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Article by Sue Smith. For permission to use this article please contact: Editor@a1cdiabetescentral.com.
Case Study: Potential Pitfalls of Using HbA1c as the Sole Measure of Glycemic Control - Two instances where persons were diabetic, but reassured they were not based on falsely low HbA1c test results. Both patients were, however, diabetic.
What are normal and target blood glucose ranges?
Diabetes Care Tips Blood Glucose Monitoring
Target HbA1c Levels
Target for most people with diabetes: At or below 7%
Time to make changes in your diabetes care plan: 8.0 (8%) or higher, or lower than 4.7%.
Interested in learning more about home HbA1c testing?
Rick Mendosa's diabetes website offers a comparison of home test kits.
Blood Sugar Conversion Tool
The BD Diabetes website explains the HbA1c test this way:
"When glucose is in the blood stream waiting to be used by the cells, some of it can become permanently attached to red blood cells. The attachment occurs at the same part of the red blood cell that carries oxygen, called the hemoglobin molecule, but does not affect the cell's ability to carry oxygen. In the absence of diabetes, the body takes glucose efficiently into the cells, so there is usually very little free glucose available to attach to hemoglobin. In that case, the hemoglobin A1c level stays around 4 to 6 percent, which means that only 4 to 6 percent of red blood cells have glucose attached to them.
"When you have diabetes ... you end up with an excess amount of free blood glucose. More glucose in the blood means more glucose attaching to the red blood cells, and that's why the HbA1c level in a person with untreated diabetes may be as high as 10 percent or more. The higher your HbA1c test results, the greater the amount of sugar in your blood. And high blood sugar over a long period of time may increase your risk of diabetes complications."
What is a CBC?
A complete blood count (CBC) is a test that measures many things:
- The number of red blood cells (RBCs)
- The number of white blood cells (WBCs)
- The total amount of hemoglobin in the blood. Hemoglobin (Hb), a protein that is the main component of red blood cells, carries oxygen from the lungs to the body's tissues, and carbon dioxide from the tissues to the lungs, to be exhaled.
- Hematocrit - the fraction of the blood composed of red blood cells
- The mean corpuscular volume (MCV) -- the size of the red blood cells (this is important because misshapen blood cells can produce false results in some types of tests)
CBC tests also provides information about:
- MCH (mean corpuscular hemoglobin)
- MCHC (mean corpuscular hemoglobin concentration)
- The platelet count is also usually included in the CBC.
Are HbA1c tests more accurate than fructosamine tests?
Falsely high glycated haemoglobin [HbA1c] because of haemoglobin OSU-Christiansborg
Are they comparable? A study in the comparison of fructosamine with glycosylated hemoglobin and plasma proteins as measures of glycemic control.
HbA1c superior to fructosamine testing in pediatric use: Fructosamine and glycated haemoglobin in the assessment of long term glycaemic control in diabetes.
Glycosylated hemoglobin test print this article
Mini Site Index
Note: You might also be interested in reading about another test to monitor glucose control called the "fructosamine test." This test may be used instead of, or in addition to an HbA1c test for women with diabetes that become pregnant, or those that have recently had a major change in their diabetes care plan. The fructosamine test measures glucose control over the previous 2-3 weeks and may be used to determine how blood glucose (blood sugar) control is being affected after recent changes in care plans sooner than an HbA1c test could show.
A fructosamine test might also be ordered when there is some medical condition that affects the average age of red blood cells (RBCs). See more detail at the bottom of this article under "When might an HbA1c not be accurate?"
What is an HbA1c test?
You are probably familiar with daily blood glucose testing. Another test for blood glucose, the HbA1c (hemoglobin A1c test, shows what your overall blood glucose was for the past 3 months. It is not performed each day, but should be done 2-4 times per year at your doctor's office. The HbA1c test measures the amount of glycosylated hemoglobin in your blood, or, more simply put, this test shows how much glucose is sticking to your red blood cells.
How is the test done?
HbA1c tests can easily be done with just a regular finger stick, just like testing for blood glucose. Home tests are now available too, but should not be used as a substitute for the one that your doctor performs. There is no risk of getting AIDS, hepatitis, or any other blood-borne disease from this test.
How often should I have an HbA1c done?
The American Diabetes Association (ADA) recommends having an HbA1c test done at your doctor's office or a lab every 3 months. Also, red blood cells (which bind with glucose) only live in the body for about 90 days, which is why the HbA1c test needs to be done several times a year.
Why can't I just perform home HbA1c tests?
You can use home test kits in between doctor visits but there are at least three good reasons why you should not use them as a substitute for the one your doctor performs:
What do the test results show?
The HbA1c test result represents an average for the previous 1-3 months of your free blood glucose level. Your blood glucose level goes up and down throughout the day and the specific amount of glucose in the blood at any given time is known as your "free blood glucose level." An HbA1c test suggests and average of glycemic control over the previous 120 days (red blood cells and the hemoglobin they contain have an average life span of 120 days).
Normally, only a small percentage of the hemoglobin (Hb) molecules in red blood cells become chemically linked (glycosylated ) to glucose (sugar). The glucose "sticks" or binds (glycosylates) to the red blood cells and these cells can be counted. The number of glycosylated blood cells increases over time, and the more often you have high blood glucose, the higher your HbA1c test results will be. Remember, the hemoglobin in red blood cells that fuses with glucose can be measured up to 120 days later so an occasional high or low here and there will not have a dramatic impact on your HbA1c, but chronic blood glucose problems will.
The HbA1c is an important tool for you and your doctor to help determine how well you are managing your care over the long-haul. It also can help predict the level of risk for developing longer-term diabetes complications. But you should also know that you can have high blood glucose on some days and low blood glucose on others, and the average could create a misleading "good" HbA1c reading. Those "bad"days can still be unhealthy and create risk factors for problems like diabetic neuropathy and retinopathy.
For example, if you run high all day long but low all night long, your average HbA1c might be in an acceptable range. But running high -- even if your HbA1c is good is still not good for you. This is why it is so important to check your blood glucose at least 5 times a day. Unless you check your blood glucose often, you may never know that you are running too high at certain times of the day until your HbA1c comes back high.
What HbA1c level is considered "good" for persons with diabetes?
Your HbA1c percentage rises as your average blood glucose level rises. The normal range for a person without diabetes is 4 to 6%. Your doctor should determine what your target HbA1c should be. For some, this means 6% or lower, for others (like the very elderly) a recommended target may be slightly higher than 7%. But for most people with diabetes the American Diabetes Association (ADA) recommends targeting for an HbA1c of 7% or less.
Generally, a result of 7.0 (7%) or lower usually means that your blood glucose is under control (keep up the good work!). If your HbA1c is 8.0 (8%) or higher, it means that your blood glucose may be too high and you are at an increased risk for developing diabetes complications. You may need a change in your meal plan, physical activity plan, or diabetes medications, or insulin.
A1CDIA Health Tip: It is important to continue to test your blood glucose on a daily basis. An HbA1c is not a substitute for daily glucose monitoring, and daily glucose monitoring is not a substitute for having an HbA1c done. Also, keep in mind that the test averages blood glucose levels. If you are hypoglycemic through the night but high during the day your diabetes is still out of control but your average on the HbA1c may be acceptable. This is because frequent lows (which are not a good thing) can bring down the overall average.
HbA1c Results and Equivalent Average Blood Glucose Level
Chart Note: (1) An HbA1c for a person with diabetes lower than 4.7 is too low (some consider lower than 5.0 as being too low). The risk of loss of consciousness, insulin shock, seizure, coma, and death from hypoglycemia for a person with diabetes increases significantly when an HbA1c falls below 5.0. According to the Diabetes Monitor, "...if your HbA1c is above 10%, there's plenty of room for improvement. If it's 6.5% that's pretty good. 5.9% would be classed as excellent."
See Minimed's chart of risks associated with low HbA1c levels.
A high HbA1c always indicates glucose control problems, but remember, a low reading may also indicate frequent hypoglycemia (low blood glucose) episodes are "balancing" the high blood glucose levels. Be sure to keep daily records of your blood glucose levels because your doctor needs information from both HbA1c levels as well as daily glucose readings to help you manage your diabetes.
When might an HbA1c test not be accurate?
If a patient's HbA1c conflicts with self blood glucose monitoring (SBGM) the HbA1c test might not be an accurate reflection of overall blood glucose control. For example, if a person has many SBGM readings in the 300-400s and an HbA1c of 6% the two do not reconcile and there may be a medical condition affecting the test results.
Your doctor may wish to check your creatinine levels and do a CBC test (complete blood count, see sidebar on left). A high creatinine level can indicate renal (kidney) impairment which can lead to anemia; people that are anemic can have false, lower HbA1c readings.
Also, incorrect blood specimen handling that causes red blood cell (RBC) destruction (hemolysis) can produce false, lower HbA1c results.
Low numbers of red blood cells (RBCs) can affect HbA1c results and may indicate, or be caused by:
Low hematocrit (see sidebar on left) may indicate, or be caused by, all of the above as well as the possibility of rheumatoid arthritis.
Low hemoglobin values may indicate, or be caused by:
Conversely, a high hematocrit and high RBC can provide falsely elevated HbA1c test results.
A1C Diabetes Central of Health Tip: Remember, your doctor needs the information from your daily diabetes log to compare with the results of your HbA1c. An HbA1c not only can help determine how well you are controlling your blood glucose during the previous months, but an HbA1c that does not seem to agree with the daily diabetes log can also serve as a tool to indicate other problems your doctor may want to investigate further.
Links to More Information about HbA1c Tests
Page Updated 07/24/2006