some reason, having iron-deficient anemia increases your A1c, and treating the anemia with iron reduces it. It's not clear exactly why. One recent finding is that the increase in A1c often found in late stages of pregnancy may be caused by iron deficiency rather than by increases in blood sugar levels, and treating the anemia makes the A1c go down.
When your A1c results don't agree with your meter readings, your doctor often assumes you're not testing properly, or you're testing at the wrong times, and believes the A1c results. But if you're testing at different times of day and your results still don't agree with your A1c, you should research the many factors that can affect the A1c to see if you can pinpoint the problem in your particular case.
If you want to know more about iron, see here.