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Coming soon to a clinical laboratory near you: a new metric to evaluate the effectiveness of diabetes therapy. In coming months, clinical labs across the country will likely begin reporting levels of glycated hemoglobin three different ways—in terms of the familiar A1C percentage that has been used for years, in increments of millimoles per mole, and with a new measure called average glucose (DOC News, May 2007, page 3).
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Of particular interest to primary care providers and their patients is the average glucose (AG) measure, which provides further information to help patients interpret the A1C reading. AG gives patients the average glucose reading in ml/dl, which—new data show—corresponds with the glucose monitoring scale they already know from home blood glucose testing. The idea is to put patients' A1C percentage in a context they will better understand.
"The ultimate goal is to make this assay more convenient, easier to understand, and more widespread and utilizable than ever before," says Richard Kahn, PhD, chief scientific and medical officer of the American Diabetes Association (ADA).
The change was unveiled at ADA's 67th Scientific Sessions, held June 22–26 in Chicago.
Since its introduction in the late 1970s, the A1C test has become accepted as a standard measure of long-term blood glucose control.
For years, clinical laboratory organizations and medical groups around the world have debated technical issues related to the measurement of A1C and what the test represents.
Three types of technology can be used to measure A1C, based on the electrical charge of glycated hemoglobin, the molecule's size, or antibody testing.
Regardless of the methods used, the results are calibrated to a standard determined by the National Glycohemoglobin Standardization Program (NGSP), based at Washington University in St. Louis.
A new reference method devised by the International Federation of Clinical Chemistry (IFCC) uses mass spectrometry to measure A1C. This is considered to be the most accurate and precise of all the methods.
The NGSP standard has been mathematically correlated to the new reference. As a result, no change in A1C tests or values will be apparent to clinicians or patients.
"The calibration is changing, not how the test is done," says Kahn. The new A1C reference is "completely transparent to users," he says.
These changes reflect several years of discussion among officials of ADA, IFCC, the American Association for Clinical Chemistry (AACC), the European Association for the Study of Diabetes (EASD), and the International Diabetes Federation (IDF).
A NEW AVERAGE
A1C is produced when a glucose molecule binds with hemoglobin, and remains attached for the remainder of the life of the red blood cell. Measurements of A1C are believed to reflect levels of blood glucose over the previous 3 months, but the exact relationship between A1C and average blood glucose levels was unknown.
"The A1C assay is perhaps the most important test we have to assess the control of diabetes," says David Nathan, MD, a professor of medicine at Harvard Medical School in Boston. "We assume that [A1C] value is closely correlated with average blood glucose values. But nobody knew for sure."
In the past, long-term studies of A1C and blood glucose levels relied on periodic tests that didn't adequately capture the daily and hourly fluctuations common to diabetes, Nathan says.
The emergence of inexpensive, reliable, and convenient continuous glucose monitoring made it possible to design a study to see how closely A1C correlates with AG levels.
Nathan is leading an international group of investigators conducting a 10-center study to more accurately define the relationship between A1C and average blood glucose levels. The study eventually will include 700 volunteers—300 with type 1 diabetes, 300 with type 2 diabetes, and 100 without diabetes—who receive continuous glucose monitoring and monthly A1C tests.
A preliminary analysis of the first 250 subjects shows that A1C test results indeed correlate closely with AG levels, according to Nathan. The relationship is sufficiently robust that a scale of average glucose levels can be derived from A1C results.
"As we thought, based on incomplete evidence, there is a tight correlation between average blood glucose and A1C measurements," Nathan says. "We can use that tight correlation to provide a transformation of the A1C level into an average blood glucose."
If the findings hold up when the study is completed, average glucose will become a routine part of lab test results for people with diabetes. Final results are expected to be reported at EASD's annual meeting in Amsterdam in September.
"Now if patients come in with an A1C level of 8%, we can tell them with confidence that that equals an average blood glucose of 180," Nathan says. "They can have that chronic metric in the same units as their day-to-day glucose monitoring."
Michael Scott, PhD, of Washington University and past president of AACC, says clinical labs in the U.S. will report A1C results in terms of the familiar percentage and the new AG value.
The millimole per mole increment may be included on lab test results, but isn't widely used in the U.S. The unit of measure is required in Germany and other European countries. "I don't expect to see American doctors use millimoles during my lifetime," Scott adds.
Kahn says ADA and other professional groups will begin promoting AG as a
measure of long-term glycemic control. "Having an A1C-derived average
blood glucose is beneficial for patients," he says. "We all agree
to push aggressively to have patients use and become more familiar with the
average glucose reading. This will be the term used with patients to denote
chronic glycemic levels." 
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