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The diagnosis of diabetes mellitus is traditionally based on demonstrating high blood glucose.

The American Diabetes Association (ADA) criteria, 1997:

1. Symptoms of diabetes plus casual/random* plasma glucose (RPG/RBS) concentration ≥200 mg/dl (11.1 mmol/l), or
2. Fasting** plasma glucose (FPG/FBS) ≥126 mg/dl (7.0 mmol/l), or
3. 2-hour postload/postprandial*** glucose (PPG/PPBS) ≥200 mg/dl (11.1 mmol/l) during an OGTT.

* Casual/random is defined as any time of day without regard to time since last meal. The classic symptoms of diabetes include polyuria, polydipsia, and unexplained weight loss.
** Fasting is defined as no caloric intake for at least 8 h.
*** The test should be performed as described by WHO, using a glucose load containing the equivalent of 75 g anhydrous glucose dissolved in water.
+ In the absence of unequivocal hyperglycemia, these criteria should be confirmed by repeat testing on a different day. The third measure (OGTT) is not recommended for routine clinical use.

In 1997, it was also mentioned that

• The FPG is the preferred test to diagnose diabetes in children and nonpregnant adults, and
• The use of the glycated hemoglobin (HbA1C) for the diagnosis of diabetes is not recommended at this time.

Now, in 2010, ADA has released new recommendations for diabetes diagnosis:

1. HbA1C ≥6.5%. The test should be performed in a laboratory using a method that is NGSP certified and standardized to the DCCT assay, OR
2. FPG ≥126 mg/dl (7.0 mmol/l), OR
3. 2-h plasma glucose ≥200 mg/dl (11.1 mmol/l) during an OGTT, OR
4. In a patient with classic symptoms of hyperglycemia or hyperglycemic crisis, a random plasma glucose ≥200 mg/dl (11.1 mmol/l).

In addition, HbA1c score of 5.7% to 6.4% indicates prediabetes.

This is a welcome development. Until now, HbA1C was used only for monitoring of therapy of diabetes. However, I have been using this, though unofficially, for quite sometime for diagnostic purposes too. HbA1c served as a very valuable tool for me in differentiating diabetes mellitus from stress/reactive hyperglycemia in non-diabetics. At times, I have also used it to diagnose diabetes in patients who had blood glucose values in equivocal ranges.

As early as in the beginning of 2009 I was discussing with one of my colleagues that HbA1c is should become a diagnostic test for diabetes, just based on logic. Little was I aware that it will come out with evidence too!

Now that it is official, the major advantage of HbA1c is that it does not require a fasting state for testing, thereby encouraging more people to undergo testing.

On the other hand, it is quite expensive. In most centers, the cost of HbA1C testing is 5-10 times the cost of a plasma glucose testing! Hopefully, with more testing, the costs will come down.

Reference: Diabetes Care January 2010 vol. 33 no. Supplement 1 S62-S69