A1C, Prediabetes and When to Test: A Plain-English Guide

What blood glucose is

Glucose is the main sugar in the blood and the body's main fuel. It comes from food and from the liver, which releases it between meals. Insulin, a hormone from the pancreas, lets cells take glucose in and keeps blood levels in a narrow range ^[1]. When cells respond poorly to insulin, a state called insulin resistance, the pancreas compensates by making more. Over time, if it cannot keep up, blood glucose rises into the prediabetes and then the diabetes range ^[2].

The three common tests

Doctors use three tests to check blood glucose status ^{[3] [2]}:

• A1C measures the share of hemoglobin in red blood cells that has glucose attached. Because red cells live about three months, it reflects average glucose over roughly that period. No fasting is needed.
• Fasting plasma glucose measures glucose after at least eight hours without food, usually in the morning.
• Oral glucose tolerance test measures glucose two hours after drinking a standard sugar solution, showing how well the body handles a glucose load.

Where the lines are

These are the thresholds NIDDK publishes ^{[3] [2]}. A diagnosis of diabetes normally needs a confirming test, and conditions such as anemia or certain hemoglobin variants can make A1C less reliable, so results are interpreted by a clinician.

What prediabetes means

Prediabetes means glucose is higher than normal but not yet in the diabetes range. It is common and often silent: many people with it do not know. It raises the risk of developing type 2 diabetes and of heart disease, but it is also the stage at which change is most effective. In the Diabetes Prevention Program, people with prediabetes who lost about 7% of their weight and exercised 150 minutes a week cut their risk of developing diabetes by 58% over about three years ^[4].

Who should be tested

NIDDK lists risk factors that make testing worthwhile: being overweight, being 35 or older, having a parent or sibling with type 2 diabetes, being physically inactive, having had diabetes during pregnancy or a baby over nine pounds, having polycystic ovary syndrome, high blood pressure or abnormal cholesterol, and belonging to certain ethnic groups with higher rates of diabetes ^[2]. If any apply, ask your clinician about testing; if results are normal, repeat testing is usually recommended every few years.

Symptoms, and why waiting for them is a mistake

High blood glucose often causes no symptoms at all until it is well into the diabetes range. When symptoms come, they include increased thirst and urination, hunger, tiredness, blurred vision, slow-healing sores and unexplained weight loss ^[1]. Waiting for these signs means missing the prediabetes stage, which is why testing based on risk factors matters more than symptoms.

Home meters versus lab tests

Home glucose meters measure glucose in a drop of blood from a fingertip. They are essential for people who manage diabetes with medicine, and useful for anyone who wants to see how meals or activity affect their glucose. They are less accurate than lab tests and are not used to diagnose diabetes.

If you use a meter to judge a supplement, be consistent: same time of day, same conditions, same meter and strips. Compare weekly averages rather than single readings, because readings vary from day to day.

Why A1C takes three months

Because A1C reflects about three months of glucose, a change in habits or medicine takes weeks to show and about three months to show fully ^[3]. That is why clinicians usually recheck A1C every three to six months, and why trials of glucose treatments last at least twelve weeks. It is also why no supplement can be judged on A1C within a 60-day refund window.

Low blood sugar, for people on medicine

People who take insulin or certain diabetes pills can have glucose that drops too low, usually defined as below 70 mg/dL. Symptoms include shakiness, sweating, hunger, irritability, confusion and a fast heartbeat. NIDDK explains how to treat it with fast-acting carbohydrate and recheck ^[5]. Anyone on these medicines who adds a supplement that may lower glucose should agree a monitoring plan with their prescriber.

What to do with your result

Questions to ask at your appointment

If you are getting tested, a few questions make the visit more useful:

• Which test are we using, and do I need to fast?
• If my result is in the prediabetes range, is there a lifestyle program you recommend?
• How often should I be retested?
• If I want to try a supplement, is there anything in it that could interact with my medicines?
• If I already take glucose-lowering medicine, what readings should prompt me to call you?

Bring a list of everything you take, including supplements. NIH recommends sharing it with every clinician you see ^[7].

If your result is borderline

A single result near a threshold, such as an A1C of 5.6% or 5.7%, is a prompt to retest and to start the habits that help, not a reason for alarm. Lab results vary a little from test to test, and a repeat measurement, sometimes with a different test, gives a clearer picture ^[3]. Borderline is also the point where everyday changes pay off most, as the Diabetes Prevention Program showed ^[4].

Bottom line

Know your numbers before you buy anything for blood sugar. A1C and fasting glucose are cheap, standard tests, and they tell you whether you need a clinician, a habit change or nothing at all. If you then decide to try a supplement, our results timeline shows how to judge it fairly.