HbA1c and fasting glucose, explained

6 min read · Last updated 21 May 2026

Lab results often read like a foreign language: two numbers, two units, and no note on which one matters more. The easiest way to hold them apart: one is a photograph, the other is the album.

Fasting glucose: this morning's photograph

Measured after at least eight hours without food, in mg/dL. It describes that moment only, and it moves with last night's dinner, your sleep, stress, and any illness you happen to have.

• Under 100 mg/dL: normal.
• 100–125 mg/dL: prediabetes.
• 126 mg/dL or above: points to diabetes, a second test is needed to confirm.

There is also the two-hour reading, after a meal or a glucose drink: under 140 mg/dL is normal, 140–199 is prediabetes, 200 or above points to diabetes.

HbA1c: the last three months

HbA1c measures the share of haemoglobin bound to glucose. Because red blood cells live around three months, the figure summarises your average blood sugar over 2–3 months, it cannot be "fixed" by fasting the day before the test.

• Under 5.7%: normal.
• 5.7–6.4%: prediabetes.
• 6.5% or above: points to diabetes.

For people already diagnosed, a common treatment target is below 7%, but the number is individual: tighter for younger people, looser for older people or anyone with frequent hypoglycaemia. Your doctor sets it.

When HbA1c can mislead

Since it is read from red blood cells, anything that affects them affects the result: anaemia, iron deficiency, pregnancy, haemoglobin disorders such as thalassaemia, chronic kidney failure, or a recent transfusion. In those situations doctors lean on direct glucose readings instead.

What is worth logging yourself

If you own a meter, three pairs of readings a week already reveal a pattern: fasting in the morning, and two hours after one meal you want to test. Write down what you ate as well; a number without context is nothing to work with at your appointment.