HbA1c & Blood Sugar Testing Explained

Two blood tests dominate conversations about blood sugar: fasting glucose and HbA1c. They answer different questions, which is why providers frequently order them together. This guide explains what each test measures, why the HbA1c reflects roughly three months of average blood sugar, and how preparation differs between the two.

What HbA1c actually measures

Hemoglobin is the oxygen-carrying protein inside red blood cells. Glucose in the blood naturally attaches to hemoglobin at a slow, steady rate, forming glycated hemoglobin, measured as HbA1c and reported as a percentage. Because red blood cells live about three months, the percentage reflects your average blood sugar exposure over that period rather than any single day.

Fasting glucose versus HbA1c

Fasting glucose is a snapshot: how much glucose is circulating at that moment, after 8 to 12 hours without calories.
HbA1c is an average: it smooths over day-to-day swings and does not require fasting.
Used together, they show both the moment and the trend, which helps providers understand your overall picture.

Preparation for each

HbA1c requires no fasting and can be drawn any time. Fasting glucose requires the usual 8 to 12 hour fast with water allowed, so when both are ordered together, follow the fasting rules and schedule a morning draw. Stay hydrated, take medications as your provider advises, and mention anything unusual such as recent illness, which can temporarily influence readings.

Why units differ between reports

HbA1c is reported as a percentage in most of North America and in mmol/mol in some other countries, while glucose appears in mg/dL or mmol/L depending on the laboratory. Compare your values only to the units and reference ranges printed on your own report, and keep past reports so trends are easy to see at your next appointment.

Factors that can skew each test

Both tests have known blind spots worth knowing about. Fasting glucose reflects only that single morning, so a stressful night or a late meal the evening before can shift it. HbA1c depends on red blood cell lifespan, so conditions such as iron deficiency anemia, recent blood loss, or certain hemoglobin variants can distort the average in either direction. Neither issue makes the tests unreliable; it just means results belong in a conversation with a provider who knows your history.

Ready to check your numbers?

Compare testing options and book a convenient location near you.

Find a Lab Near You

Frequently asked questions

Does the HbA1c test require fasting?

No. HbA1c reflects your average blood sugar over roughly three months and can be measured any time of day without fasting.

What is the difference between HbA1c and fasting glucose?

Fasting glucose is a moment-in-time reading after 8 to 12 hours without calories, while HbA1c is a longer-term average of blood sugar over about three months. Providers often order both.

How often should HbA1c be checked?

It depends on your health status and your provider's plan. Many people with normal results are checked periodically at routine visits, and more frequent testing is used when monitoring changes.

Can anemia affect HbA1c results?

Conditions that affect red blood cells, such as certain anemias, can shift HbA1c values. Tell your provider about any blood conditions so they can interpret the result correctly.

Medical disclaimer: This guide is for general education only and is not medical advice. Test preparation and reference ranges vary by laboratory and by person. Always follow the instructions from your ordering provider and the testing location, and consult a qualified healthcare professional about your results.

Affiliate disclosure: MyLabService may earn a commission when you book a test through links on this page, at no extra cost to you. This never affects our editorial content.

HbA1c and Blood Sugar Testing