Anyone with diabetes will be familiar with finger-prick testing for monitoring blood glucose to see how well they are managing their disease.
This kind of regular testing is essential for most people with diabetes, but what role does an occasional hemoglobin A1C blood test play in controlling blood sugars, and how does it work?
Contents of this article:
What is the A1C test?
The abbreviation A1C is used in the US (sometimes with a lower-case 'c' - A1c) and is short for glycated hemoglobin (sometimes called 'glycosylated' hemoglobin or glycohemoglobin). The other abbreviations in use are:
The A1C is a less frequent diabetes test than home monitoring. A sample of blood is taken up to four times a year for lab analysis.
- HbA1c (widely used internationally)
The A1C test is a blood test used to measure the average level of glucose in the blood over the last two to three months. This test is used to check how well blood sugar levels are being controlled in a person with diabetes and can also be used in the diagnosis of diabetes.1
Hemoglobin is the protein in red blood cells which is responsible for transporting oxygen around the body. When blood glucose levels are elevated, some of the glucose binds to hemoglobin and, as red blood cells typically have a lifespan of 120 days, A1C (glycated hemoglobin) is a useful test because it offers an indication of longer term blood glucose levels.2
The particular type of hemoglobin that glucose attaches to is hemoglobin A, and the combined result is call glycated hemoglobin. As blood glucose levels rise, more glycated hemoglobin forms, and it persists for the lifespan of red blood cells, about four months.2
Therefore, the A1C level directly correlates to the average blood glucose level over the previous 8-12 weeks; A1C is a reliable test that has been refined and standardized using clinical trial data.3
There are two key things to know about the application and accuracy of the A1C test of glycemic control:1
- It reflects average plasma glucose levels over the previous 8 to 12 weeks
- The blood test, performed at the doctor's clinic and sent for laboratory assessment, can be done at any time of the day and does not require special preparation.
What A1C level defines diabetes?
In 2009, the A1C test began to be recommended as a means of diagnosing diabetes, in addition to its use in monitoring management of the disease.1
The World Health Organization (WHO) recommends the A1C test for diagnosis partly because of the convenience compared to the two traditional means of measuring plasma glucose levels which require a blood sample to be taken:
- After an eight-hour period of fasting (fasting plasma glucose or FPG test)
- Two hours after a special sweet drink (the oral glucose tolerance test or OGTT).
Here are some key points about the A1C test. More detail and supporting information is in the body of this article.
- A1C is an abbreviation for glycated hemoglobin, which forms when glucose attaches to a protein in red blood cells.
- As blood glucose levels rise, so does the percentage of glycated hemoglobin, so A1C is used to measure diabetes control.
- The test is used for both monitoring and diagnosing diabetes, and is usually performed using a blood sample taken at the doctor's clinic.
- A1C is a reliable indicator of glycemic control as it reflects the average blood glucose level over the 8-12 weeks leading up to the test.
- A1C is more convenient than other tests that measure glucose levels directly because it does not need any special preparation such as fasting or consuming a special glucose drink.
- The test is standardized and every patient with diabetes is recommended to have an A1C test between two and four times a year.
- An initial diagnosis of diabetes is made if A1C (glycated hemoglobin) is 6.5% or higher. Prediabetes, a risk factor for diabetes, is defined as A1C of 5.7-6.4%. A result below 5.7% indicates a healthy blood sugar level.
- The typical target for people with diabetes is to get their A1C below 7%, but there are different targets for different individuals and stages of the disease.
- Results from A1C testing are easily converted into an equivalent blood glucose reading as given by home monitoring.
In addition to the convenience of taking an A1C measurement, the reason this test has become the diagnostic standard is because it overcomes the short-term variability of glucose tests by taking a measure of the last 8 to 12 weeks.
The A1C tests that have been standardized for used by clinics are also at least as reliable at measuring glucose directly (it has almost the same sensitivity and specificity).1
The WHO and the American Diabetes Association recommend a diagnosis of diabetes if a patient has:1,3
- An A1C of 6.5% or above.
However, diabetes is not automatically ruled out if a patient has an A1C lower than 6.5%; a lower value accompanied by clear symptoms of diabetes and/or indications of hyperglycemia through direct glucose tests can also lead to a diagnosis of diabetes.1
Certain conditions may confound the results of an A1C test - anemia, kidney disease, or certain blood disorders (thalassemia), for example.
A diagnosis of prediabetes (a risk factor for later diabetes) may be made in cases where A1C measures 5.7-6.4%.4
Prediabetes is associated with obesity, high blood pressure and dyslipidemia (high cholesterol or triglycerides in the blood plasma, or low levels of high-density lipoproteins); lifestyle and dietary changes will be recommended to try to address prediabetes and reduce the likelihood of full-blown diabetes developing.3
A normal blood glucose profile in someone without diagnosed diabetes is reflected by an A1C result below 5.7%.4
What are the target A1C levels for people with diabetes?
When people have an A1C result of 6.5% or more and are diagnosed with diabetes, the goal of diabetes treatment is to lower blood glucose and keep it below 7%. Treatment usually involves lifestyle and dietary changes and/or drugs or insulin injections.
In the majority of people, the risk of complications from diabetes (such as cardiovascular disease) increases in line with higher A1C percentages. However, the relationship between A1C percentage and the risk of diabetes is curvilinear, i.e. it rises disproportionately.3
Using data from over 44,000 individuals from 16 cohort studies (with an average follow-up time of 5.6 years), researchers determined the risk of developing diabetes within 5 years as:8
- 9-25% for those with an A1C of 5.5-6.0%
- 25-50% for those with an A1C of 6.0-6.5%
The relative risk of developing diabetes for people with an A1C of 6.0-6.5% was 20 times higher than for those with an A1C of 5.0%.8
For people with diabetes, the target A1C level is aimed at reducing the risk of complications related to damage to blood vessels - both small blood vessels (microvascular complications) and to the arteries, including the coronary arteries (macrovascular disease):3
- Reaching and maintaining an A1C of 7% or lower reduces these risks and is the usual target that is set for people with diabetes, except for pregnant women.
Some people with diabetes are recommended to aim to get their A1C level even lower, such as under 6.5%, as long as this does not lead to significant hypoglycemia or other adverse treatment effects. Such a goal may be proposed for those who have a short duration of diabetes, a long life-expectancy, or no significant cardiovascular disease.3
In other cases, less stringent A1C goals - under 8%, perhaps - may be needed, such as for people with diabetes and severe hypoglycemia, those not expected to live for many more years, and those who have advanced diabetes complications.3
Where glycemic goals have been difficult to achieve in spite of a patient's best efforts and optimal medical interventions, relaxing the A1C target may also be deemed appropriate in those with long-standing diabetes.3
How does A1C relate to average glucose level?
The A1C test result can be equated to the average of levels taken through traditional self-monitoring of blood glucose. The estimated average glucose (eAG) is a figure calculated from a number of directly measured blood glucose levels over time. It is expressed as milligrams per deciliter (mg/dl), the same way as individual readings obtained through home monitoring.5
The American Diabetes Association has produced a web-based converter to turn A1C readings into eAG readings, and vice versa.6
Doctors may use this to help make sense of the A1C reading for patients accustomed to the glucose measurements taken at home.
An A1C reading can be converted to an eAG reading as follows:5
- 6.0% = 126 mg/dl
- 6.5% = 140 mg/dl
- 7.0% = 154 mg/dl
- 7.5% = 169 mg/dl
- 8.0% = 183 mg/dl
- 8.5% = 197 mg/dl
- 9.0% = 212 mg/dl
- 9.5% = 226 mg/dl
- 10.0% = 240 mg/dl.
How often should people with diabetes go for the A1C test?
A large study of A1C testing in the UK (involving 79,409 patients between 2008 and 2011), found that A1C testing once every three months was optimal for helping people with diabetes to maintain good control over their blood glucose levels.7
The A1C test averages out the blood sugar levels over the previous two to three months. It helps to monitor success in managing diabetes and reaching targets for glycemic control.
The study, published in the journal Diabetes Care in 2014, found that testing A1C four times a year (quarterly) was associated with a 3.8% drop in A1C levels in people who had an A1C of 7% or higher when initially diagnosed with diabetes.7
This improvement was in contrast to those tested just once a year who actually experienced a small rise, of 1.5%, in blood glucose levels. In addition, compared to those tested just once a year, half as many people in the quarterly-testing group suffered a significant rise in blood sugar levels.7
The recommendations of the American Diabetes Association, followed by US providers, gives the following minimum standard for frequency of offering A1C testing:3
- At least twice a year for patients who are meeting treatment goals and who have stable control over blood glucose levels
- Quarterly for patients whose treatment program changes and/or who are not meeting blood glucose goals.
The guidelines also urge doctors to provide A1C testing at the point of care so that there is an immediate opportunity to make appropriate changes to treatment.
New A1C target for children with type 1 diabetes. The American Diabetes Association lowered the target A1C level recommended for children under 19 years of age. Children with type 1 diabetes should aim to keep their blood glucose levels to an A1C level below 7.5%. The targets were higher before the new guidance issued in June 2014, which aims to reduce complications of diabetes in children.
New study shows routine blood glucose measurements can accurately estimate hemoglobin A1c. The researchers' data-based model accurately estimated A1C from self-monitored blood glucose readings. Their model, they concluded, would be "readily implementable into home SMBG meters." They added: "Real-time HbA1c estimation could increase patients' motivation to improve diabetes control."