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:

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)

• 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
  • Pregnancy

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.