HbA1c is an indicator of your long-term glucose levels. This lab test is used to aid in the diagnosis of diabetes and prediabetes. In general, the lower the HbA1C within the normal range, the better. As the number climbs, it increases the risk of diabetes and chronic disease. Read on to find out why you should regularly check your values and whether they are in the normal range.
What is HbA1c?
Definition
Glycated hemoglobin (HbA1c) is an important indicator of long-term glucose levels and has been recommended for use in the diagnosis of diabetes mellitus (DM) by the American Diabetes Association (ADA) and World Health Organization (WHO) [1].
HbA1c is created when glucose binds to hemoglobin, which is found in red blood cells [2].
How High Blood Sugar Creates HbA1c
Red blood cells (RBCs) are exposed to glucose in the blood. The higher the blood glucose level gets the more HbA1c forms throughout the lifespan of a red blood cell [3].
The lifespan of a red blood cell varies from person to person but averages out to approximately 117 days in men and 106 days in women. Therefore, the HbA1c is an index of average blood sugar levels over the preceding 2 to 3 months. Of this 2 – 3 month period, the immediately preceding 30 days contribute 50% to HbA1c [3].
Normal adult hemoglobin consists predominantly of HbA1, of which HbA1c comprises approximately 5% [4].
However, HbA1c does not always correlate with blood glucose levels. It is possible to have high blood glucose but low HbA1c due to the presence of hemoglobin variants, inflammation, or other factors that increase red blood cell turn over.
If your red blood cells live longer than 90 days or they are smaller (as measured by MCV in a blood test), then your HbA1C will be artificially higher.
HbA1c Test
Procedure
A healthcare professional will take a blood sample from your arm, which will be sent to the lab to measure your HbA1c levels. Finger prick tests can use a single drop of blood to get a reading, which is often more convenient for children or people who have difficulty giving larger blood samples.
Your doctor will usually order this test as part of a routine health checkup or if you are at risk of diabetes. If you have diabetes, they will order the test to monitor your health status or your response to medication [5].
In most cases, you don’t need to prepare for the test in any particular way or fast, unless you are also testing your fasting blood glucose at the same time. Check with your health care provider to be sure.
How Often Should You Test HbA1c?
According to the American Diabetes Association, screening for diabetes is recommended in people over 45 (every 3 years), or at any age if you have certain risk factors, including [6]:
- Being overweight, obese, or physically inactive
- Having a close (first or second degree) relative with diabetes
- Belonging to a certain race/ethnic group (Native Americans, African-Americans, Hispanic Americans, Asians/South Pacific Islanders)
- Having signs of insulin resistance or conditions associated with insulin resistance, such as high blood pressure (hypertension), low good cholesterol and/or high triglycerides (dyslipidemia), and polycystic ovary syndrome
- Having had diabetes in pregnancy (gestational diabetes)
Diabetic patients with stable blood glucose levels should test HbA1c approximately twice a year [7].
Your doctor may suggest quarterly monitoring (every 3 months) if you have newly diagnosed or pregnancy-associated diabetes, rapidly changing HbA1c levels, changes in treatment or insulin dose, or if you have a history of hypoglycemia (low blood sugar) [8].
Testing HbA1c five or more times per year is redundant because HbA1c is a measure of average blood sugar over three months [8].
Advantages of HbA1C Over Other Blood Glucose Tests
- The HbA1c test can be taken at any time and does not require fasting [9].
- It is unaffected by physical activity or stress prior to testing [9].
- As a marker of long-term blood sugar levels, it is unaffected by short-term hormonal surges [9].
- As a test, HbA1c has a low variability, both when measured in a single individual, and among people [9].
- HbA1c is more stable than glucose at 37°C [9].
- Symptomatic patients can be tested immediately at the same visit, saving costs [9].
However, some health conditions that affect red blood cells and hemoglobin (e.g. anemia) can falsely increase or decrease HbA1c levels [10].
Unit Conversion
HbA1c is usually measured in percent or mmol/mol (mili-mols per mol).
Measuring HbA1c percentages are considered to be the “old way” and most labs nowadays report its values as mmol/mol.
If you need to convert between the two, the Australian Diabetes Society recommends the following formula [11]:
Old (%) = 0.0915 New (mmol/mol) + 2.15%
New (mmol/mol) = 10.93 Old (%) – 23.5 mmol/mol
As an example, using this formula, 5% HbA1c equals 31 mmol/mol (54.7-23.5).
HbA1c Normal Range
In Healthy People
In healthy people, HbA1c values range between 4.0% and 5.6% (20 – 38 mmol/mol) [12].
Integrative medicine doctors often tighten those parameters to as low as 5.2% (33 mmol/mol) as prevention measures, though this is not part of international guidelines and isn’t supported by research.
Screening for Prediabetes
People with HbA1c values of 5.7 – 6.4% (39 – 46 mmol/mol) are considered at high risk for diabetes [13].
In 2015, the American Diabetic Association (ADA) suggested that an HbA1c of 5.7 – 6.4% (39 – 46 mmol/mol) was reasonable for the diagnosis of pre-diabetes and that patients with HbA1c > 6.0% (>42 mmol/mol) should be considered to be at very high risk of developing diabetes [1].
Diagnosing Diabetes
Values ≥ 6.5% (48 mmol/mol) can indicate diabetes [2].
However, HbA1c as a single measurement may not be adequate to diagnose diabetes [14].
Your doctor will interpret this test, taking into account your medical history and other test results.
What Causes Normal HbA1c Levels to Vary?
1) Seasonal changes
HbA1c levels were reported to be higher in colder (January-March) than in warmer (July-September) months in a study of over 600 diabetics [15].
This variation amounts to about 0.3% (3.3 mmol/mol). These differences may be due to differences in exercise, carbohydrate intake, and the number of illness events between summer and winter months [16].
2) Ethnic differences
HbA1c levels are often reported to be different depending on race, ethnicity, and ancestry.
African-Caribbean and South Asian subjects were reported to have HbA1c levels 0.27 – 0.4% higher compared to those with European ancestry, despite lower fasting blood glucose in oral glucose tolerance tests [9, 17].
In South Africa, Black African patients with diabetes were more likely to reach the HbA1c target (<7%) and less likely to have eye, kidney, or heart disease complications [18].
3) Genetic Variants of Hemoglobin
HbA1c results can be inaccurate in persons with elevated amounts of hemoglobin variants. These variants (Hemoglobin S, C, E, and F) are more prevalent in non-whites [19].
Hemoglobin S (the cause of sickle cell anemia) is found in approximately one in 12 African Americans, one in 100 Hispanics and other races [19].
Hemoglobin C is found in approximately one in 50 African Americans [19].
Hemoglobin E (a cause of beta thalassemia) is associated with Asian descents, particularly southeast coastal Asia, with up to 30% in some areas [19].
Hemoglobin F is the normal hemoglobin typically found in a fetus and in low concentration in adults. In some genetic diseases like Hereditary Persistence of Fetal Hemoglobin, Sickle Beta Thalassemia, and some other diseases, hemoglobin F can be elevated [19].
4) Gender Differences
Men have to undergo greater metabolic disturbances and put on more fat to develop HbA1c-defined diabetes than do women [20].
5) Age
HbA1c increases with age, even in the absence of diabetes [21, 22, 23, 9].
People aged over 70 have about 0.5% higher HbA1c compared to those under 30 [21].
Age results in an increase of 0.074% – 0.094% in HbA1c per decade. This age-dependent increase in HbA1c is greater in those with a family history of diabetes, and higher BMI [21, 23, 22].
One study argues against the increase of HbA1c with age though [24].
6) Recent Blood Donation
HbA1c decreases after donating blood. Therefore, HbA1c levels are likely to be underestimated after blood donation in patients with diabetes [25].
This is as a result of lower hemoglobin after giving blood and doesn’t mean that your blood sugar levels are lower.
7) Pregnancy
Normal Pregnancy
HbA1c levels fluctuate very slightly during pregnancy. HbA1c levels decrease during the second trimester of a normal non-diabetic pregnancy and rise during the third trimester [26].
Pregnancy Risks
A study suggests that HbA1c of 5.9 – 6.7% (41 – 49 mmol/mol) identifies a higher-risk group of women with gestational diabetes [27].
In other studies, HbA1c>5.9% (41 mmol/mol) was associated with an increased risk of large infants, cesarean section, and high blood pressure (preeclampsia) [28, 29].
Post-partum Weight Loss in Diabetic Women
Pre-pregnancy BMI and weight retention after delivery were positively associated with HbA1c levels during the first year after delivery in 136 women with type 1 diabetes. Interventions to modify the behaviors associated with these bodyweight factors before pregnancy and after delivery may help women with type 1 diabetes maintain good blood sugar and HbA1c levels after pregnancy [30].
Takeaway
HbA1c is a measure of your average blood glucose levels over the past 3 months. Doctors usually order the HbA1c test to check for or monitor prediabetes or diabetes. Because blood sugar and insulin fluctuations are directly associated with most chronic disease, HbA1C is one of the most useful tests for preventative health as well. Healthy people should keep their levels in the normal range, which is 4-5.6 % (20 – 38 mmol/mol). Slightly higher levels point to prediabetes, while high levels (above 6.5% or 48 mmol/mol) indicate diabetes. However, your doctor will make a diagnosis taking into account your medical history, symptoms, and other test results. HbA1c may slightly vary depending on the season (higher in warm months), a person’s ethnicity, gender, concurrent diagnoses, and genetic makeup. Levels should remain close to normal in a healthy pregnancy.
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