|There will be a new international standard for doing A1c testing in order to eliminate some local lab differences in method. In order to make the reporting more understandable to persons with diabetes and their health care providers, the results of the A1c test will be reported both in the old units of A1c as a % reading as well as a mathematically derived Average Glucose over a 3 month period. At present many people with diabetes believe that an A1c of 7% correlates with an Average Glucose of 7 mmol/L which is of course incorrect. An A1c of 7% corresponds to an Av Glucose of 8.6 mmol/L (154 mg/dL). The concept of an Average Glucose is easier for the person with diabetes to understand and relate to the results on home glucose monitoring.
The equation: AG (average glucose in mmol/L) = 1.583 x HbA1c - 2.52. (Note that 1 mmol/L = 18 mg/dL) . Thus an A1c of 7% represents an Average Glucose of 1.583x7-2.52=8.5 mmol/L (or 8.5x18=153 mg/dL)
With this system, providers will have three numbers: the usual A1c percentage, the new IFCC version in mmol/L, and the new estimated average glucose.
The implications are that, hopefully, patients will find it easier to integrate this information into their management behaviors and improve control because the average glucose scale matches that of glucose meters. Also, manufacturers of A1c equipment will need to update their software. Prior to this new information, the A1c was tied to the results of the DCCT, where a 6% was equal to 7.5 mmol/L (135mg/dL). This came about when they checked the A1c and then looked at a couple of thousand finger sticks and averaged them out. Now with the use of hundreds of thousands of readings, not just with the finger sticks but also with the use of continuous blood glucose monitors, we have more accurate results. The equation yields a linear correlation over a wide range of A1c.
The new numbers:
A1c = GLU mmol/L
6% = 7 mmol/L (126 mg/dl)
7% = 8.6 mmol/L (155 mg/dl)
8% = 10 mmol/L (182 mg/dl)
9% = 11.7 mmol/L (211 mg/dl)
10% =13.3 mmol/L (239 mg/dl)
Now because of this new equations we have to ask which measure is most important, the A1c or blood glucose? The ADAG trial showed no difference between Glucose Meter and CGM data. Perhaps A1c is not the gold standard, at least for correlating cardiovascular disease with glucose levels. After all, data presented at EASD showed that patients with the same A1c can have different ‘area under the curve’ PPG. For these patients, high PPG values may be a better indicator of inflammation and CVD risk than A1c. It should be the combination of data that will most help patients understand how the A1c and the average blood glucose are both important.
At the Diabetes Clinic we will be reporting A1c in conventional % units as well as the ADAG Average Glucose reading.