The glucose vs. HbA1c controversy

Which test should be used for the diagnosis of diabetes: glucose or HbA1c? Over the last few years, an increasing number of countries have moved from fasting plasma glucose to HbA1c as the method of choice. The measurement of glucose, which goes back more than 100 years, far pre-dates HbA1c, and it can be argued that it measures the analyte that is widely known as being central to diabetes. Hemoglobin does not have so strong a pedigree as a diabetes diagnostic. The red protein that transports oxygen from the lungs to the tissues and carbon dioxide back to the lungs is an artifact, or at most an indirect measure, of average glucose. HbA1c testing has historically been recommended only to determine glucose control in those who already have been diagnosed as diabetic. Why, then, are we seeing an increasing move toward measuring HbA1c rather than fasting plasma glucose as the diagnostic test for diabetes?

Some relevant background

The red blood cell has an average circulating life of some 120 days. The cell membrane allows some reagents to cross into the cell. These so-called penetrating solutes include glucose, urea, bicarbonate, phosphate, and water. Hemoglobin, which is highly concentrated within the cell, reacts with free glucose to form glycated hemoglobin. The main glycation site is at the N-terminal valine of the beta chain. The term HbA1c refers to glycation at this specific site. Hemoglobin is also glycated at a number of €-amino lysines such as ß-66, α-16, and ß-17, and also on the alpha N-terminal valine.¹

In 1969 Samuel Rahbar, an Iranian scientist, was the first to report the linkage between diabetes and HbA1c.² He showed a band migrating ahead of HbA (toward the cathode) using agar gel electrophoresis at pH 6.2. This band had the same chromatographic mobility as the HbA1c peak on a Bio-Rex 70 column. He reported that normal, non-diabetic subjects had HbA1c levels in the range of four percent to six percent of the total hemoglobin, whereas patients with diabetes showed levels from 7.5 percent to 10.6 percent.

The Diabetes Complications and Control Trial (DCCT)³ and the UK Prospective Diabetes Studies⁴ on type 1 and type 2 diabetic subjects respectively showed the value of intensive treatment to maintain blood glucose levels close to the normal range, as opposed to conventional treatment. The intensively treated group had a significantly reduced risk of developing complications such as retinopathy, nephropathy, and neuropathy compared with the conventionally treated group. Furthermore, the intensively treated group showed a drop in median HbA1c from 9.1 percent to 7.4 percent in the DCCT study and from 7.9 percent to 7.0 percent in the UK Prospective Diabetes Study.

Since the mid-1990s the NGSP⁵ has focused its efforts on standardizing the measurement of HbA1c and has seen dramatic improvements in the assay. The IFCC⁶ has overseen the development of a primary reference material against which calibrators can be standardized.

Arguments in favor of HbA1c

There are real concerns about the accuracy of blood glucose measurements. The test needs to be performed promptly after collection since glucose will decrease up to 10mg/dL per hour unseparated at room temperature. Frequently there are delays in the blood sample reaching the testing laboratory, often with samples not having been kept chilled. There are also significant day-to-day variations in fasting blood glucose from the same subject, with a coefficient of variation up to 8.3 percent.⁷ In contrast, the coefficient of variation for HbA1c measurements is now consistently below 3.5 percent.

Patients may also fast for a few days prior to their appointment to give a non-representative average of fasting glucose level. Whether or not patients try to cook the results that way, it is argued that HbA1c is more convenient since it does not require a fasting sample and there is less day-to-day variation.

Selvin et al⁸ looked at the prognostic value of HbA1c compared with fasting glucose in a population of non-diabetic adults to identify those at risk for diabetes or cardiovascular disease. They found that a fasting glucose in the pre-diabetic range of 100 mg/dL to 125 mg/dL had no predictive value for coronary heart disease, but an HbA1c between 6.0 percent and 6.4 percent showed an 88 percent risk of developing coronary heart disease. In other words, HbA1c was found to be a better predictor of cardiovascular disease than glucose.

Arguments in favor of glucose

Skeptics of HbA1c point out that all HbA1c assay methods have some bias. This means that reliance on a particular instrument or method may consistently give values that are higher or lower than the actual level. Potentially a diagnosis for diabetes using HbA1c could be missed or falsely given with levels near the critical decision point.

The presence of hemoglobinopathies also may give a false result.⁵ Instruments using boronate affinity methods are largely unaffected by the common variants HbC, HbS, HbE, and HbD. Newer HPLC instruments are generally unaffected, although HbE remains a problem with some.

Any condition that decreases erythrocyte age will lower the HbA1c independently of glycemia. Iron deficiency has been shown to shift HbA1c levels slightly upward.⁹ Again, this may be a problem with lower HbA1c levels where diagnostic decisions are made.

It has been proposed that some individuals have HbA1c values that are higher or lower than expected from measurements of average blood glucose or fructosamine concentrations. This has been termed the glycation gap. However, this is probably more an issue of the limitations of the fructosamine assay, including its dependence upon the albumin level, and the problems in determining an average glucose level, rather than a glycation issue.¹⁰

The verdict?

If it were an easy question, it would have an easy answer; in fact, the controversy continues. But the general trend has been a shift from glucose to HbA1c for the diagnosis of diabetes, and signs are the trend will continue

The main problem with glucose relates to it being a fasting sample that needs to be transported on ice to the laboratory and tested promptly. In contrast, HbA1c does not require a fasting sample and is stable during transportation to the laboratory. The analyzers, thanks to the efforts of NGSP and IFCC, have very good CVs.

As with any laboratory diagnostic test, the result must be interpreted in light of the subject’s clinical situation. This may mean other tests may be required to confirm the diagnosis. In many cases, glucose and HbA1c may work together for the diagnosis of diabetes.

References

1.  Shapiro R, McManus MJ, Zalut C , Bunn HF. Sites of nonenzymatic glycosylation of human hemoglobin A. J. Biol. Chem. 1980;255(7):3120-3127.
2. Rahbar S, Blumfield O, Ranney HM. Studies of an unusual hemoglobin in patients with diabetes mellitus. Biochem Biophys Res Comm. 1969;36(5)838-843.
3. The Diabetes Control and Complications Trial Research Group. The effect of intensive treatment of diabetes on the development and progression of long-term complications in insulin-dependent diabetes mellitus. 1993; NEJM 329(14):977-986.
4. American Diabetes Association. Implications of the United Kingdom Prospective Diabetes Diabetes Care.2002; Vol 25 Supplement 1:28-32.
5. NGSP: Harmonizing HbA1c testing. www.ngsp.org. Accessed September 18, 2015.
6. International Federation of Clinical Chemistry and Laboratory Medi6.cine. www.ifcchba1c.net. Accessed September 20, 2015.
7. Paxton A. Diabetes debate: HbA1c or glucose. www.captodayonline.com/diabetes-debate-hba1c-or-glucose. Accessed September 20, 2015.
8. Selvin E, Steffes MW, Zhu H et al. Glycated hemoglobin, diabetes, and cardiovascular risk in nondiabetic adults. NEJM. 2010;362(9):800-811.
9. Ahmad J, Rafat D. HbA1c and iron deficiency: a review. Diabets Metab Syndr. 2013;7(2):118-122.
10. Sacks DB, Nathan, DM, Lachin JM. Gaps in the glycation gap hypothesis. Clin Chem. 2011;57(2):150-152.