In people with a blood disorder who need regular transfusions, the common blood sugar test (HbA1c) often says they have high blood sugar, but their fasting blood sugar test says it's normal — so the two tests don’t match up well.

People with a blood disorder called thalassemia who need regular blood transfusions often show high blood sugar on HbA1c tests, but their actual blood sugar is normal—so the test might be giving false alarms in these patients.

In people with a blood disorder called thalassemia who need regular blood transfusions, their long-term blood sugar marker (HbA1c) goes up when their daily fasting sugar goes up — so even though their HbA1c is usually high, it still gives some clue about their blood sugar changes.

People with a serious blood disorder called thalassemia who need regular blood transfusions have about the same fasting blood sugar as healthy people, even though their long-term blood sugar marker (HbA1c) is high.

Older people with a blood condition called thalassemia who need regular blood transfusions tend to have higher HbA1c levels — a number often linked to blood sugar — even if their actual blood sugar isn't high.

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.