Blood Sugar Testing Explained: Fasting Glucose, A1C, and Diabetes Risk
Written by YiQiu Hu, NMD
Arizona-licensed naturopathic physician and virtual concierge medicine provider
Published August 2026 | Medically reviewed August 2026
Blood sugar is one of the most commonly measured—and sometimes misunderstood—values on routine laboratory testing.
You may see a glucose result on a comprehensive metabolic panel, have a separate fasting glucose test, or receive a hemoglobin A1C measurement during preventive blood work. Although all of these tests provide information about blood sugar regulation, they do not measure exactly the same thing.
A fasting glucose provides a snapshot of your blood sugar at a particular moment after a period without food. An A1C provides a broader estimate of your average blood sugar over approximately the previous three months.
Together, these measurements can help identify normal glucose regulation, prediabetes, diabetes, and changes in metabolic health that may deserve closer attention.
Understanding the difference between these tests can make your laboratory results much easier to interpret.
If you are new to blood work in general, start with our Understanding Laboratory Testing: A Complete Guide for Patients.
Why Blood Sugar Testing Matters
Your body uses glucose as an important source of energy.
After you eat carbohydrates, your digestive system breaks many of them down into glucose, which enters your bloodstream. The pancreas then releases insulin, a hormone that helps glucose move from your blood into cells where it can be used or stored.
Normally, your body keeps blood glucose within a relatively controlled range.
Problems can develop when the body becomes less responsive to insulin, does not produce enough insulin, or has another condition that interferes with glucose regulation.
Over time, blood sugar may gradually increase.
This process does not necessarily cause noticeable symptoms early on. A person can feel completely well while laboratory testing begins to show abnormal glucose regulation.
That makes glucose testing an important component of appropriately selected preventive blood work.
Current diagnostic criteria use several laboratory approaches—including fasting plasma glucose and hemoglobin A1C—to help identify prediabetes and diabetes. The appropriate test depends on the individual clinical situation.
What Is a Fasting Glucose Test?
A fasting plasma glucose test measures the concentration of glucose in your blood at one specific point in time.
For diagnostic testing, fasting generally means having no caloric food or beverages for at least eight hours before your blood is drawn. Plain water is typically permitted unless your physician or laboratory gives you different instructions.
Because food can temporarily raise blood glucose, fasting provides a more standardized way to evaluate how your body is regulating glucose without the immediate effect of a recent meal.
Fasting glucose may be ordered by itself or may appear as the glucose measurement within a comprehensive metabolic panel.
What Does a Normal Fasting Glucose Mean?
For nonpregnant adults, commonly used diagnostic categories are:
Normal fasting glucose: below 100 mg/dL
Prediabetes range: 100–125 mg/dL
Diabetes range: 126 mg/dL or higher
A fasting glucose of 126 mg/dL or higher does not always mean diabetes should immediately be diagnosed from a single laboratory result. When someone does not have unequivocal symptoms or other clear evidence of hyperglycemia, abnormal diagnostic testing is generally confirmed with repeat testing.
This distinction is important.
Laboratory results should be interpreted as part of a clinical evaluation rather than treating one number as an automatic diagnosis.
What Can Affect a Fasting Glucose Result?
Fasting glucose is useful, but it represents only one moment in time.
Blood glucose can vary from day to day and can be affected by factors such as:
Recent food intake
Whether you actually fasted
Illness
Physical or emotional stress
Certain medications
Sleep
Recent exercise
Changes in diet or body weight
Hormonal changes
Other medical conditions
Because of this biological variability, one unexpectedly elevated fasting glucose may need to be repeated or compared with another measure such as A1C.
This is one reason physicians look at laboratory patterns rather than interpreting every abnormal result independently.
Our guide to interpreting common blood test results explains this principle in more detail.
What Is Hemoglobin A1C?
Hemoglobin A1C—also called A1C, HbA1c, glycated hemoglobin, or glycosylated hemoglobin—is a blood test that provides information about your average blood glucose over roughly the previous three months.
Glucose circulating in your blood attaches to hemoglobin, the oxygen-carrying protein inside red blood cells.
The more glucose that is present in the bloodstream over time, the greater the percentage of hemoglobin that becomes glycated.
The laboratory reports this as your A1C percentage.
Unlike fasting glucose, you generally do not need to fast for an A1C test.
What Do A1C Results Mean?
For nonpregnant adults, the standard diagnostic categories are:
Normal A1C: below 5.7%
Prediabetes: 5.7% to 6.4%
Diabetes: 6.5% or higher
As with fasting glucose, an A1C reaching the diabetes threshold generally requires confirmation when there are no clear symptoms of diabetes or unequivocal hyperglycemia.
An A1C within the prediabetes range does not mean that diabetes is inevitable.
Instead, it identifies abnormal glucose regulation and a higher future risk of developing type 2 diabetes. Within the prediabetes range, risk generally rises as the A1C becomes higher.
That makes prediabetes an important opportunity for further evaluation and risk reduction rather than simply a laboratory label.
Fasting Glucose vs. A1C: What Is the Difference?
The easiest way to understand these two measurements is to think about snapshot versus trend.
Fasting glucose is a snapshot
Fasting glucose tells you approximately what your blood sugar was at the time your blood was drawn after fasting.
It can change relatively quickly.
A1C reflects a longer period
A1C provides an estimate of average glucose exposure over approximately three months.
It is therefore less influenced by what happened during one particular morning.
Neither measurement is universally better than the other.
They answer slightly different questions, and using them together can sometimes provide a clearer picture of glucose regulation.
Can Your Fasting Glucose and A1C Disagree?
Yes.
This is actually fairly common.
Someone may have a fasting glucose within the normal range but an A1C in the prediabetes range. Another person may have an elevated fasting glucose while the A1C remains below the diagnostic threshold.
Different tests identify abnormalities in glucose regulation differently, and diabetes can develop gradually.
When test results disagree, the appropriate next step may include repeating testing, reviewing previous laboratory trends, considering factors that could affect either measurement, or using another diagnostic test.
A difference between fasting glucose and A1C should therefore prompt interpretation—not panic.
A1C Is Useful, but It Is Not Perfect
A1C is extremely useful because it provides information about longer-term glucose exposure without requiring fasting.
However, certain circumstances can make A1C less reliable.
A1C depends partly on the normal lifespan and characteristics of your red blood cells. Conditions that alter red blood cells or hemoglobin can therefore produce results that do not accurately reflect actual glucose exposure.
Examples can include:
Some types of anemia
Significant iron deficiency
Recent blood loss
Recent blood transfusion
Certain hemoglobin variants
Sickle cell disease or related hemoglobin disorders
Hemodialysis
Erythropoietin treatment
Advanced kidney disease in some circumstances
Certain liver conditions
Pregnancy-related changes
If your A1C seems inconsistent with your fasting glucose, home glucose measurements, symptoms, or medical history, your physician may consider whether the A1C is accurately representing your glucose levels.
What Is Prediabetes?
Prediabetes means blood glucose regulation has become abnormal but has not yet reached the diagnostic criteria for diabetes.
It is generally identified by:
A1C of 5.7% to 6.4%
Fasting plasma glucose of 100–125 mg/dL
Two-hour oral glucose tolerance test result of 140–199 mg/dL
Prediabetes matters because it indicates an increased likelihood of developing type 2 diabetes and is also associated with broader metabolic and cardiovascular risk.
However, prediabetes is not simply an unavoidable waiting room before diabetes.
Identifying abnormal glucose regulation early creates an opportunity to evaluate the factors contributing to metabolic risk and make changes before blood sugar progresses further.
This is one of the reasons laboratory testing can be valuable for the early detection of chronic disease and health risks.
What Increases the Risk of Type 2 Diabetes?
Type 2 diabetes usually develops through a combination of factors rather than one single cause.
Risk can be influenced by:
Age
Family history of diabetes
Excess body weight or central adiposity
Physical inactivity
Previous prediabetes
History of gestational diabetes
Polycystic ovary syndrome
High blood pressure
Abnormal cholesterol or triglycerides
Cardiovascular disease
Certain medications
Sleep and lifestyle factors
Certain racial and ethnic backgrounds associated with higher population-level diabetes prevalence
Your laboratory values therefore make the most sense when considered alongside your overall metabolic risk profile.
A fasting glucose of 103 mg/dL, for example, may deserve different consideration in someone with multiple metabolic risk factors than in a person whose remaining evaluation is reassuring.
The number matters.
But the patient matters more.
Who Should Be Screened for Prediabetes and Diabetes?
There is not one screening schedule that applies perfectly to every adult.
Current American Diabetes Association guidance recommends routine screening beginning at age 35 for most adults, with earlier testing for adults who have overweight or obesity plus additional diabetes risk factors. Screening intervals depend on previous results and individual risk.
The U.S. Preventive Services Task Force recommends screening asymptomatic adults ages 35 to 70 who have overweight or obesity and notes that earlier screening may be appropriate for certain higher-risk populations.
Rather than focusing only on one age cutoff, the practical question is:
Does this person have enough risk that knowing their glucose status could meaningfully influence their care?
Your physician may recommend earlier or more frequent testing based on your health history.
What Other Tests Can Evaluate Diabetes?
Fasting glucose and A1C are not the only ways to evaluate diabetes.
Oral Glucose Tolerance Test
An oral glucose tolerance test, or OGTT, evaluates how the body handles a measured glucose load.
For a standard two-hour test in nonpregnant adults:
Below 140 mg/dL is considered normal
140–199 mg/dL is in the prediabetes range
200 mg/dL or higher reaches the diabetes threshold
The OGTT can sometimes identify abnormalities that fasting glucose or A1C may miss.
Random Plasma Glucose
When a patient has classic symptoms of significant hyperglycemia, a random plasma glucose may also be used diagnostically.
A random plasma glucose of 200 mg/dL or higher in someone with classic symptoms can support the diagnosis of diabetes.
These tests are selected based on the clinical situation rather than being routinely necessary for everyone.
What Symptoms Can Occur With High Blood Sugar?
Prediabetes and early type 2 diabetes may cause no noticeable symptoms.
As blood glucose becomes significantly elevated, possible symptoms can include:
Increased thirst
Frequent urination
Increased hunger
Unexplained weight loss
Fatigue
Blurry vision
Recurrent infections
Slow-healing wounds
Numbness or tingling
Dehydration
Significant symptoms—particularly when accompanied by markedly abnormal blood sugar—deserve prompt medical evaluation.
Blood sugar testing is not intended to replace evaluation of symptoms that may require more immediate care.
Does One Elevated Blood Sugar Mean You Have Diabetes?
Usually not.
One elevated laboratory result needs context.
For example, glucose can temporarily increase during acute illness, significant physiological stress, or medication exposure.
Likewise, laboratory and biological variability means repeating an unexpected result may sometimes produce a different measurement.
In a patient without unequivocal symptoms of hyperglycemia, current diagnostic guidance generally uses a second abnormal result to confirm diabetes. The repeat measurement can involve the same test or another accepted diagnostic test.
This is why seeing a laboratory result highlighted in red should lead to a conversation with your healthcare provider rather than an immediate self-diagnosis.
Why Blood Sugar Trends Matter
One of the most useful aspects of routine laboratory testing is the ability to compare results over time.
Imagine that your fasting glucose values over several years were:
88 → 92 → 97 → 104 mg/dL.
The final result is important because it has crossed into the impaired fasting glucose range.
But the trend may be just as informative.
It suggests that glucose regulation has gradually changed.
The same principle applies to A1C.
A person whose A1C progresses from 5.2% to 5.5% to 5.8% is showing a different metabolic pattern from someone whose A1C has remained stable at 5.2% for many years.
This does not mean every small increase represents disease.
Normal biological and laboratory variation still occurs.
But meaningful trends can help identify when it is appropriate to look more closely at nutrition, body weight, physical activity, sleep, medications, family history, and other metabolic factors.
This is why keeping previous laboratory records can be extremely valuable.
What Should You Do if Your Glucose or A1C Is Elevated?
The first step is to determine whether the result represents a persistent problem.
Depending on the circumstances, your physician may consider:
Repeating fasting glucose
Repeating A1C
Comparing previous laboratory results
Confirming that fasting instructions were followed
Reviewing medications that may affect glucose
Evaluating symptoms
Reviewing family history
Assessing blood pressure and cardiovascular risk
Reviewing cholesterol and triglycerides
Evaluating body weight or waist circumference
Considering an oral glucose tolerance test
Considering whether A1C may be unreliable
Evaluating other medical conditions contributing to abnormal glucose
The appropriate evaluation depends on how abnormal the result is and what else is happening with your health.
Can Prediabetes Improve?
Yes.
For many patients, glucose regulation can improve—particularly when abnormalities are identified before more advanced diabetes develops.
Depending on the individual, strategies may include changes in:
Nutrition
Physical activity
Body weight
Muscle mass and resistance exercise
Sleep
Alcohol intake
Medication use
Overall metabolic health
Structured lifestyle interventions have been shown to reduce progression from prediabetes to type 2 diabetes, which is why both the ADA and USPSTF emphasize intervention when prediabetes is identified.
The appropriate strategy should still be individualized.
A patient who is sedentary and carrying excess weight may need a very different plan from an active patient with a strong genetic predisposition, medication-related glucose elevation, or another medical condition affecting glucose metabolism.
Blood Sugar Is Only One Part of Metabolic Health
Glucose and A1C are important, but neither measurement tells the entire story.
Your physician may also consider:
Blood pressure
LDL cholesterol
HDL cholesterol
Triglycerides
Body weight
Waist circumference
Physical activity
Nutrition
Family history
Smoking status
Kidney function
Liver health
Medications
Sleep
Other cardiovascular risk factors
This broader perspective is especially important because diabetes, hypertension, abnormal lipids, obesity, fatty liver disease, and cardiovascular disease frequently share overlapping risk factors.
Routine laboratory testing works best when these values are interpreted together rather than treating each result as an unrelated problem.
You can explore the broader role of testing in our Routine Laboratory Testing Resources.
Do You Need Additional or Specialty Testing?
Not necessarily.
For most people being screened for diabetes, standard validated laboratory tests such as fasting plasma glucose and A1C provide clinically useful information.
More testing does not automatically produce a better evaluation.
Additional testing may become appropriate when:
Standard results are conflicting
A1C may be unreliable
Symptoms do not match routine laboratory findings
A clinician is investigating another metabolic disorder
Diabetes type is uncertain
Another medical condition could be affecting glucose
A specific clinical question cannot be answered with routine testing
The objective should be to order tests that answer meaningful clinical questions rather than simply collecting as many biomarkers as possible.
For more information, read Routine Blood Work vs. Specialty Laboratory Testing: What's the Difference?.
How to Think About Your Own Blood Sugar Results
When you receive fasting glucose or A1C results, try not to focus only on whether the laboratory marked the result as high.
Instead, consider several questions:
What exactly was measured?
Fasting glucose and A1C provide different kinds of information.
Was I actually fasting?
If the glucose test was intended to be fasting, preparation matters.
How does this compare with previous results?
Changes over time may provide important context.
Do glucose and A1C tell the same story?
If they do not, your physician may want to understand why.
Do I have diabetes risk factors?
Family history, body composition, physical activity, medications, pregnancy history, blood pressure, cholesterol, and other factors can influence your overall risk.
Does the result need confirmation?
An abnormal diagnostic result may need to be repeated.
What would we do differently based on this information?
Laboratory testing is most valuable when the result leads to an appropriate next step.
The Bottom Line
Fasting glucose and hemoglobin A1C are two of the most useful laboratory tools for evaluating blood sugar regulation.
Fasting glucose provides a snapshot of your glucose level after fasting.
A1C provides a broader picture of average glucose exposure over approximately three months.
For nonpregnant adults:
Fasting glucose below 100 mg/dL is generally considered normal.
Fasting glucose from 100–125 mg/dL is considered prediabetes.
Fasting glucose of 126 mg/dL or higher reaches the diabetes diagnostic threshold.
A1C below 5.7% is generally considered normal.
A1C from 5.7%–6.4% is considered prediabetes.
A1C of 6.5% or higher reaches the diabetes diagnostic threshold.
But numbers alone do not tell the entire story.
Your laboratory results should be interpreted alongside previous measurements, symptoms, medications, family history, metabolic risk factors, and your overall health.
The purpose of blood sugar testing is not simply to label a number as normal or abnormal.
It is to identify meaningful changes early enough to make better decisions about your health.
If you would like to better understand your laboratory results, our Patient's Guide to Understanding Blood Work and Laboratory Testing provides additional guidance.
Patients interested in a more proactive evaluation can also learn about our Preventive Care & Health Optimization services, which may incorporate individualized laboratory testing, risk assessment, results interpretation, and long-term health planning.
Medical Sources & Editorial Standards
This article was written for patient education and reviewed using current evidence-based guidance on glucose testing, prediabetes, and diabetes.
Important clinical references include the American Diabetes Association, including its current Standards of Care in Diabetes, the National Institute of Diabetes and Digestive and Kidney Diseases, and the U.S. Preventive Services Task Force recommendation on screening for prediabetes and type 2 diabetes.
Diagnostic criteria and screening recommendations may change as new evidence becomes available. Testing should be individualized according to age, symptoms, medical history, pregnancy status, medications, risk factors, and previous results.
Ask Dr. Hu's educational resources are written and medically reviewed by licensed Arizona naturopathic physicians and are intended to help patients better understand their healthcare options. Educational information does not replace individualized medical evaluation, diagnosis, or treatment.
About the Author
YiQiu Hu, NMD is an Arizona-licensed naturopathic physician and the founder of Ask Dr. Hu. His clinical approach combines conventional medical evaluation, preventive healthcare, laboratory assessment, lifestyle medicine, and individualized treatment planning.
Dr. Hu provides personalized medical care for adults throughout Arizona, with an emphasis on helping patients understand laboratory findings, identify meaningful health risks, and develop practical strategies for long-term health and disease prevention.