DEAR DR. ROACH: I had an eye issue last year that led to blood tests resulting in an elevated A1C.

I said that it wasn’t my blood; I am plant-based, exercise regularly and don’t drink sodas or alcoholic beverages. Of course, they wanted to start me on medication for prediabetes, but I opted to see if I could fix it with dietary changes first.

I was told to limit bread, pasta and rice, among other things vegans eat, and proceeded to lose a lot of weight. (I don’t have much to spare.) During my recent blood test, my A1C was still high at 6.1%. Looking into it a bit further, my physician’s assistant realized that low iron can impact A1C readings, and I have always had low iron. I take a supplement almost daily with an orange.

Please tell your readers about this so as not to scare them into medicine that they might not need. There still may be something going on with me, and I will be getting more tests to be sure that this can be corrected.

But I am somewhat relieved that it is not prediabetes.

— M.K.

ANSWER: The A1C level measures the amount of sugar that is found on red blood cells. For most people, it’s an excellent indication of how high their blood sugar has been over the past few months. Half of red blood cells die after 60 days in most people, so the normal ranges of A1C are appropriate for people who have red blood cells that live a normal life.

Several conditions lead to red blood cells that live much shorter than this. Sickle cell disease, autoimmune hemolytic anemias and severe heart valve disease are among the diseases that have markedly shortened the lifespan of red blood cells. This can cause a person’s A1C level to be falsely low. A person with poorly controlled diabetes may have a normal A1C because the red blood cells don’t live long enough to accumulate as many sugar molecules on it.

People with iron deficiency have slower red blood cell turnover or longer-lived red blood cells. This means that there is more time to accumulate sugar and a falsely elevated A1C level.

Before we had the A1C test, we had blood sugar levels and glucose tolerance tests to make the diagnosis of prediabetes and diabetes. In people with abnormalities in their red blood cells, these are more reliable than the A1C level.

Readers may email questions to ToYourGoodHealth@med.cornell.edu.

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