Unlike a fasting glucose test, an A1C test usually does not require you to stop eating before your blood sample is taken. However, the result should not be interpreted alone without considering your symptoms, medical history, medications and other glucose measurements.

Important: This guide is for general education. It does not diagnose diabetes or replace advice from a qualified healthcare professional.

Quick Answers About the A1C Test

The National Institute of Diabetes and Digestive and Kidney Diseases states that the test reflects average blood glucose over about three months and can be used for type 2 diabetes, prediabetes and diabetes management.

What Does an A1C Test Measure?

Glucose circulates through your bloodstream and attaches to hemoglobin, the oxygen-carrying protein inside red blood cells. This attachment is called glycation.

The A1C test measures the percentage of hemoglobin that has glucose attached to it. When average blood sugar is higher, a larger percentage of hemoglobin becomes glycated. When average blood sugar is lower, the percentage is generally lower.

For example:

• An A1C of 5.4% means that approximately 5.4% of the measured hemoglobin is glycated.
• An A1C of 7.0% indicates a higher average glucose level than an A1C of 5.4%.
• An A1C of 9.0% generally reflects substantially higher average glucose exposure.

Red blood cells typically circulate for about three months, which is why the HbA1c test provides a longer-term picture than a single finger-prick glucose reading. Recent glucose levels have a greater influence on the result than levels from several months earlier.

A1C is not a real-time blood sugar reading

A glucose meter tells you what your blood sugar is at a particular moment. A continuous glucose monitor can show current levels, patterns and changes throughout the day.

An A1C test provides a broader average. It does not clearly show:

• Sudden blood sugar spikes
• Episodes of low blood sugar
• Differences between daytime and overnight levels
• How often glucose moves outside your target range
• Day-to-day glucose variability

Two people can have the same result while experiencing very different daily patterns. One person may have relatively steady glucose, while another may alternate between high and low readings.

That is why the test should be considered alongside home glucose readings, continuous glucose monitoring information and symptoms when these are available. The CDC describes A1C as an important tool, but not a replacement for daily monitoring when daily monitoring is needed.

How Does an A1C Test Work?

The test uses a blood sample to measure glycated hemoglobin. A laboratory instrument separates or identifies the hemoglobin and calculates how much of it has glucose attached.

The basic process is:

1. A healthcare professional collects a blood sample.
2. The sample is analyzed using an HbA1c measurement method.
3. The laboratory reports the result, usually as a percentage.
4. The result may also include an estimated average glucose value.
5. Your healthcare professional interprets the result with your medical history and other tests.

For diagnostic use, standardization is important. NIDDK advises that blood used for diagnosis should be analyzed with an NGSP-certified method so that results are comparable with established research standards.

How Is the A1C Test Performed?

There are two common ways to collect a sample.

1. Blood drawn from a vein

A healthcare professional inserts a small needle into a vein, usually in your arm. A small amount of blood is collected in a tube and sent to a laboratory.

The process usually takes only a few minutes. You may feel a brief pinch when the needle is inserted.

A venous laboratory sample is generally preferred when the test is being used to help establish a diagnosis.

2. Finger-prick testing

Some clinics use a finger-prick sample and a point-of-care machine. The result may be available during the same appointment.

Finger-prick testing can be convenient for monitoring an existing condition. However, a rapid result should not automatically be treated as diagnostic confirmation.

What to expect after the sample

Most people can return to normal activities immediately. Possible minor effects include:

• Temporary soreness
• Mild bruising
• A small amount of bleeding
• Dizziness in people who are sensitive to blood draws

Tell the person collecting your sample if you have previously fainted during blood tests or take medication that affects bleeding.

Do You Need to Fast for an A1C Test?

No. In most cases, you do not need to fast before an A1C test. You can normally eat and drink before the sample is collected, and the test can be performed at any time of day.

However, your healthcare professional may order other blood tests at the same appointment. Some of those tests may require fasting.

Examples include:

• Certain fasting glucose tests
• Some lipid or cholesterol tests
• Tests ordered for a particular medical condition

Practical preparation tip

Before your appointment, ask:

“Am I only having an A1C test, or are other fasting blood tests being performed?”

Do not fast unnecessarily. Also, do not assume that eating is allowed simply because HbA1c testing itself does not require fasting. Follow the instructions provided for the complete set of tests.

Why Might a Doctor Order an A1C Test?

A healthcare professional may order the test for several reasons.

Screening for prediabetes

Prediabetes means blood sugar is higher than the normal range but not yet in the diabetes range. It often causes no obvious symptoms.

Finding it early may give a person time to discuss physical activity, nutrition, weight management, sleep, medication and other risk-reduction steps with a healthcare professional.

Helping diagnose type 2 diabetes

An A1C test may be used alone or with another glucose test to help diagnose type 2 diabetes.

If you do not have clear symptoms and the result falls within the diabetes or prediabetes range, NIDDK advises confirming it on another day with a repeat HbA1c measurement or another accepted diabetes test.

Monitoring diabetes management

For someone already diagnosed with diabetes, the test helps show whether average glucose levels have been near the agreed target over recent months.

A doctor may use the result to discuss:

• Medication effectiveness
• Nutrition and activity patterns
• Treatment adherence
• Risk of hypoglycemia
• Glucose-monitoring results
• Whether the current plan needs adjustment

Do not change insulin or other diabetes medication based only on an online chart or calculator. Medication decisions should be made with a qualified healthcare professional.

What Do A1C Test Results Mean?

Results are commonly reported as a percentage in the United States.

General diagnostic ranges

These are general diagnostic thresholds, not personal treatment goals. A result in the diabetes range normally requires confirmation when classic symptoms are absent.

Example

Suppose a person without clear diabetes symptoms receives a result of 6.6%.

This does not necessarily mean the person should immediately consider the diagnosis final. A clinician may:

1. Review symptoms and risk factors.
2. Check whether a condition could have affected the result.
3. Repeat the hemoglobin A1C test.
4. Order fasting plasma glucose or an oral glucose tolerance test.
5. Compare the results before confirming a diagnosis.

A diagnosis and a treatment target are different

The 6.5% diagnostic threshold is not automatically the target for every person already living with diabetes.

Treatment goals may differ according to:

• Age
• Pregnancy
• Duration of diabetes
• Risk of low blood sugar
• Current medication
• Kidney or cardiovascular disease
• Other medical conditions
• Personal circumstances

The American Diabetes Association notes that a goal near or below 7% is commonly used for many people with diabetes, but individual targets should be established with the healthcare team.

How Is A1C Reported?

Percentage

In the United States, the result is normally shown as a percentage, such as:

• 2%
• 1%
• 4%
• 0%

IFCC units

In many countries, HbA1c may also be reported in millimoles per mole, written as mmol/mol.

For example:

Estimated average glucose

Some laboratory reports include estimated average glucose, or eAG. This converts the result into the units commonly used by glucose meters.

These values are estimates. They should not be interpreted as proof that blood sugar remained at that exact number throughout the day. A person may have significant highs and lows while still producing the same average.

Laboratory A1C Test vs Point-of-Care Test

The terms describe where and how the sample is analyzed.

NIDDK’s patient guidance states that office or clinic point-of-care results should not be used for diagnosis. Its professional guidance adds an important qualification: certain point-of-care assays may be certified or approved for diagnostic use, but they should only be considered in laboratories certified to perform the appropriate complexity of testing and maintain proficiency.

Practical takeaway

A rapid clinic result can be useful, particularly for monitoring. When a result could create a new diagnosis, ask:

• Was this a laboratory measurement?
• Was an NGSP-certified method used?
• Does the result need to be repeated?
• Should it be compared with another glucose test?

How Often Should an A1C Test Be Performed?

Testing frequency depends on why the test is being performed and how stable the person’s glucose management is.

People without diabetes

There is no single schedule for everyone. A clinician may recommend screening based on age, body weight, family history, previous gestational diabetes and other risk factors.

When an initial result is normal, the next testing date depends on the person’s overall risk.

People with prediabetes

The CDC says a healthcare professional may recommend repeating testing every one to two years, depending on the individual situation.

People living with diabetes

Many people with stable diabetes management have testing at least twice per year.

Testing may be performed approximately every three months when:

• Treatment has recently changed
• Glucose targets are not being met
• Medication has been added or adjusted
• Results are changing unexpectedly
• A clinician needs to assess a new treatment plan

NIDDK and the American Diabetes Association state that people with diabetes commonly have the test at least twice annually and may need it more often when goals are not being met or treatment has changed.

Why testing every few weeks is usually not useful

The measurement reflects a period of approximately two to three months. Repeating it too soon may not show the full effect of a treatment or lifestyle change.

However, recent glucose levels have more influence on the result than older levels. A healthcare professional may therefore repeat the measurement earlier when there is a specific clinical reason.

When Should the A1C Test Not Be Used Alone?

The test is useful, but it is not the correct stand-alone diagnostic method in every situation.

Type 1 diabetes

NIDDK advises that it should not be used to diagnose type 1 diabetes. A person with symptoms such as excessive thirst, frequent urination, vomiting, unexplained weight loss, abdominal pain, rapid breathing or severe tiredness may need urgent glucose and ketone testing.

Gestational diabetes

The test is not the standard method for diagnosing gestational diabetes. Glucose challenge or oral glucose tolerance testing is normally used during pregnancy.

It may sometimes be used early in pregnancy to help identify previously undiagnosed diabetes, but it should not replace recommended pregnancy glucose testing.

Sudden symptoms

Because the result represents a longer-term average, it may not fully reflect a rapid change occurring over hours or days.

Seek prompt medical care when symptoms suggest very high or very low blood sugar, regardless of a previous result.

Conditions That Can Affect A1C Test Accuracy

Anything that changes hemoglobin, red-blood-cell survival or the relationship between glucose and hemoglobin can potentially affect the result.

Blood loss and blood transfusion

Recent significant blood loss may produce a falsely low result because newer red blood cells have had less time to accumulate glycated hemoglobin.

A blood transfusion may also make the result difficult to interpret because the sample includes red blood cells from another person.

Hemolytic anemia

Hemolytic anemia shortens red-blood-cell survival and can produce a lower measurement than expected.

Iron-deficiency anemia

Iron deficiency may be associated with a higher HbA1c result in some situations. The effect may change after iron deficiency is treated.

Hemoglobin variants

Variants such as hemoglobin S, C, E or D may interfere with some measurement methods.

A variant does not automatically make every A1C test inaccurate. The effect depends on the specific variant and the laboratory method. Reliable methods are available for many people with hemoglobin variants.

Kidney failure

Kidney disease can affect interpretation through anemia, altered red-cell survival, treatment effects and chemically modified forms of hemoglobin. The amount of interference varies by condition and testing method.

Liver disease

Liver disease may change red-blood-cell production or survival and can affect the relationship between measured HbA1c and actual glucose levels.

Pregnancy

Pregnancy changes blood volume, iron status and red-blood-cell turnover. Early and late pregnancy may affect accuracy, and recommended gestational diabetes tests should still be completed.

Certain medicines

Some medicines can affect red blood cells, hemoglobin or glucose levels. Tell your clinician about prescription medicines, over-the-counter products and supplements.

Recent major change in glucose

The result is weighted toward recent weeks but still reflects a longer period. It may lag behind a sudden improvement or worsening.

Signs That the Result May Need Further Review

Ask your healthcare professional to review the result carefully when:

• It is very different from glucose-meter readings.
• It does not match continuous glucose monitor data.
• It changes sharply without an obvious reason.
• It is below 4% or above 15%.
• You recently had significant bleeding.
• You recently received a blood transfusion.
• You have anemia or a known hemoglobin variant.
• You are pregnant.
• You have kidney or liver disease.
• Your symptoms do not match the reported number.

NIDDK recommends considering possible interference when HbA1c results conflict with other glucose information or change unexpectedly.

A1C Test vs Other Diabetes Tests

One test is not universally better than every other test. They answer different questions.

The A1C test is convenient and reflects longer-term exposure, while fasting glucose and oral glucose tolerance testing directly measure blood glucose under defined conditions. In some people, these tests produce different diagnostic classifications, so clinicians may repeat or compare them.

Questions to Ask Your Doctor

Bring these questions to your appointment:

1. What does my result mean in my situation?
2. Does this result need to be repeated?
3. Was the sample analyzed by a certified laboratory method?
4. Could anemia, blood loss or a transfusion affect my result?
5. Could I have a hemoglobin variant?
6. Does the result agree with my home glucose or CGM data?
7. What personal target is appropriate for me?
8. How often should I repeat the test?
9. Do I need fasting glucose or an oral glucose tolerance test?
10. Could any of my medicines affect the result?
11. What symptoms require urgent attention?
12. What changes should I make before my next appointment?

Appointment tip

Take the following information with you:

• Recent laboratory reports
• Glucose-meter records
• CGM summaries
• Current medication list
• Details of recent illness
• History of anemia or transfusion
• Dates of major treatment changes

This information can help your healthcare professional interpret the measurement more accurately.

Key Takeaways

• An A1C test estimates average blood glucose over approximately two to three months.
• Fasting is normally not required.
• It can help screen for prediabetes, diagnose type 2 diabetes and monitor diabetes management.
• A result in the diabetes range usually needs confirmation when clear symptoms are absent.
• It should not be used alone to diagnose type 1 diabetes or gestational diabetes.
• Laboratory testing with an appropriately standardized method is preferred for diagnosis.
• Point-of-care testing is mainly useful for convenient monitoring unless strict diagnostic laboratory requirements are met.
• Anemia, blood loss, transfusion, pregnancy, kidney disease and hemoglobin variants may affect accuracy.
• Personal treatment targets should be discussed with a qualified healthcare professional.

Medical Review and Editorial Information

Written by: [Author’s full name and relevant qualification]

Medically reviewed by: [Licensed physician, endocrinologist, certified diabetes care and education specialist, or other appropriately qualified reviewer]

Published: [Month Day, Year]

Last medically reviewed: [Month Day, Year]

Next scheduled review: [Month Year]

Editorial note

This page should be reviewed whenever recognized diabetes-testing guidance changes. Corrections should be documented through a publicly accessible corrections policy.