Measurement of Hemoglobin A_1c

Evaluation of glycemia is used for the diagnosis and management of patients with diabetes. Glucose and hemoglobin A_1c (HbA_1c) provide complementary information and both are used to assess an individual’s glycemic status. The concentration of glucose in the blood indicates the subject’s glycemia at the time of blood sampling. However, blood glucose concentrations are modified by numerous factors, ranging from food ingestion and exercise to stress and medication (1). By contrast, the concentration of HbA_1c in the blood reflects the average glucose over the preceding 8–12 weeks. Thus, HbA_1c provides an additional criterion for assessing glucose control that is free of the wide diurnal fluctuations that occur with blood glucose. HbA_1c has several additional attributes, which render it valuable in the setting of diabetes. These include, but are not limited to, the following: the subject does not need to be fasting, blood can be sampled any time of the day, the sample is stable, and there is very little biological variability (1). These factors, in conjunction with the documentation that HbA_1c predicts the development of microvascular (2,3) and macrovascular (4) complications of diabetes, have led to the widespread adoption of HbA_1c as integral to the management of patients with diabetes. Guidelines from several prominent clinical organizations recommend that HbA_1c be measured at regular intervals in all patients with diabetes (5,6).

The quality of analytical methods for HbA_1c initially lagged considerably behind the evidence of its clinical value. Early assays lacked standardization, substantially limiting the use of HbA_1c in patient care. Considerable effort was invested to effect standardization, with schemes developed in the 1990s in Japan (7), Sweden (8), and the U.S. (9). All HbA_1c results were reported as a percentage of hemoglobin. The subsequent development, almost a decade later, of a reference method for HbA_1c (10,11) led to different units (12). This situation has generated considerable controversy as to how HbA_1c should be reported. In this review, the background leading up to HbA_1c standardization and the development of different units are summarized. The formula for converting patient results from one set of units to the other is provided, and the current state of HbA_1c reporting in several countries is indicated.

Identification of HbA_1c

In normal adults, hemoglobin usually contains HbA (∼97% of the total) (Table 1), HbA₂ (∼2.5%), and HbF (∼0.5%). HbA is made up of four polypeptide chains, two α- and two β-chains. Several posttranslational modifications of hemoglobin have been observed. These include carbamylation, acetylation, sulfation, and glycation. Glycation is the nonenzymatic attachment of a sugar to amino groups of proteins. The phenomenon of glycation, also termed “browning” or the “Maillard reaction,” has been known for over 100 years (13). Early evidence documenting that hemoglobin is glycated was the demonstration in 1955 that small amounts of hemoglobin could be separated from HbA by their migration on electrophoresis in a starch slab (14). Three years later, three minor heme proteins, termed HbA_1a, HbA_1b, and HbA_1c on the basis of their elution, were observed to be resolved when normal human adult hemoglobin was subjected to cation-exchange chromatography (15). (In this review, the term “glycated hemoglobin” is used to refer to the set of all glycated hemoglobins, and “HbA_1c” is used to refer to a specific molecular form as described in the text.) The clinical significance of this finding remained obscure for 10 years until Rahbar (16) detected an unusual hemoglobin on electrophoresis of blood from patients with diabetes. This hemoglobin, which was identified as HbA_1c, was found to be increased twofold in patients with diabetes compared with healthy individuals (17). At essentially the same time, analysis revealed that HbA_1c has a hexose attached covalently to the NH₂-terminal valine residue of the β-chain of HbA (Table 2) (18). Several years later, HbA_1c was defined by the International Union of Pure and Applied Chemistry as the fraction of the β-chains of hemoglobin that has a stable hexose adduct on the NH₂-terminal amino acid valine (19).

Table 1.

Nomenclature of selected hemoglobins

Table 2.

Species of modified HbA

It is important to emphasize that glycation of hemoglobin may also occur at sites other than the end of the β-chain, such as the NH₂-terminal valine residue of the α-chain as well as lysine residues on the α-chain or β-chain (20). These glycated hemoglobins are referred to as glycated HbA₀ or total glycated hemoglobin (GHb) (Table 1). The components of other forms of HbA have been identified. HbA_1a1 and HbA_1a2, which make up HbA_1a, have fructose 1,6-diphosphate and glucose 6-phosphate, respectively, attached to the NH₂ terminus of the β-chain (Table 2). The structure of HbA_1b, solved by mass spectrometry, contains pyruvic acid linked to the NH₂-terminal valine of the β-chain, probably by a ketimine bond (21).

Measurement of HbA_1c

Commercial assays to measure HbA_1c became available in 1978 (22,23), and the test gained popularity during the 1980s. The first mention of glycated hemoglobin by the World Health Organization was in 1985 when the potential value of its measurement in diabetes was indicated (24). In 1988, the American Diabetes Association (ADA) recommended in its Standards of Medical Care that HbA_1c determination should be performed at least semiannually for routine monitoring of patients with diabetes (25). The commercialization of HbA_1c assays led to the development of a plethora of methods to measure glycated hemoglobin. The general concept underlying these methods is to separate the glycated from the nonglycated hemoglobin and quantify the amount of each. Techniques that have been used to achieve this separation include those based on charge differences (ion-exchange chromatography, high-performance liquid chromatography [HPLC], electrophoresis, and isoelectric focusing), structural differences (affinity chromatography and immunoassay), or chemical analysis (photometry and spectrophotometry) (26). Analysis by electrophoresis or chemical techniques has become obsolete in most countries. The methods most commonly used today are HPLC and immunoassays (27).

Unfortunately, the early glycated hemoglobin assays suffered from several deficiencies, most notably a lack of standardization. The diverse methods, coupled with the different forms of glycated hemoglobin that were measured, produced a very wide variation in results. For example, in 1993 only 50% of clinical laboratories in the U.S. were reporting glycated hemoglobin as HbA_1c (28). The remaining laboratories were measuring, and reporting, HbA₁ (21%) or total GHb (29%). One study compared seven glycated hemoglobin methods and observed that results for a single sample varied from 4.0 to 8.1% among the different methods (29). Many people in the diabetes community were unaware of these differences in reporting. The publication of the Diabetes Control and Complications Trial (DCCT) (2) in 1993 provided the impetus necessary to initiate a resolution to this problem.

Documentation of the clinical value of HbA_1c

The DCCT evaluated the effect of intensive insulin therapy (compared with conventional insulin therapy) in patients with type 1 diabetes (2). The study documented that maintaining lower blood glucose concentrations (assessed by HbA_1c) resulted in a delayed onset and reduced the rate of progression of microvascular complications. (All of the glycated hemoglobin measurements in the DCCT were performed in a single laboratory by an HPLC assay that measured HbA_1c. This approach obviated the issue of test variability and established HbA_1c as the species of glycated hemoglobin that should be reported.) The risk of retinopathy increased continuously with increasing HbA_1c, and a single measure of HbA_1c predicted the progression of retinopathy 4 years later. Further analysis of the DCCT data revealed that the mean HbA_1c was the dominant predictor of retinopathy progression, and a 10% lower HbA_1c concentration (e.g., from 9 to 8.1%) was associated with a 45% lower risk (30). Extended follow-up demonstrated that the incidence of cardiovascular disease was reduced by 42% in patients with lower HbA_1c (4). Thus, the DCCT unequivocally established the value of measuring HbA_1c in patients with diabetes.

Evidence that lowering HbA_1c in patients with type 2 diabetes reduces complications was provided in 1998 with the publication of the UK Prospective Diabetes Study (UKPDS) (3). To ensure that HbA_1c results in the UKPDS were comparable to those in the DCCT, an ion-exchange HPLC method calibrated to the DCCT was used. Mean HbA_1c was 7.0% in the intensive group compared with 7.9% in the conventional group (3). Notwithstanding the seemingly small difference in HbA_1c concentrations between the two groups, significant differences in the rate of complications were found. Analogous to the DCCT, the UKPDS showed that intensive blood glucose control reduced the risk of microvascular complications. Risk reductions of 37% for microvascular disease, 21% for deaths related to diabetes, and 14% for myocardial infarction were observed for each 1% reduction in HbA_1c (e.g., from 9 to 8%) (31). Ten-year follow-up demonstrated that the risk of myocardial infarction was significantly lower in patients who had lower HbA_1c at the end of the UKPDS (32). Thus, both the UKPDS and DCCT (large, prospective, multicenter, clinical studies) documented that a small change in HbA_1c values translates into a large alteration in the risk of diabetes complications in patients with type 1 or type 2 diabetes. It was evident that the state of measurement of HbA_1c in routine patient samples was untenable, and accurate, standardized HbA_1c testing was essential.

Standardization to DCCT/NGSP numbers

There are >100 methods currently available to measure glycated hemoglobin, and it is vital that they are standardized to report the same (or at least a very similar) result for a single blood sample. Moreover, as mentioned earlier, both the DCCT and UKPDS measured exclusively HbA_1c and not other forms of glycated hemoglobin. As a result of the DCCT, the American Association for Clinical Chemistry (AACC) established a committee in 1993 to standardize glycated hemoglobin testing (9). The NGSP (previously called the National Glycohemoglobin Standardization Program) was created 3 years later to execute the protocol developed by the AACC committee. The goal of the NGSP is to standardize glycated hemoglobin test results to those of the DCCT and UKPDS, which established direct relationships between HbA_1c concentrations and outcome risks in patients with diabetes. The concept is that all clinical laboratories that measure patient samples should report an HbA_1c value equivalent to that reported in the DCCT and UKPDS. A network of laboratories, located in the U.S., Europe, and Japan, has been established to achieve this standardization (28). A brief description of the process follows.

The Central Primary Reference Laboratory (CPRL) measures HbA_1c with a Bio-Rex 70 cation-exchange HPLC, which is the method used in the DCCT (28). Three primary reference laboratories (PRLs), which use the same method, serve as backup to the CPRL. The eight second reference laboratories (SRLs) assist manufacturers with calibrating their assays so they will report a value equivalent to that measured in the DCCT. (A calibrator is a material of known concentration that is used to adjust a measurement procedure.) Thus, an HbA_1c result of 7.0% in a patient’s blood performed by that assay in a routine clinical laboratory would be essentially identical to a result of 7.0% in the DCCT or UKPDS. If an HbA_1c method meets strict accuracy criteria, the manufacturer receives a certificate that is valid for 1 year (28). In 2011, there were 112 methods that had NGSP certification. The ADA recommends that laboratories use only HbA_1c assays that are certified by NGSP as traceable to the DCCT reference (33). These assays are listed on the NGSP website (http://www.ngsp.org) and are updated at least annually. The HPLC method used in the CPRL and PRLs is not suitable for routine measurement of patient samples. By contract, the SRLs all use commercially available methods, identical to those used in clinical laboratories. National standardization schemes were developed in Japan and Sweden (7,8). The Japanese and Swedish values were rarely adopted by other countries. Due to its link to the DCCT and UKPDS, the NGSP system was, by an overwhelming margin, the most popular global HbA_1c standardization system. NGSP-certified methods are currently used worldwide in the clinical laboratories that measure patient samples, and these results are directly traceable to the DCCT and UKPDS.

The efforts of the NGSP resulted in a considerable improvement in the performance of HbA_1c measurement by routine clinical laboratories (28). The fraction of laboratories reporting glycated hemoglobin measurements as HbA_1c increased from 50% in 1993 to 80% in 1996 to ∼99% in 2004. This was accompanied by a concurrent improvement in accuracy and reduced variability among laboratories (28).

IFCC standardization

A working group on HbA_1c standardization was established by the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) in 1995. The strategy adopted by this committee was different from that of the NGSP. Instead of standardizing to a comparison method, the primary objective of the IFCC committee was to develop a true reference method for HbA_1c. This goal was achieved (10). In brief, the approach is to digest hemoglobin with an enzyme (termed endoproteinase Glu-C) that cleaves a hexapeptide off the NH₂ terminus of the β-chain of hemoglobin. The glycated hexapeptides are separated from the nonglycated hexapeptides by reverse-phase HPLC. The peptides are then quantified by either mass spectrometry or capillary electrophoresis. HbA_1c is measured as the ratio of glycated to nonglycated hexapeptide. Results were initially reported as a percentage. The IFCC Working Group also developed primary reference materials of pure HbA_1c and HbA₀ (10). These are purified from human whole blood, blended together, and used to calibrate the primary reference measurement system. Secondary reference materials are also produced, and manufacturers use these to calibrate their instruments. A network of reference laboratories (15 at the time of this report) works together to form an IFCC reference system (34). The IFCC reference method is technically demanding, time consuming, and very expensive and is not designed for routine analysis of patient samples. It serves as a reference measurement procedure with well-defined metrologic traceability to a “higher-order method.”

The NGSP and IFCC networks have complementary roles in the HbA_1c standardization process. The IFCC provides manufacturers with traceability to a standard of higher metrologic order, and the NGSP defines the limits of acceptability for method performance. The two networks together form a solid basis to establish the accuracy and reliability of HbA_1c measurement in a patient’s sample performed in a clinical laboratory anywhere in the world.

Comparison between networks

Analyses using pooled whole-blood samples were conducted to compare the values of HbA_1c obtained by the IFCC and NGSP networks. A linear relationship was observed between HbA_1c results of the IFCC reference method and the NGSP network (11). The calculated regression equation is NGSP = 0.09148(IFCC) + 2.152. This equation is termed the “master equation” and permits conversion between the two sets of values. Importantly, the HbA_1c values measured by the IFCC method are significantly lower than the NGSP values. Moreover, the difference is not constant. For example, an NGSP value of 6.0% is 4.2% in IFCC numbers (difference of 1.8), and 10.0% NGSP is 8.6% IFCC (difference of 1.4). Comparisons of the IFCC network with the Swedish and Japanese standardization schemes revealed a unique regression equation for each network, with lower values obtained by the IFCC method in all cases (11,34). The most likely explanation for the higher values with the HPLC-based standardization schemes is that the HbA_1c peak on the chromatogram contains other substances in addition to HbA_1c. These observations introduced a conundrum and have generated considerable controversy as to how HbA_1c should be reported. The major arguments promulgated for and against each set of units are summarized in Table 3.

Table 3.

Comparison of IFCC and NGSP units for reporting HbA_1c

Reporting HbA_1c

Three preeminent clinical diabetes organizations, the ADA, the European Association for the Study of Diabetes (EASD), and the International Diabetes Federation (IDF), attempted to resolve the dispute by agreeing to consider reporting HbA_1c with estimated average glucose (eAG) (35,36). A prospective multinational study documented a linear relationship between HbA_1c and mean blood glucose (37). Nevertheless, many experts concluded that the large variability between HbA_1c and mean blood glucose precludes the use of eAG (38–42). By contrast, others believe that eAG is useful in communicating the extent of glycemic control with patients (43–45). In response to a questionnaire included in the College of American Pathologists HbA_1c proficiency survey in April 2012, 1,153 (35.7%) of 3,233 participating clinical laboratories (∼90% of which are located in the U.S.) indicated that they report eAG together with HbA_1c results.

Another factor has exerted considerably more influence on how HbA_1c is reported. A decision was made in 2007 to report IFCC results in the International System of Units (SI) rather than percent (12,19). Thus, IFCC values are now expressed as millimoles of HbA_1c per mole of HbA₀. The Committee on Nomenclature, Properties, and Units of the IFCC proposed a new term for HbA_1c, namely Haemoglobin beta chain(Blood-N-(1-deoxyfructos-1-yl)haemoglobin beta chain; substance fraction) (19). This term has not gained wide acceptance. However, the SI units have (see “Current reporting” section below). A considerable advantage of reporting IFCC values in SI units (as opposed to reporting IFCC values as percent) is that it will avoid the confusion that would almost certainly have ensued from using the same units (namely percent) but different reference intervals. For example, NGSP-certified HbA_1c concentrations of 6.5 and 7.0% (previously 4.8 and 5.3% in the original IFCC reporting system) now correspond to 48 and 53 mmol/mol, respectively (Table 4).

Table 4.

Comparison of HbA_1c values

In 2004, a working group with representatives from the ADA, EASD, and IDF was established to harmonize HbA_1c reporting (35,36). There was unanimous agreement among the members of the group, termed the ADA/EASD/IDF Working Group of the HbA_1c Assay, that the same HbA_1c values should be reported throughout the world. The initial report was followed in 2007 by a consensus statement on the worldwide standardization of HbA_1c (46). The publication called for HbA_1c results to be reported worldwide in IFCC units (mmol/mol) and derived NGSP units (%), using the IFCC-NGSP master equation. A subsequent publication appeared 3 years later (47) and reiterated that HbA_1c results should be reported by clinical laboratories worldwide in SI and derived NGSP units. Regrettably, this ideal has not been realized.

Current reporting

In 1998, the European Union introduced a directive on in vitro diagnostics requiring that laboratory tests be traceable to a “higher-order method” (48). This factor, in conjunction with the use of SI units for reporting almost all other laboratory tests in many countries, has resulted in the adoption of SI units for HbA_1c by some countries in Europe and the Antipodes (Table 5). Most of the countries that have decided to convert to SI units have had a period of dual reporting where both NGSP and IFCC units have been used before switching to single reporting of SI units. The two countries that had their own national standardization schemes, namely Sweden and Japan, both chose to discontinue reporting results using these numbers. Sweden elected to adopt SI units exclusively, whereas Japan is currently reporting both NGSP and Japan Diabetes Society numbers, before switching to only NGSP numbers in 2013 (Table 5). After considerable deliberation, Canada has recently decided to continue using NGSP values. Although no formal statement has been issued, the U.S., which continues to use “traditional” (non-SI) units for all blood tests, is extremely unlikely to switch to SI units for HbA_1c. Most countries have not yet made a decision whether to adopt SI units, but it is likely that at least some will convert. A concern has been expressed by some experts from both less-developed countries and newly industrialized countries that efforts, and limited resources, should be directed toward adopting higher-quality (and standardized) analytical methods, expanding the use of HbA_1c in patient care, and educating health care providers. The concern is that introducing SI units will both divert resources from these necessary tasks and generate confusion.

Table 5.

Units for reporting HbA_1c in selected countries

Implications of different HbA_1c units

The hope expressed by members of the ADA/EASD/IDF Working Group of the HbA_1c Assay that the same HbA_1c values be reported globally (36) has clearly not been realized (Table 5). This situation is most unfortunate and has consequences for all those in the field of diabetes. It is essential that countries that choose to report HbA_1c in SI units introduce extensive education programs that will adequately explain the new units to all health care providers. Because management of diabetes requires the active participation of the patient (49), the change in reporting must also be clearly communicated to patients. It is important that if HbA_1c units are altered, the modification should be coordinated so that it is instituted throughout the entire country to avoid having different units used in a single country. Medical and scientific journals should require that authors provide both SI and DCCT units for all HbA_1c results. This dual reporting will enable readers to evaluate results and compare them to prior publications.

Physicians and investigators need to be aware that the conversion of HbA_1c results between DCCT/NGSP (%) and IFCC units is not as simple as changing glucose values from traditional to SI units or vice versa. The glucose conversion is based on the molecular mass of glucose (C₆H₁₂O₆), which is 180.16 g/mol. Therefore, to change glucose from mg/dL to mmol/L, one divides by 18.016 (usually rounded off to 18), and values are multiplied by 18 to switch from mmol/L to mg/dL. For example, 126 mg/dL is equivalent to 7.0 mmol/L, and 40 mg/dL is 2.2 mmol/L. By contrast, the conversion of HbA_1c results is more complex, partly because it is the conversion of a ratio of two measurements made in different assays rather than of a single concentration. At an NGSP HbA_1c of 4%, the IFCC values are fivefold higher (20 mmol/mol), whereas at 12%, the IFCC results are ninefold greater (108 mmol/mol) (Table 4), precluding the use of a simple multiplication or division factor to transform values. Inspection of Fig. 1 reveals that although the relationship between HbA_1c measured in NGSP units and IFCC units is a straight line, that line has a slope that differs significantly from 1 and an intercept that differs from 0. Therefore, conversion between NGSP and IFCC units requires a simple linear equation, the master equation (NGSP = 0.09148(IFCC) + 2.152 or IFCC = 10.93(NGSP) – 23.50). It is reassuring that the master equation has been shown to be stable for >11 years (34), and comparisons between the NGSP and IFCC networks continue to be conducted twice a year. These ongoing assessments, which validate the stability and reliability of the networks, will ensure that results can be converted from DCCT/NGSP units to SI units and vice versa. Tables and/or calculators that convert units should be readily accessible to health care providers and patients. To assist in this endeavor, the NGSP has posted both a table and a calculator on its website (http://www.ngsp.org/convert1.asp). A calculator is also available at http://www.hba1c.nu/eng2.html. For countries that select SI units for reporting HbA_1c, it is imperative that the tenet “primum non nocere” (first do no harm) is adhered to and that care of patients with diabetes is not compromised.

Figure 1.

Comparison of HbA_1c values between the NGSP and IFCC networks. HbA_1c was measured in 10 pooled blood samples by the NGSP (mean value of 7 network laboratories) and IFCC (mean value of 13 network laboratories) networks. ◆ is the regression line, and the solid black line is the y = x line (line of identity).