In the blood, glucose binds irreversibly to hemoglobin molecules within red blood cells. The amount of glucose that is bound to hemoglobin is directly tied to the concentration of glucose in the blood. Since red blood cells have a lifespan of approximately 90 days, measuring the amount of glucose bound to hemoglobin can provide an assessment of average blood sugar control during the 60 to 90 days prior to the test. This is the purpose of the glycated hemoglobin tests, most commonly the hemoglobin A1c (HbA1c) measurement. Since the test results give feedback on the previous two to three months, getting an HbA1c test done every three months will give you good data on your average blood sugars. If you get an HbA1c every six months, you'll miss out on three months worth of information that could help you manage your diabetes.
Variations in Standards
There are several methods for measuring HbA1c, and results from one method cannot be compared directly with results from another method. You must compare your measurement with the standards for the method of measurement that was used, usually against the highest non-diabetic value. The National Glycohemoglobin Standardization Program aims to to standardize glycohemoglobin test results so that clinical laboratory results are comparable to those reported in the Diabetes Control and Complications Trial.
Note also that different patterns of blood sugar values can result in similar HbA1c values. For example, one person may have an irregular pattern of blood sugars, with lots of highs and lows, and an average blood sugar level of 200 mg/dl. Another person may have a very regular blood sugar level of around 200 mg/dl all the time. Though both will have similar HbA1c levels, they have very different situations that require different actions to remedy. Frequent home blood glucose monitoring will provide the detailed information about the pattern of blood sugars to help each person decide how to adjust the insulin to improve their average control.
HbA1c and Blood Sugar Values
Your HbA1c value can tell you what your average blood sugar has been for approximately the previous three months. The chart to the right shows the approximate relationship between HbA1c and average blood sugar values, as reported in the DCCT.
A1c (%) Mean Plasma Glucose mg/dl mmol/l 6 135 7.5 7 170 9.5 8 205 11.5 9 240 13.5 10 275 15.5 11 310 17.5 12 345 19.5
From Diabetes Care - 26 (Supplement 1): Table 1
HbA1c Measurement Methods
There are several methods of performing an HbA1c measurement:
- Traditional Laboratory
Laboratory methods, run by commercial laboratories and hospitals and large clinics. There are several varieties of methods used by these labs; the most accurate is called HPLC. You don't have to worry about the methodology that the lab uses, since their equipment is highly reliable, rechecked at least daily, and is monitored by outside Federal agencies. The ranges reported by different labs as being "normal (non-diabetic)" will vary; be sure to get the range as well as your test value in order to interpret the test results. While traditional laboratory testing does produce excellent results, it usually requires a large blood draw (typically from the arm, into a vial) and takes days to over a week to get results back to you.
- Immediate Feedback During Your Clinic Visit
The DCA VantageTM Analyzer (previously known as the DCA 2000+) provides HbA1c values in 6 minutes from one drop of capillary blood obtained via a finger stick. Also, Abbott sells the Afinion AS100 Analyzer System, which provides a 3 minutes HbA1c result. If your diabetes team has one of these products (or something similar), you can review your HbA1c values during the clinic visit, rather than receiving a report days or weeks later. A study in 1999 showed that immediate feedback on HbA1c improves control. For health care providers who do not have a point-of-care HbA1c device, single use tests like the A1cNow+ offer a way to provide immediate feedback to the patient during an office visit.
- In-Home Testing
You can now perform an HbA1c test at home. With some tests, you get results back in the mail. With the A1CNow® SELFCHECK System, you get the results in eight minutes, right in your home. If you are unable to get to your doctor for regular HbA1c tests, these products may be of interest. The companies listed below sell FDA-approved in-home A1c collection kits.
- A1CNow® SELFCHECK by Bayer Healthcare - Diabetes Care
Two-test kit for $29.99
FDA-approved for in-home use without a prescription
Provides a result in eight minutes, unlike mail-in test kits
Available online at Walgreens.com and other online pharmacies
- A1c At Home
One test $24.95
FlexSite Diagnostics, Inc., 3543 SW Corporate Parkway, Palm City, FL 34990
- Accu-Base Hemogloboin A1c Sample Collection Kit
One test $25.95 each including lab analysis and reporting
Diabetes Technologies, Inc., PO BOX 1954, Thomasville, GA 31799-1954
Phone (888) 872-2443 or (229) 227-1245, Fax (229) 227-1752
- Appraise Hemoglobin A1c at Home Test
$19.95 including lab fees
Heritage Labs, 1111 West Old 56 Highway, Olathe, KS 66061
For More Information
- The ADA's Estimated Average Glucose Calculator will help you learn how HbA1c relates to average glucose levels.
- If You Have Diabetes, Know Your Blood Sugar Numbers (PDF)
- A1c Test from the American Diabetes Association
- HbA1c Test Results (%) and Their Blood Sugar Equivalents (mg/dl)
- The UK NHS offers New HbA1c Leaflets which explain the coming change in how A1c results will be reported in the UK beginning in June 2009.
- hange in measurement of HbA1c in the UK.
- HbA1c Converter.
- Management of Diabetes Mellitus: Perspectives of Care Across the Life Span edited by Debra Haire-Joshu, MSEd, MSN, PhD, RN. Pg. 14.
For More Information
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