Know Your A1C: What This Blood Test Can Tell You About Your Risk for Diabetes and Cardiovascular Disease

A simple blood test can diagnose diabetes, but it also can tell you so much more, including your risk for heart attack and stroke.

Type 2 Diabetes: Who Is at Risk?

Diabetes, which causes chronically high blood sugar levels, is the seventh leading cause of death in the United States, according to the Centers for Disease Control and Prevention. It can also result in serious health complications, including heart disease, blindness, kidney failure and lower-extremity amputations. The CDC reports that close to 29.1 million people are currently living with diabetes in America, meaning about 1 of every 11 people has it. There are different types of diabetes, but Type 2 diabetes accounts for about 90 to 95 percent of all diagnosed diabetes cases.

Risk factors for Type 2 diabetes include older age, obesity, a family history of diabetes, prior history of gestational (pregnancy) diabetes, impaired glucose tolerance, physical inactivity and race/ethnicity. African-Americans, Latinos, American Indians and some Asian-Americans and Pacific Islanders are at particularly high risk for Type 2 diabetes.

What Is Prediabetes?

People with prediabetes have glucose (i.e., blood sugar) levels that do not meet the criteria for diabetes but are too high to be considered normal. These individuals have an increased risk for the development of diabetes and other serious health problems, including heart disease and stroke. According to the CDC, 86 million American adults, or more than 1 of 3 people, have prediabetes. Without lifestyle changes, such as eating healthy foods, getting regular physical activity and maintaining a healthy weight, 15 to 30 percent of these individuals will develop Type 2 diabetes within five years.

[See: Got Diabetes? Why You Must Protect Your Feet.]

What Is the A1C Blood Test?

The term A1C is short for HbA1c, or hemoglobin A1C. It refers to glycated hemoglobin, which develops when hemoglobin -- a protein within red blood cells that carries oxygen -- becomes coated with glucose or sugar in the blood. The amount of glucose that combines with this protein is directly proportional to the total amount of sugar in a person's system, and so the higher blood glucose levels are, the higher the A1C level. Red blood cells have a life span of 120 days; by measuring A1C, clinicians are able to determine average blood sugar levels over approximately two to three months. A1C is particularly important in people with diabetes because the higher the A1C level, the greater the risk of developing diabetes-related complications. After a diabetes diagnosis, A1C is also used for gauging how well treatment controls blood sugar levels. In the U.S., A1C results are given as a percentage of hemoglobin that is glycated.

How Does an A1C Test Differ From a Blood Glucose Level?

An A1C measurement is a marker of average blood sugar levels over a period of two to three months, so it is a more stable test assessing longer-term blood sugar control. This means less day-to-day fluctuations to A1C levels due to stress and illness. A1C is often tested using blood samples from the arm, but samples can also be taken from a finger prick. Fasting is not required before A1C testing like it is for the blood glucose test.

On the other hand, the blood glucose level gives us the concentration of glucose in the blood only at the time of the test.

Health care providers measure both A1C and blood glucose to ensure good diabetes control, which informs them of the long-term and day-to-day control of blood sugar levels.

How Do We Diagnose Diabetes and Prediabetes?

Both diabetes and prediabetes may be diagnosed based on either A1C or blood glucose criteria. Blood glucose criteria could either be a blood glucose level measured after an overnight fast or a two-hour blood glucose value after eating 75 grams of sugar.

An international committee of experts from the American Diabetes Association, the European Association for the Study of Diabetes and the International Diabetes Federation recommends that the A1C test be the primary test used to diagnose prediabetes and Type 2 diabetes.

What Are the A1C Criteria for Diabetes and Prediabetes Diagnosis?

A1C can indicate if people have prediabetes or diabetes based on the following:

An A1C level of 6.5 percent or more measured on two separate occasions indicates you have diabetes.

If your A1C test returns a reading of 5.7 to 6.4 percent, this indicates you have prediabetes and are at an increased risk of Type 2 diabetes. At this point, you need to talk to your doctor about appropriate lifestyle changes that could reduce your risk of developing full-blown Type 2 diabetes in the future.

It is important to note that normal ranges for A1C levels may vary from one lab to another, so patients who may wish to interpret their own A1C results need to keep this in mind, especially when using a lab that is different from the one used for previous testing.

What Is the Target A1C Level?

For most people with previous diagnoses of diabetes, a target A1C level of 7 percent or less is a common treatment target. However, this is a general target, and health care teams do tailor targets to meet individual goals. A1C values are not indicators of specific diabetes complications, meaning any complications could arise with any A1C value. However, the closer someone's value is to the normal A1C range, the better. A person's recommended A1C target should take into account his or her ability to achieve the target without any risk of serious health complications caused by blood sugar levels that are too low (called hypoglycemia).

[See: The Best Diets to Prevent -- and Manage -- Diabetes.]

What Are the Benefits of Lowering A1C?

Studies have shown that reducing A1C by 1 percent in people with diabetes reduces the risk of serious health complications involving small vessels of the eyes and kidneys, as well as nerves by almost 25 percent.

Also, a study published in the journal BMJ revealed that people with Type 2 diabetes who reduce their A1C level by 1 percent are:

-- 19 percent less likely to suffer cataracts

-- 16 percent less likely to suffer heart failure

-- 43 percent less likely to suffer amputation or death due to blood vessel diseases

A lower A1C level in the blood means a lower amount of sugar in the blood on average, which reduces the risk of developing complications caused by high blood sugar levels.

Who Should Get an A1C Test?

-- Everyone with Type 2 diabetes should be offered an A1C test at least once a year.

-- Some may require an A1C test more often. This is especially true for patients who had a recent change in medication(s) or if a health care team wishes to more frequently monitor a patient's diabetes status to get it under control.

-- For those without diabetes, experts recommend that anyone 45 or older should consider getting tested for A1C, especially if they are overweight. If they are younger than 45 but are overweight and have one or more additional risk factors for diabetes, they should consider getting tested.

What Are the Limitations to A1C Testing for Diabetes?

While A1C tests are usually reliable and widely used, it's important to acknowledge that the test may not be accurate in people who:

-- Have insufficient hemoglobin due excessive bleeding (may have a falsely low A1C reading).

-- Have iron-deficiency anemia (may have a falsely high A1C test).

-- Have hemoglobin genetic variations or uncommon forms of hemoglobin, commonly found in African-Americans and people of Mediterranean or Southeast Asian heritage.

-- Have had a recent blood transfusion or have other forms of hemolytic anemia (may have falsely low A1C results)

-- Are pregnant.

[See: How to Cope With Gestational Diabetes.]

What Can You Do to Protect Yourself From Diabetes?

Anyone can benefit from a reduction of long-term diabetes complications, such as heart attack, stroke, kidney failure and diabetic nerve pain, by controlling their A1C levels through adopting healthy lifestyle practices. The benefit of reducing A1C should not be underestimated. To reduce A1C levels, you can:

Eat healthy. By keeping your post-meal blood glucose low, A1C can gradually be reduced in patients with diabetes and prediabetes. Those with diabetes and prediabetes need to eat foods that are high in nutrition and avoid excess calories. A healthy diet is rich in fruits, vegetables, fiber, lean protein and "good" monounsaturated and polyunsaturated fats in moderation. Saturated fats, refined "simple" carbohydrates and processed foods should be limited. For instance, switching white bread and white rice for whole-grain and brown rice will help reduce blood glucose spikes after a meal. Understanding what to eat and what to avoid can be challenging. Talk to a registered dietitian if you need help with food choices and meal planning. Tracking daily food intake using a diet diary or calorie-counter app can help keep things in check.

Be physically active. By keeping physically active, blood glucose is moved from the blood into cells to produce energy for the body, which lowers blood glucose levels. Also, physical activity improves our body's sensitivity to insulin, a hormone needed to transport glucose into cells. This means that less insulin is needed to transport large amounts of glucose. Everyone should incorporate physical activity into their daily routine. For those without diabetes, being physically active will help to prevent the onset of prediabetes and Type 2 diabetes. For those with diabetes, it will help them maintain good blood sugar levels. The American Diabetes Association recommends aiming for 30 minutes of moderate- to vigorous-intensity aerobic exercise at least five days a week, or a total of 150 minutes per week. Moderate intensity means that you are working hard enough that you can talk but not sing during the activity, while vigorous intensity means you can't say more than a few words without pausing for a breath during the activity.

Maintain a healthy weight. Losing weight through diet and exercise if you are overweight will significantly improve blood sugar levels, meaning a good A1C measurement.

Monitor your numbers. Carefully monitor both blood sugar and A1C levels if you have diabetes. Your medical team will most likely recommend regular A1C testing to monitor your overall diabetes control over a period of two to three months. However, A1C should never replace blood sugar level monitoring. For instance, people on insulin and other medications that cause hypoglycemia need regular blood glucose monitoring to ensure blood glucose doesn't get too low.

Dr. Martin Tibuakuu is a postdoctoral research fellow at the Johns Hopkins University School of Public Health and a fellow at the Johns Hopkins Ciccarone Center for the Prevention of Heart Disease. His research interests focus on general preventive cardiology and on environmental exposures such as air pollution and cardiovascular health.