What can cause a false or misleading A1c reading?

A1c is a reliable test for most people most of the time, but it isn't infallible. Certain conditions and circumstances can make the number read higher or lower than your true average glucose — which matters if your A1c doesn't match what your glucose meter or CGM has been showing.

Conditions affecting red blood cells

Because A1c depends on hemoglobin inside red blood cells, anything that changes how long those cells circulate can distort the result:

• Anemia and blood loss — can falsely lower A1c, since newer red blood cells haven't been exposed to glucose as long.
• Recent blood transfusion — introduces donor red blood cells with a different glucose history, skewing the result temporarily.
• Hemoglobin variants (such as sickle cell trait) — can interfere with some A1c lab methods, sometimes requiring an alternative test.
• Kidney disease — can shorten red blood cell lifespan, generally lowering A1c relative to true average glucose.

Other situations worth knowing about

• Pregnancy — red blood cell turnover changes during pregnancy, and A1c is generally considered less reliable for diagnosis in this context; other tests are typically used instead.
• Recent rapid glucose changes — A1c reflects an average, so it can look "normal" even alongside significant swings between highs and lows.
• Certain supplements (high-dose vitamin C or E) — have been reported to interfere with some A1c testing methods in some studies.

What to do if something seems off

If your A1c consistently doesn't match what your device-measured glucose has been showing, that's worth raising with your clinician directly rather than trying to reconcile it yourself — they can determine whether an alternative test (like fructosamine, which reflects a shorter timeframe) is more appropriate for your situation.