HbA1c — 2B or not 2B?

There is an interesting commentary in a recent issue of the Journal of the American Medical Association on the value and limitations of HbA1c for the diagnosis of diabetes. The authors point out the limitations of HbA1c in establishing a diagnosis of diabetes and suggest some alternatives that have not yet been fully evaluated.

They provide an important list of hematologic conditions that affect the measurement, including variable red cell life span (RDW), anemia, hemolysis, reticulocytosis and hemoglobinopathies. Medications (erythropoietin, dapsone), hypothyroidism and mechanical heart valves are also on their list of possible confounding factors. There are also racial differences in the relationship between HbA1c and a diagnosis of diabetes.

To me, a more important question is whether HbA1c is a reliable measure of glycemic control in an individual patient, and my answer is: “It depends.” As far as I can gather, the HbA1c is reliable in patients with fairly stable home blood glucose values, but too few patients measure blood glucose often enough to know this. It’s not the patient’s fault — their insurance carrier is very niggardly about the number of times a day a patient is allowed to measure blood sugar. If only the carrier would recognize the longer-term health care costs in those patients! All too often the patient leaves the glucose meter at home — that’s like going to the pediatrician without the baby.

Serial HbA1c is far less reliable in patients with widely fluctuating blood glucose values. Continuous glucose monitoring (CGM) is being utilized more frequently to document widely fluctuating blood glucose values, and patients learn a lot from the graphical illustration of how well or not well they are taking care of their diabetes.

Point-of-care HbA1c instruments are available and are also being used more often in the clinic. It makes sense to have the data available while the patient is still in the clinic rather than wait 24 hours for the lab result to come back. Caveat emptor! If you are of a mind to get a handheld HbA1c device for your clinic, I encourage you to first read the editorial and article published in Clinical Chemistry about 1 year ago. Only two of the eight available devices were reliable!

For more information:

Bruns DE. Clin Chem. 2010;56:4-6.
Hempe JM. Diabetes Care. 2010;33:1449-1451.
Lenters-Westra E. Clin Chem. 2010;56:44-52.
Rubinow KB. JAMA. 2011; 305:1132-1133.