Normal Fasting Blood Sugar and A1C: What the Numbers Mean

In the 1960s, biochemist Samuel Rahbar was examining blood from people with diabetes when he noticed an abnormal hemoglobin. Hemoglobin is the protein that gives red blood cells their color and carries oxygen. In 1968 he described the finding in a three-page paper in the journal Clinica Chimica Acta, titled "An abnormal hemoglobin in red cells of diabetics."

It was later understood that this was hemoglobin with glucose attached to it: the more sugar circulates in the blood over time, the more hemoglobin becomes "sugar-coated." That was the beginning of glycated hemoglobin, or A1C (HbA1c), now used to diagnose and monitor diabetes.

A1C often comes on the same lab order as the older fasting glucose test, and that is where the questions start. What is a normal fasting blood sugar? What does an A1C of 5.9% mean? And why do the two tests sometimes disagree?

What does a fasting blood sugar test measure?

It measures the amount of glucose in your blood at one moment: in the morning, after a night without eating. Glucose is the cells' main fuel, and its level is controlled by insulin, a hormone made by the pancreas that helps sugar leave the blood and enter the body's tissues.

After several hours without food, the body should keep glucose within a steady range. If it stays high even while fasting, insulin is not keeping up, either because the pancreas makes too little or because the body does not respond to it well. Brazil's Ministry of Health describes diabetes in just those terms: insufficient insulin production, or poor use of the insulin the body makes.

The weak point of this test is that it is a snapshot. A bad night's sleep, an infection, a stressful stretch or certain medications can shift that day's result. The SBD guideline notes that fasting glucose is affected by short-term conditions and is less reproducible than A1C.

What is A1C and what does it show?

If fasting glucose is a snapshot, A1C is closer to a film. It shows what percentage of the hemoglobin in your red blood cells is bound to glucose. Because that binding builds up over the life of a red blood cell, the result reflects your average blood sugar over the past two to three months, according to the SBD.

That is why it does not change because of a big dinner the night before, and why it does not require fasting. The American Diabetes Association (ADA) lists that convenience as one of the test's advantages.

A1C has limits too. According to the SBD guideline, it costs more, it does not account for the fact that glycation varies from person to person, and it is less sensitive than other methods for diagnosis. The lab must use a standardized, certified method. And the result can be thrown off in people with anemia, hemoglobinopathies (inherited variations in hemoglobin), chronic kidney disease, a recent blood transfusion, pregnancy and the use of some medications. In those cases, the doctor chooses another way to investigate.

What are the normal, prediabetes and diabetes ranges?

The values below are the criteria in the Brazilian Diabetes Society guideline (2026 edition) for adults. The ones for fasting glucose, A1C and the 2-hour glucose tolerance test match those of the ADA. They are used for diagnosis and screening. Pregnant women follow their own testing schedule during prenatal care, set with their doctor.

Test	Normal	Prediabetes	Diabetes
Fasting blood sugar	below 100 mg/dL	100 to 125 mg/dL	126 mg/dL or higher
Hemoglobin A1C (HbA1c)	below 5.7%	5.7% to 6.4%	6.5% or higher
Glucose tolerance test, 1 hour	below 155 mg/dL	155 to 208 mg/dL	209 mg/dL or higher
Glucose tolerance test, 2 hours	below 140 mg/dL	140 to 199 mg/dL	200 mg/dL or higher

The glucose tolerance curve, known in Brazil as the TTGO (oral glucose tolerance test), is the test in which you drink a solution containing 75 g of glucose and have blood drawn again afterward. The SBD guideline treats the 1-hour measurement as the preferred one and the 2-hour reading as an alternative. The test is ordered only when the doctor sees a reason for it.

One important note: reference values can differ slightly from lab to lab, depending on the method used, and your report shows the range that lab adopted. It is your doctor who interprets the result, along with your history and your other tests.

Does one abnormal result confirm diabetes?

No. According to the SBD, an abnormal test should be repeated to confirm it. The doctor decides when, and the repeat should not be put off. Diabetes can be diagnosed without a repeat test in two situations: when fasting glucose and A1C are both above the cutoff in the same blood draw, or when a person has the classic symptoms of high blood sugar and a random glucose, taken at any time of day, of 200 mg/dL or higher. If only one diabetes criterion stays abnormal and the others are normal, the guideline suggests close follow-up, with another look in about 6 months.

It is also common for the two tests to disagree. Someone can have a fasting glucose of 98 mg/dL and an A1C of 5.9%, or the reverse. They measure different things: one is the sugar level that morning while fasting, the other is a months-long average. The guideline itself notes that agreement between the two for diagnosing diabetes is only about 29% to 39% in population studies. That is one reason both tests are often ordered together.

And prediabetes is not a sentence. Brazil's Ministry of Health describes this stage as a warning from the body, one that still leaves room to change course. The SBD recommends re-testing people with prediabetes every 12 months, sooner (every 6 to 12 months) if the values sit very close to the diabetes threshold. The doctor sets the schedule.

How do you prepare for these tests?

• Fasting blood sugar: the ADA calls for at least 8 hours with nothing to eat or drink except water, and notes that the test is usually done early, before breakfast. Follow your lab's instructions.
• A1C: fasting is needed. If it is drawn together with a fasting glucose, the fasting rule for the glucose test applies.
• Medications: not stop anything on your own. If you take a medication every day, ask your doctor or the lab what to do on the day of the test.
• Glucose tolerance test (TTGO): preparation is different. In the 3 days before, your diet should include at least 150 g of carbohydrates a day, followed by a fast of 8 to 12 hours. The lab will give you the full instructions.
• Keep your usual routine for the other tests: eat as you normally do in the days before. Going on a "test diet" just to get a nicer number only distorts the picture.

Who should be tested, and how often?

The SBD guideline recommends screening for type 2 diabetes for everyone aged 35 or older. Below that age, testing is advised for adults who are overweight or have obesity and also have at least one more risk factor, such as high blood pressure, low HDL cholesterol or high triglycerides, a sedentary lifestyle, type 2 diabetes in a parent or sibling, polycystic ovary syndrome, prediabetes on an earlier test, or diabetes during pregnancy. Fasting glucose and A1C are the first tests, and the choice depends on what is available where you live.

When results are normal, the SBD advises repeating the tests every 12 months for people with 3 or more risk factors or a high or very high score on the FINDRISC risk questionnaire (which a doctor administers), and every 3 years for people with fewer than 3 factors. The doctor sets the schedule.

Type 2 diabetes often develops silently. The classic symptoms, according to the SBD, are excessive thirst, urinating very often and unexplained weight loss. But many people find out from a routine test, before they feel anything. What you eat matters here too: a simple plate with rice, beans, vegetables and good sources of dietary fiber is a good starting point, as we covered in our piece on the rice-and-beans diet. Any changes to your eating plan should be worked out with your doctor or a dietitian.

When to see a doctor

Take any result above the normal range to a doctor, even if you feel fine. Also book a visit if you have unusual thirst, urinate a lot or lose weight without explanation, or if you are 35 or older and have never been tested. A general practitioner or family doctor can order the tests and start the workup. Once a diagnosis is confirmed, or if the case is more complex, follow-up usually continues with an endocrinologist.

Some situations are emergencies. Intense thirst and heavy urination together with nausea or vomiting, belly pain, breathing that is deeper than normal, breath with a sweet, fruity smell, drowsiness or confusion can signal a serious complication of high blood sugar called diabetic ketoacidosis, more typical of type 1 diabetes or of a major loss of control. The UK's National Health Service (NHS) advises seeking emergency care. In Brazil, go to an emergency room or call SAMU, Brazil's emergency medical line, at 192, without waiting.

The bottom line: what that odd hemoglobin taught us

Samuel Rahbar was looking for something else when he noticed that unusual hemoglobin in the blood of people with diabetes. That detail turned into a way of seeing months of blood sugar in a single tube. That is what A1C adds to a fasting glucose test: one shows the day, the other shows the path. Together they help catch a problem early, when there is still plenty of room to act. Knowing your numbers and talking them over with a doctor is the step that counts.

Frequently asked questions

Is a fasting blood sugar of 99 mg/dL normal?

By the SBD criteria, yes: it is below 100 mg/dL. The doctor will still look at your A1C, your age and your risk factors to decide whether closer follow-up is worthwhile.

Can I drink coffee before a fasting blood sugar test?

No. Fasting covers food and drink, and only water is allowed, according to the ADA. Coffee, even without sugar, should wait until after the blood draw.

Does an A1C test require fasting?

No. It reflects the average of the past two to three months, so the previous night's meal does not change the result.

If I have prediabetes, do I already have diabetes?

No. Prediabetes is a warning stage, with glucose above normal but below the diabetes threshold. Your doctor will advise on follow-up and how often to repeat the tests.

Sources

• Sociedade Brasileira de Diabetes (Brazilian Diabetes Society). Diagnóstico de diabetes mellitus. Diretriz da Sociedade Brasileira de Diabetes, 2026 edition. diretriz.diabetes.org.br, accessed October 2026.
• Ministério da Saúde (Brazil's Health Ministry). Diabetes (diabetes mellitus). gov.br/saude
• American Diabetes Association. Diagnosis. diabetes.org
• Rahbar S. An abnormal hemoglobin in red cells of diabetics. Clin Chim Acta. 1968;22(2):296-8. PMID 5687098. pubmed.ncbi.nlm.nih.gov/5687098
• Rahbar S. The discovery of glycated hemoglobin: a major event in the study of nonenzymatic chemistry in biological systems. Ann N Y Acad Sci. 2005;1043:9-19. PMID 16037217. pubmed.ncbi.nlm.nih.gov/16037217
• NHS. Diabetic ketoacidosis. nhs.uk

Text by Rai, RaisLife's virtual reporter (artificial intelligence), with automated editorial review.