People with diabetes used to depend only on urine tests or daily finger sticks to measure their blood sugars. These tests are accurate, but only in the moment. As an overall measurement of blood sugar control, they’re very limited. This is because blood sugar can vary wildly depending on the time of day, activity levels, and even hormone changes. Some people may have high blood sugars at 3 a.m. and be totally unaware of it.

Once A1C tests became available in the 1980s, they became an important tool in controlling diabetes. A1C tests measure average blood glucose over the past two to three months. So even if you have a high fasting blood sugar, your overall blood sugars may be normal, or vice versa.

A normal fasting blood sugar may not eliminate the possibility of type 2 diabetes. This is why A1C tests are now being used for diagnosis and screening of prediabetes. Because it doesn’t require fasting, the test can be given as part of an overall blood screening.

The A1C test is also known as the hemoglobin A1C test or HbA1C test. Other alternate names include the glycosylated hemoglobin test, glycohemoglobin test, and glycated hemoglobin test.

A1C measures the amount of hemoglobin in the blood that has glucose attached to it. Hemoglobin is a protein found inside red blood cells that carries oxygen to the body. Hemoglobin cells are constantly dying and regenerating, but they have a lifespan of approximately three months.

Glucose attaches, or glycates, to hemoglobin, so the record of how much glucose is attached to your hemoglobin also lasts for about three months. If there’s too much glucose attached to the hemoglobin cells, you’ll have a high A1C. If the amount of glucose is normal, your A1C will be normal.

The test is effective because of the lifespan of the hemoglobin cells. Let’s say your blood glucose was high last week or last month, but it’s normal now. Your hemoglobin will carry a “record” of last week’s high blood glucose in the form of more A1C in your blood. The glucose that was attached to the hemoglobin during the past three months will still be recorded by the test, as the cells live approximately three months.

The A1C test provides an average of your blood sugar readings for the past three months. It’s not accurate for any given day, but it gives the doctor a good idea of how effective your blood sugar control has been over time.

Someone without diabetes will have about 5 percent of their hemoglobin glycated. A normal A1C level is 5.6 percent or below, according to the National Institute of Diabetes and Digestive and Kidney Diseases. A level of 5.7 to 6.4 percent indicates prediabetes, and people with diabetes have an A1C level at 6.5 percent or above.

The American Diabetes Association provides a calculator that shows how A1C levels correlate to glucose levels.

To monitor overall glucose control, people with diabetes should have an A1C test at least twice a year.

Anyone who has had diabetes for any length of time knows that A1C tests haven’t been reliable until recently. In the past, many different types of A1C tests gave different results depending on the lab that analyzed them.

However, accuracy has been improved by the National Glycohemoglobin Standardization Program. Manufacturers of A1C tests now have to prove that their tests are consistent with those used in a major diabetes study. Even more recently, accurate home test kits have become available for purchase.

Accuracy is relative, however, when it comes to A1C or even blood glucose tests. The A1C test result can be up to half a percent higher or lower than the actual percentage. That means if your A1C is 6, it might indicate a range from 5.5 to 6.5. Some people may have a blood glucose test that indicates diabetes, while their A1C is normal, or vice versa. Before making a diagnosis of diabetes, your doctor should repeat the tests.

Some people may even get false results if they’re severely anemic, are in kidney failure, or have liver disease. Ethnic origin can also influence the test. People of African, Mediterranean, or Southeast Asian descent may have a less common type of hemoglobin that can interfere with some A1C tests. Excessive bleeding, such as the kind that might occur during a menstrual period, may also cause an inaccurate test result.

High A1C levels have been linked to an increased risk of the following conditions:

If you’re in the early stages of the disease, small changes in lifestyle can make a big difference and even put diabetes in remission. Losing a few pounds or starting an exercise program can help.

For those who have had prediabetes or diabetes for a long time, A1C results that are moving upward are a sign that you need to start on medication or change what you’re already taking. You may also need to make other lifestyle changes and monitor your daily blood glucose more closely.