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.

Your HbA1c number isn't just about your average blood sugar — it also depends on how long your red blood cells live. The longer they stick around, the more sugar builds up on them, which can raise your HbA1c even if your blood sugar is stable.

If your red blood cells live longer than usual, your A1C test might show high blood sugar even if your actual sugar levels are normal.

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 with type 2 diabetes has good HbA1c numbers but their red blood cells don’t live as long, their blood sugar might actually be higher than the test shows — meaning the test could be giving them a too-rosy picture of their sugar control.

If your red blood cells live longer or shorter than average, your A1C test might not give a true picture of your average blood sugar levels.