Blood sugar, or glucose, serves as the fuel your body uses to generate energy. The level of glucose in your blood remains fairly stable, slightly rising after eating and declining a small amount between meals or after exercising. Blood glucose can be measured in many ways. Some tests measure glucose directly, while others measure the amount of glucose that is attached to a specific protein.
Random Blood Glucose
The average level of glucose in the bloodstream of a normal adult ranges from 70 to 130 mg/dL before meals and less than 180 milligrams per deciliter, or mg/dL, 2 hours after starting a meal, according to the American Diabetes Association. A random blood glucose test can be done at any time. Therefore, the amount of glucose in the blood varies, depending on when you last ate. People with diabetes typically check their glucose level several times each day to determine if their level is too far above or below the average range, enabling medication adjustments as needed.
Fasting Blood Glucose
A fasting blood glucose blood test allows doctors to assess your level without the variation caused by eating. Eating increases your blood sugar levels by varying amounts, depending on the amount and composition of the meal and how long ago you ate. Fasting is defined as no food or drink for at least 8 hours before having your blood drawn for the test. According to the American Diabetes Association, an average range for normal fasting blood glucose is 70 to 99 mg/dL. A fasting blood glucose blood test is one of several tests used to diagnose diabetes.
The blood test A1c -- more specifically called glycated hemoglobin A1c -- is another way to determine your blood sugar level. Instead of determining the amount of blood sugar present at the time of the test, this method determines the average blood sugar level over an extended period, typically the past few months. This is done by measuring glycated hemoglobin, which is formed when sugar attaches to the hemoglobin protein found in red blood cells. The percent of hemoglobin with sugar bound to it depends on the amount of sugar in your bloodstream. The more sugar in your blood, on average, the higher the percentage of glycated hemoglobin in your bloodstream. The normal range for A1c is about 4.0 to 6.0 percent. The American Diabetes Association identifies people with an A1c result from 5.7 to 6.4 percent as having an increased risk for diabetes.
The estimated average glucose, or eAG, blood test is similar to the A1c blood test, except the eAG is reported in the same units that most people are used to seeing for typical blood sugar blood tests. The A1c result is converted from percent glycated hemoglobin to the estimated average blood glucose in mg/dL. For example, the normal A1c range of 4.0 to 6.0 percent is converted to an eAG of 68 to 126 mg/dL. However, an eAG result should not be compared to fasting or random blood glucose results, as eAG is an average of glucose levels over time.
- Tietz Textbook of Clinical Chemistry and Molecular Diagnostics; Carl Burtis, et al.
- Diabetes Care: Standards of Medical Care in Diabetes -- 2014
- Clinical Chemistry: Theory, Analysis, and Correlation; Lawrence Kaplan, et al.