Until recently, the A1C test was not recommended as a means to diagnose diabetes. But that's changed.
For years, physicians and patients have been advised to use the hemoglobin A1C test (also called "HbA1c" or more recently, "A1C") as a tool to monitor diabetes control. As is well-known, the A1C test measures the average blood glucose level (BGL) over several months. If your A1C is elevated, then it's safe to assume that your average BGL has been high. There are potential glitches in the logic, however, if your A1C is normal: your A1C might be normal because your blood glucose has been normal throughout the past few months, or your A1C might be normal because you have had lots of elevated BGLs, and lots of hypoglycemic levels, and the highs and lows balanced out.
(Parenthetically, there are also a few very rare situations where the A1C might be inaccurate: abnormal hemoglobins fouled up some of the early assays and caused falsely-elevated values; on the other hand, problems with red blood cell survival, such as anemias and a few medications, can cause depressed values for A1C. But usually, the A1C test is a very accurate tool to follow the progress of a patient's glucose control, and it correlates well with likelihood of the microvascular complications of diabetes: lower A1C indicates lower risk of developing diabetic eye disease, kidney disease, and nerve disease.)
Normal A1C levels in most labs is usually considered to be somewhere around 5.0%, with different laboratories reporting different ranges. (The lab that does my A1C testing currently uses 4.8-5.9%; I've seen 4%-6% and "less than 6%" on earlier lab reports.)
The question for years has been: why not use the A1C test to screen for diabetes, or use elevated values to diagnose diabetes? There's been lots of debate about this for years. One obvious concern: what level of A1C to use as the cut-point?
Last year, an international expert committee with members from several diabetes organizations (the American Diabetes Association (ADA), the International Diabetes Federation (IDF), and the European Association for the Study of Diabetes (EASD)) presented their recommendations about using the A1C for the diagnosis of diabetes. Like most committees, their report, which was announced at the ADA meeting in June 2009, and which is available on-line, generated its share of controversy. The committee weighed the pro's and con's of using the A1C test, and suggested that if the A1C is 6.5% or higher, that it's high enough to make a diagnosis of diabetes. They also advised that the diagnosis should be conﬁrmed with a repeat A1C test, unless clinical symptoms and glucose levels of 200mg/dl (11.1mmol/l) or more are present.
They also agreed that glucose levels can still be used to make the diagnosis. As one committee member pointed out, the A1C "may be too expensive to use in the developing world. Some of these countries have severe chronic anemia, hemolytic anemia, and so on, where we will have to fall back on traditional tests."
Based on this report, the ADA has recently changed its mind from a year ago, and the criteria for the diagnosis of diabetes now include elevated A1C levels. The information, published as part of the ADA's Clinical Practice Recommendations for 2010, in a document about the Diagnosis and Classification of Diabetes Mellitus indicates four ways to make the diagnosis, three using BGL as before, and now a new one based on elevated A1C levels:
1. A1C ≥6.5%. The test should be performed in a laboratory using a method that is NGSP certified and standardized to the DCCT assay.*
*In the absence of unequivocal hyperglycemia, criteria 1-3 should be confirmed by repeat testing.
After years of having patients come to the laboratory fasting, or having them swallow tons of gooey sweet syrup during a glucose tolerance test, the diagnosis of diabetes can now be made with a simple blood test, done at any time of day, with no concern whether the patient is fasting. Isn't science wonderful?