Also called: Fasting Hemoglobin, HbA1C Test, GHb Test, Diabetic Control Index, Hemoglobin A1C Test, Glycosylated Hemoglobin Test, Total Fasting Hemoglobin Test, Hb A1c
A glycohemoglobin test is a blood test used primarily to monitor diabetes. It measures a patient’s average glucose (blood sugar) level over the past two to three months.
Along with other glucose tests, glycohemoglobin (GHb) tests can indicate how well a patient’s diabetes has been controlled and whether the average blood glucose level is too high. The test can also verify self-testing results and blood test results from a physician, determine the effectiveness of a diabetes management plan and predict a patient’s risk of developing complications.
Many physicians refer to a glycohemoglobin test simply as an A1C test. This is actually the most common type of glycohemoglobin test ordered by physicians. Traditionally requiring laboratory analysis, glycohemoglobin tests can now be performed in a doctor’s office or even at home with results available within minutes.
Glycohemoglobin, or glycosylated hemoglobin, is a term used for glucose that has glycated (bonded) with hemoglobin, the part of the blood that carries oxygen to the cells of the body. High levels of glycosylated hemoglobin are proportional to high blood glucose (hyperglycemia). Excess glucose can damage the nerves, blood vessels, heart, kidneys, eyes and other organs.
Once glucose attaches itself to hemoglobin, it remains there for the life of the red blood cell, about 120 days. As a result, the evidence of high glucose is still detectable as glycosylated hemoglobin, even after blood glucose levels have returned to normal. Whereas most other glucose tests provide a “snapshot” of the amount of glucose in a person’s blood at any one time, glycohemoglobin tests provide more of an indirect measurement of glucose levels for the past few months.
Most people with diabetes require a glycohemoglobin test two to four times a year. Changes in diet, activity or daily medications are not required before testing.
About the glycohemoglobin test
The glycohemoglobin test (also known as the GHb, A1C or HbA1C test) measures the amount of glycosylated hemoglobin in the blood. Hemoglobin is a protein in red blood cells that transports oxygen to the cells in the body. Hemoglobin combines (glycates) with glucose (blood sugar) to make glycosylated hemoglobin, which is proportional to the amount of glucose in the blood.
Whereas glucose self-monitoring is a method of measuring day-to-day glucose control, a glycohemoglobin test provides an overall view of how a diabetes management plan is working. The test measures a person’s average glucose control for the previous two to three months.
The test reveals past glucose levels because once glucose attaches itself to hemoglobin, it remains there for the life of the red blood cell, about 120 days. So even after blood glucose has returned to normal, evidence of high glucose is still detectable as glycosylated hemoglobin.
There are many types of hemoglobin. The ones most commonly tested are glycosylated hemoglobin A1 (Hb A1) and three of its components – Hb A1A, Hb A1B and Hb A1C. Many physicians refer to the glycohemoglobin test as A1C, because this is the individual test most commonly ordered.
Glycohemoglobin levels are measured as a percentage of total hemoglobin. The average person has between 4 and 6 percent glycosylated hemoglobin. A diabetic individual with high blood glucose (hyperglycemia) will have a higher percentage. The amount depends on the person’s average glucose level for the weeks leading up to the test.
The American Diabetes Association (ADA) recommends that people with diabetes have a glycohemoglobin test when they are diagnosed or when they begin treatment. After the initial test, additional glycohemoglobin tests are recommended:
|For Patients Who:|
|Twice a year||Control their diabetes with diet, exercise and antidiabetic agents|
|Every three months||Control their diabetes with insulin Have poor glucose control Are starting a new medication or other treatment|
to six weeks
|Are pregnant or trying to conceive|
All people with diabetes can benefit from a glycohemoglobin test. Benefits include:
- Confirming other test results. Glycohemoglobin tests can verify self-testing results and blood test results from a physician.
- Determining the effectiveness of a diabetes management plan. If results are above a patient’s target percentage, the physician can adjust the meal plan, exercise plan or medications.
- Predicting the patient’s risk of developing complications. High levels of glycosylated hemoglobin are equivalent to high levels of blood glucose. Hyperglycemia increases a person’s risk of developing complications, such as:
- Kidney disease (diabetic nephropathy)
- Nerve damage (diabetic neuropathy)
- Cardiovascular disorders (including heart disease, stroke, atherosclerosis and diabetic angiopathy)
- Eye diseases (diabetic retinopathy, glaucoma, cataracts)
- Kidney disease (diabetic nephropathy)
- Encouraging the patient. Good test results can show people the effectiveness of their decisions on diet, exercise and quitting smoking.
Although glycohemoglobin tests can provide an idea of how well a patient’s diabetes management plan is working, there are disadvantages if only these tests are used:
- Day-to-day glucose control is not measured. Patients must still rely on daily self-testing with a glucose meter to spot patterns of high and low glucose.
- Dosage of medication cannot be adjusted based on glycohemoglobin test results. Patients taking insulin or certain antidiabetic agents need daily results to adjust insulin doses and prevent low and high glucose.
- Many factors can cause inaccurate results, including:
- Diseases that affect hemoglobin, such as anemia
- Diseases of the liver and kidneys
- Vitamins C and E
- High levels of lipids
- A blood transfusion or severe blood loss within three months of testing
It is important for patients to be aware that laboratories measure hemoglobin levels in different ways. As a result, normal values may vary slightly from lab to lab. However there is an effort to standardize the test so that all results are comparable to those of the Diabetes Control and Complications Trial (DCCT). The National Academy of Clinical Biochemists recommends that U.S. labs use methods certified by the National Glycohemoglobin Standardization Program. Patients should be aware of the normal range for the lab their physician is using.
Although the glycohemoglobin test is the primary blood test used to monitor diabetes management, another test called the fructosamine test may be used if information about glucose control is needed for the past few weeks rather than months.
When nondiabetics have a physical examination, they often undergo routine blood testing to screen for diabetes and check cholesterol. This blood work may include a glycohemoglobin test in addition to a fasting plasma glucose test or an oral glucose tolerance test, either of which is a preferred method of diagnosing diabetes.
Types and differences of GHb tests
Traditionally, glycohemoglobin (GHb) tests have been performed in laboratories. A blood sample would be taken in a physician’s office and the sample would be sent to a laboratory for analysis. However, more options are now available. Glycohemoglobin tests can now be performed in physicians’ offices and at home.
Laboratory tests are considered very accurate. However, they require a large blood sample and can take days to provide results. Now there are desktop instruments that let physicians test glycohemoglobin in their offices. A drop of blood from a patient’s fingertip is tested, with results available within minutes. The physician can then discuss the results with the patient.
Kits are also available for glycohemoglobin testing at home. Most require patients to take their own blood sample and mail it to a lab for results. At least one version allows patients to see a digital reading in less than 10 minutes, but it is a one-time-use test.
When used properly, home tests can provide accurate results. Self-tests, though, have a greater risk of error than laboratory conducted tests.
Patients using a home test should have a strong sense of how to perform the test and what the results will mean. People can benefit from performing the test under the supervision of a physician or a certified diabetes educator to ensure the test is being done correctly. Patients should always discuss their home test results with their physician. Home monitoring should not replace the regular glycohemoglobin test performed by a physician.
Before, during and after GHb tests
There is no special preparation for a glycohemoglobin test. The blood sample can be taken anytime, without the fasting that some glucose tests require.
When the test is performed in a physician’s office for laboratory analysis, blood is usually drawn from a vein (venipuncture) from the inside of the elbow or the back of the hand. First the puncture site is sterilized with antiseptic and an elastic band is wrapped around the patient’s upper arm. As a result, the pressure restricts blood flow through the vein and causes the veins below the band to fill with blood.
A needle is then inserted into a vein. Occasionally more than one puncture is necessary to locate a vein. Some people may feel a moderate level of pain when the needle is inserted, but most only feel a prick or stinging sensation. Once the needle is inserted into a vein, a tube is attached to collect the blood as it begins to flow out. The elastic band is then removed.
After the necessary amount of blood is collected, the needle is withdrawn and a small cotton ball or pad is applied with light pressure over the puncture site. After several minutes, the cotton will be discarded or replaced, and a small bandage will be placed on the puncture wound. The entire process takes less than 10 minutes. The sample is sent to a laboratory for analysis, and results are returned in several days.
Despite the precautions taken to avoid bruising and soreness, it does sometimes occur. Typically, this is not a cause for concern. To minimize soreness, patients may immediately apply a warm compress to the puncture site and repeat the application every three hours until the discoloration or pain subsides.
Glycohemoglobin tests performed in a physician’s office or at home do not require a venipuncture, but rather drops of blood from a finger prick. Some home tests require the user to mail a blood sample to a lab for analysis. With this test the patient uses a lancet provided in the kit to produce a couple of drops of blood. The patient than places the blood on a special collection card and mails the card to the company’s lab for analysis. The results are then returned to the user.
Some tests provide immediate results to the patient and physician. A physician can collect a drop of blood from a finger prick into a supplied capillary holder. Light pressure and sterile gauze are then applied to the puncture site to stop the bleeding. A bandage is usually not necessary. The sample is then inserted into a cartridge, and the cartridge is placed in an analyzer. Results are available in 15 minutes. The patient can then discuss the results with the physician.
At least one over-the-counter home test provides results in less than 10 minutes. The test requires the user to prick a finger and collect drops of blood into a provided pipette. The patient then dilutes the sample by placing it in a provided test solution and mixing it. The diluted sample is then applied to the provided monitor. Light pressure and sterile gauze are applied to the puncture site to stop the bleeding. A bandage is usually not necessary. Results are available in eight minutes. Some glucose meters also offer glycosylated hemoglobin testing.
Home tests should not replace testing by the physician. Patients performing home tests should always discuss results with their physician.
After the test, the patient may feel some throbbing. There is a slight risk of infection at the puncture site.
Understanding glycohemoglobin test results
A typical goal for people with diabetes, and one suggested by the U.S. National Institute of Diabetes and Digestive and Kidney Diseases, is a glycohemoglobin test result of less than 7 percent. A level of 7 percent means that 7 percent of the hemoglobin is combined with glucose (blood sugar). It also means that the average plasma blood glucose level for the previous two to three months was about 170 milligrams per deciliter (mg/dL).
The International Diabetes Federation (IDF), which includes organizations in 150 nations, issued new guidelines in 2005 urging that people with type 2 diabetes aim for a maximum glycohemoglobin result of 6.5 percent to reduce the risk of diabetic complications. However, the IDF acknowledged that most diabetic individuals fell short of this target. The American College of Endocrinology and the American Association of Clinical Endocrinologists also recommend aiming for 6.5 percent or less. In 2005, though, they released a nationwide study of more than 157,000 people with type 2 diabetes that showed only one-third meeting this goal.
Patients can convert their glycohemoglobin test results as follows:
Blood Glucose Level
|6 percent||135 mg/dL|
|7 percent||170 mg/dL|
|8 percent||205 mg/dL|
|9 percent||240 mg/dL|
|10 percent||275 mg/dL|
|11 percent||310 mg/dL|
|12 percent||345 mg/dL|
Some studies have indicated that black and Hispanic people could have greater levels of glycohemoglobin than people of other racial and ethnic groups who have similar levels of glucose. An international effort is under way to ensure the accuracy of using glycohemoglobin to estimate blood glucose in various populations.
When a patient’s test result is higher than the physician-recommended goal, the physician may make changes to the patient’s diabetes care plan. A change in treatment plan is usually recommended when results are higher than 8 percent. A physician may recommend changes in meal plan, exercise plan or medication. If changes are made, the patient may need to have glycosylated hemoglobin retested in three months to monitor the effectiveness of the new treatment.
A goal of 6.5 percent or 7 percent is not realistic or appropriate for everyone. Patients who are pregnant or trying to become pregnant may have a recommended target of closer to 6 percent, if they are not suffering from too many episodes of hypoglycemia (low blood glucose). Children under the age of 7, the elderly and people with certain health conditions may have goals higher than 7 percent to prevent hypoglycemia.
Patients with hyperglycemia are encouraged to lower their glucose levels to prevent or delay the onset of diabetic complications. Research studies have shown that for every 1 percent reduction in glycohemoglobin test results, the risk of developing eye, kidney, nerve and blood vessel disease is significantly reduced.
Because of the rising incidence of diabetes in recent years, New York City in 2006 began requiring most laboratories to report results of glycohemoglobin tests to the city’s health department. The stated purpose of this requirement is to improve residents’ care and reduce diabetic complications. Patients may be contacted by mail or telephone unless they submit a form to opt out of the registry. Concerns about privacy have kept this concept from catching on elsewhere.
Questions for your doctor
Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about the glycohemoglobin test:
- How often should I have a glycohemoglobin test?
- What does this test assess? When should I have other glucose blood tests instead?
- What is my target glycohemoglobin level?
- Do you test glycohemoglobin in your office, or will my blood sample be sent to a lab? When will my test results be available? Who will explain them to me, and how?
- What do my test results indicate?
- How do my glycohemoglobin results translate to plasma glucose?
- Do my test results show a need for adjustments in my diabetes management plan?
- Am I good candidate for using a glycohemoglobin self-testing kit?
- Do you recommend I use a glucose meter that also reveals glycohemoglobin?
- If I check my own glycohemoglobin, how should I communicate the results to you? How often will I still need to have a glycohemoglobin test performed by you?