HbA1c shows how much sugar has been sticking to your blood cells over the past few months — it's like a long-term blood sugar report card.

If your red blood cells live longer, they soak up more sugar over time, which can make your HbA1c blood test go up — even if your average blood sugar hasn't actually changed.

If your body is healthier and your red blood cells live longer, your HbA1c blood sugar test might show higher levels — not because your blood sugar is high, but because the sugar has more time to stick to older red blood cells.

If someone has certain blood conditions like sickle cell trait, their A1c test might show lower blood sugar levels than they actually have, which could trick doctors into thinking diabetes is under control when it's not — especially if they don't also check regular blood sugar tests.

Some people with chronic inflammation and certain blood and pancreas conditions might have normal blood sugar readings on standard A1c tests, even though their actual blood sugar is high — because inflammation makes red blood cells die faster, so there's less time for sugar to stick to them.