Finding the right tests…
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Finding the right tests…
Glucose
A fasting blood sugar test measures the glucose in your blood after you have gone without food or any drink except water for at least eight hours, usually overnight. Unlike a glucose tolerance test, this is a single draw — there is no sugary drink to swallow and no second or third sample taken an hour later. Your doctor asks for it on its own, either at a routine checkup or to look at your baseline glucose before deciding whether a fuller glucose tolerance series is needed. Because nothing you recently ate is still being digested, the result reflects how steady your blood sugar stays between meals, not how your body handles a sugar load.
This test reports the amount of glucose, in milligrams per decilitre, in your blood plasma at the moment of the draw. Because you have fasted, the result reflects your baseline glucose control — how well insulin and your liver keep sugar steady between meals — rather than your response to a meal or a glucose drink. A single vial of blood, usually taken from a vein in your arm, is enough. The lab report lists your value against the standard reference range for a fasting sample, which your doctor will interpret alongside your history.
A fasting glucose test is often the first check a doctor reaches for when screening someone who has no symptoms but has risk factors for diabetes — a family history, extra weight around the waist, or a sedentary routine. It is also the test used to keep track of blood sugar in someone already diagnosed with diabetes, since repeating it every few months shows whether diet, exercise, or medicine are keeping glucose under control.
A fasting value that comes back higher than expected does not confirm diabetes on its own. Guidelines call for a second fasting sample on a different day, or a follow-up test such as an oral glucose tolerance test or HbA1c, before a diagnosis is made. A fasting number below the normal range, on the other hand, points toward hypoglycemia, which can happen with certain medicines, prolonged fasting, or some hormonal conditions.
Because the result depends on true fasting, anything that breaks the eight-hour window — a sip of tea, a stick of gum, even sweetened black coffee — can push the number up and blur the picture your doctor is trying to see.
A fasting glucose of 70 to 99 mg/dL is generally considered normal, based on CDC and MedlinePlus reference ranges. A reading of 100 to 125 mg/dL suggests prediabetes, and 126 mg/dL or higher on two separate tests points to diabetes. Ranges can vary slightly by lab, so always check the range printed on your own report.
You need to avoid all food and drink except plain water for at least eight hours before the sample is taken, which is why most people schedule this test first thing in the morning. Even a small snack or a sweetened drink during this window can raise your reading.
Yes, plain water does not break your fast and won't affect the result. Whether to take your regular medicines beforehand is a question for your doctor, since some drugs are best taken after the blood draw.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
No. Guidelines generally call for the fasting test to be repeated on a different day, or confirmed with another test such as an oral glucose tolerance test or HbA1c, before diabetes is diagnosed.
A fasting test is a single blood draw and stops there. A glucose tolerance test also starts with a fasting draw, but then you drink a glucose solution and give one or more further samples over the next few hours to see how your body handles the sugar load.
A fasting test gives a cleaner baseline because no recent food is affecting the result, which makes it more reliable for screening and diagnosis. A random test is used instead when someone needs an immediate check regardless of when they last ate.
Yes. Acute illness, infection, and significant stress can all temporarily raise blood sugar, even in someone who does not have diabetes. If you're unwell on the day of testing, it's worth telling your doctor before they interpret the result.