The diagnosis of diabetes is to a large extent based on detecting elevated levels of sugar (glucose) in the blood.
Hemoglobin A1c (HbA1c) is a laboratory measure frequently used for this purpose. The test is also useful to monitor treatment in patients with established diabetes.
Approximately 8 percent of the US populations suffer from type 2 diabetes, with as many as 40 percent of those undiagnosed (1).
Worldwide, the prevalence of type 2 diabetes is estimated at 6.4 percent in adults but varies somewhat among countries with the rate of undetected diabetes as high as 50 percent in some areas (2).
The term diabetes describes several disorders of abnormal carbohydrate metabolism that are characterized by high levels of blood glucose (hyperglycemia). Diabetes is associated with a relative or absolute impairment in insulin secretion, along with varying degrees of peripheral resistance to the action of insulin (3).
The Difference Between Type 1 and Type 2 Diabetes
The prevalence of both type 1 and type 2 diabetes continues to increase worldwide, with type 2 diabetes much more common and accounting for over 90 percent of patients with diabetes.
Type 1 diabetes used to be called juvenile onset or insulin-dependent diabetes because it often presents in childhood and it is characterized by the inability of the pancreas to produce the insulin.
Insulin is necessary for the cells of the body to be able to utilize glucose for energy production. Without insulin, glucose accumulates in the blood leading to hyperglycemia. Due to the absence of insulin, most patients with type 1 diabetes need to be treated with insulin.
Conversely, type 2 diabetes, formerly called adult-onset or non-insulin-dependent diabetes, can occur at any age. Although it is most common among adults, the prevalence is rising among children.
Type 2 diabetes is characterized by insulin resistance. Insulin resistance is defined as a diminished response to a given concentration of insulin. Initially, the pancreas responds by producing more insulin. For this reason, people with insulin resistance often have high blood levels of insulin. However, as diabetes develops, the beta cells of the pancreas often become unable to produce more insulin and its blood levels drop.
Type 2 diabetes is commonly associated with obesity or the metabolic syndrome. Many patients with the disorder also have high blood pressure and lipid abnormalities.
How Is Diabetes Diagnosed?
Early identification of diabetes opens the door for interventions to limit the progression of the disease and to prevent or delay its complications.
The most common symptoms of diabetes are thirst leading to increased fluid intake (polydipsia), excessive and frequent urination (polyuria), blurred vision, abnormal skin sensation (paresthesia), tiredness and unexplained weight loss.
However, many patients with diabetes are asymptomatic causing the disease often to remain undetected.
The diagnosis of diabetes is usually based on detecting elevated levels of blood glucose. This may be done by measuring fasting blood glucose, HbA1c or by a glucose tolerance test.
Fasting blood glucose level is the concentration of glucose in blood at the moment of the test. Although such a test is informative, it has several limitations.
A glucose tolerance is test in which glucose is administered, and blood samples taken afterward to measure how quickly the sugar is cleared from the blood. Although often useful, it is a cumbersome test and not very useful for screening or to monitor therapy.
What is HbA1c
A widely used blood test to detect diabetes is the measurement of glycated hemoglobin, also called A1C, hemoglobin A1C, glycohemoglobin, or HbA1C. An HbA1c reading can be taken from blood from a finger but is often taken from a blood sample drawn from a vein in the arm.
By measuring HbA1c, it is possible to get an overall picture of what the average blood sugar levels have been over a period of weeks or months.
The test is based on the fact that glucose enters red blood cells and becomes irreversibly attached to hemoglobin within the cells. The rate at which glucose binds to hemoglobin is dependent on the blood glucose concentration.
Approximately 1 percent of red blood cells are destroyed each day, and an equal umber of new ones enters the circulation. Therefore, HbA1c reflects the mean blood glucose over the lifespan of the red blood cell (about 120 days) and correlates well with the mean blood glucose over the previous 8-12 weeks.
Thus, measurements of HbA1c are helpful to detect diabetes as well as monitoring therapy in those already being treated for diabetes. The higher the HbA1c, the greater the risk of developing diabetes-related complications.
As HbA1c reflects long-term control of blood glucose, it is often used to monitor the effects of diet, exercise, and drug therapy in people with diabetes. If blood sugar levels have been high in recent weeks, HbA1c will also be higher.
HbA1c can be expressed as a percentage (DCCT unit) or as a value in mmol/mol (IFCC unit).
The American Diabetes Association (ADA), European Association for the Study of Diabetes (EASD) and International Diabetes Federation (IDF) have agreed that, in the future, HbA1c is to be reported in the International Federation of Clinical Chemistry (IFCC) units (4).
How Is HbA1c Interpreted in Clinical Practice?
In healthy people, the HbA1c level should be less than 6% (42 mmol/mol) of total hemoglobin.
The term prediabetes is often used to describe a situation where the blood sugar level is higher than normal but not yet high enough to be classified as type 2 diabetes. Without intervention, patients with prediabetes are likely to develop diabetes.
The 2010 American Diabetes Association (ADA) standards of care for diabetes, based largely on the opinion of an international expert committee, added HbA1c as diagnostic criteria for diabetes (5).
According to the ADA, prediabetes is defined as HbA1c 5.7–6.4% (39–47 mmol/mol) and diabetes is defined as HbA1c > 6.5% (48 mmol/mol)
For patients with established diabetes, the treatment goals vary somewhat. The International Diabetes Federation and the American College of Endocrinology recommend achieving HbA1c values below 6.5% (48 mmol/mol), while the American Diabetes Association recommends HbA1cA below 7% (53 mmol/mol) for most patients (5).
In general, it is recommended that treatment of diabetes be directed at keeping an individual’s HbA1c level as close to normal as possible without episodes of hypoglycemia (low blood glucose levels).
Here’s how HbA1c levels are interpreted in clinical practice:
HbA1c levels between 5.7% and 6.4% (39–47 mmol/mol) indicate increased risk of diabetes (prediabetes)
Diabetes is defined as HbA1c > 6.5% (48 mmol/mol)
For people with diabetes, an HbA1c of 6.5% (48 mmol/mol) is considered good control
For people at risk of low blood sugar (hypoglycemia), a target HbA1c of 7.5% (59 mmol/mol) may be more appropriate