Blood Sugar Tests: Fasting Glucose and A1c

Blood sugar tests measure fasting glucose and hemoglobin A1c (HbA1c) to screen for and monitor diabetes and prediabetes. Fasting glucose reflects blood sugar at one moment, while A1c shows average blood sugar over the past two to three months, helping assess overall control over time.

What blood sugar tests cover

This panel typically includes two markers that give a picture of blood sugar control from different angles.

• Fasting glucose (FBS) measures the amount of sugar in the blood after not eating for at least 8 to 12 hours. It shows blood sugar at a single point in time and is often used as a first screening step.
• A1c (hemoglobin A1c) measures the percentage of hemoglobin that has sugar attached to it. Because red blood cells last about two to three months, A1c reflects average blood sugar over that period rather than a single moment, and it does not require fasting.

Why clinicians order these tests

Clinicians often order fasting glucose (FBS) and A1c (hemoglobin A1c) together as part of a routine checkup, especially for adults over 45 or those with a family history of diabetes, obesity, or metabolic syndrome. These tests can also be ordered when someone has symptoms that may be linked to high or low blood sugar, such as increased thirst, frequent urination, unexplained weight change, or fatigue. For people already diagnosed with diabetes or prediabetes, repeat testing helps monitor how well blood sugar is being managed over time and whether current lifestyle changes or medicines are having the desired effect.

How to read your results

Reference ranges can vary somewhat between laboratories, so always check the specific range printed on your report. For adults, a fasting glucose (FBS) result is generally considered in the normal range between 70 and 99 mg/dL, with fasting glucose from 100 to 125 mg/dL described as the prediabetes range, and 126 mg/dL or higher, confirmed on repeat testing, described as the diabetes range. For A1c (hemoglobin A1c), a level below 5.7% is generally considered normal, 5.7% to 6.4% falls in the prediabetes range, and 6.5% or higher, confirmed on repeat testing, is in the diabetes range.

Because A1c reflects average blood sugar over the past two to three months while fasting glucose reflects a single point in time, looking at both markers together, alongside the units used (mg/dL or mmol/L for glucose, percent for A1c), can give a fuller picture than either result alone. The table below this article lists the specific ranges and bands for each marker.

Common patterns across these markers

When fasting glucose (FBS) and A1c (hemoglobin A1c) are both above the normal range, this pattern may be linked to prediabetes or diabetes, since both markers reflect aspects of blood sugar control. A fasting glucose result in the prediabetes or diabetes range with a normal A1c, or the reverse, can happen and may be linked to timing of the test, recent illness, certain medicines, or day-to-day variation in blood sugar; this pattern is often followed up with repeat testing rather than treated as a single definitive result.

Both markers moving higher together over time, especially alongside weight gain, high blood pressure, or unusual thirst or urination, may be linked to metabolic syndrome or developing diabetes and is generally discussed with a clinician. These patterns are not diagnostic on their own and depend on the full clinical picture, including symptoms and other test results.

Preparing for blood sugar testing

Fasting glucose (FBS) usually requires not eating or drinking anything except water for at least 8 to 12 hours before the blood draw, so this test is often scheduled in the morning. A1c (hemoglobin A1c) does not require fasting and can be done at any time of day. It can help to mention any medicines, supplements, or recent illnesses to the clinician or lab, along with any planned or recent changes in diet, since these can sometimes affect results.

Next steps

Any result outside the expected range on a blood sugar test is worth discussing with a healthcare provider, who can consider it alongside symptoms, other lab results, and personal and family health history. Seek urgent care if you experience symptoms of very low or very high blood sugar, such as confusion, fainting, rapid breathing, or extreme thirst combined with vomiting. Uploading a lab report to MediWing can help translate these results into plain language and track how fasting glucose and A1c change over time, though it does not replace medical advice.