Bio LabFasting blood sugar vs HbA1c
Reviewed by Dr. Sanaa Ghoneim, Medical laboratory specialist 21 October 2026
The key difference is simple: fasting blood sugar measures the glucose in your blood at the moment the sample is taken, after fasting, while HbA1c reflects your average blood sugar over roughly the past two to three months. That is why fasting blood sugar needs fasting and HbA1c usually does not. The two tests complement each other, and doctors often order them together.
Fasting blood sugar: a snapshot of today
The fasting blood sugar test measures glucose in your blood after a period without food, usually 6 to 8 hours. Fasting matters because eating raises blood sugar for a while, so a recent meal would make the reading less useful.
The result describes the day of the draw. That means it can be affected by things such as poor sleep, stress, illness, or a fasting period that was not followed properly.
Preparation:
- Usually 6 to 8 hours without food, with water allowed.
- No tea, coffee, juice or chewing gum.
- If you take diabetes medicines or insulin, ask your doctor before the test day whether to take your morning dose before or after the sample, and do not stop your treatment on your own. If you feel signs of low sugar while fasting (shaking, sweating, dizziness), stop fasting, have something sugary, and tell us.
HbA1c: an average over a longer period
Glucose in the blood attaches to the hemoglobin inside red blood cells. Red cells live for a fairly long time, so measuring this share gives a picture of your average blood sugar over roughly the past two to three months.
Its advantage is that a single meal or a single day has little effect on it. However, some conditions can affect the reading, such as anemia, a recent blood transfusion or blood loss, or pregnancy, so tell your doctor about any of these.
Preparation:
- Usually no fasting needed, and it can be done at any time.
- If fasting blood sugar or other fasting tests are ordered with it, follow the fasting instructions for those.
Why your doctor may order both
Each test answers a different question. Fasting blood sugar shows how your sugar is on the day of the draw, and HbA1c shows how it has been over the recent period. Together they give your doctor a clearer picture, whether checking for diabetes or following up treatment. Your doctor decides which tests you need and how often to repeat them.
If both tests are ordered together
The simplest plan is to fast and have the sample taken in the morning, so both tests come from the same draw. We confirm the tests, fasting and timing with you on WhatsApp before the visit, and a technician takes the sample at your home.
About your results
Discuss the results of both tests with the doctor who ordered them, who can explain them in light of your health and decide on the next step.