Introduction
Blood oligosaccharides are attached to many proteins after translation, forming glycoproteins.
Glycosylation refers to an enzyme-mediated modification that alters protein function,
for example, their life span or their interactions with other proteins (1). By contrast,
glycation refers to a monosaccharide (usually glucose) attaching nonenzymatically
to the amino group of a protein. Glycated hemoglobin is formed by the condensation
of glucose with select amino acid residues, commonly lysine, in hemoglobin to form
an unstable Schiff base (aldimine, pre-HbA1c) (Fig. 1). The Schiff base may dissociate
or may undergo an Amadori rearrangement to form a stable ketoamine.
Figure 1
Formation of glycated protein. A reversible interaction between a primary amino group
(depicted as NH2) of a protein and the carbonyl group of d-glucose yields a labile
intermediate, called a Schiff base. This can undergo a slow and spontaneous Amadori
rearrangement to form a stable ketoamine. HbA1c is formed if glucose attaches to the
N-terminal valine of the β-chain of hemoglobin. If the glucose attaches to proteins
in the plasma, fructosamine or glycated albumin results. RBC, red blood cell.
Glycated hemoglobin, particularly HbA1c, has for decades been widely incorporated
into the management (and, more recently, the diagnosis) of patients with diabetes.
An important attribute is that glycation occurs continuously over the lifetime of
the protein, so the concentration of the glycated protein reflects the average blood
glucose value over a period of time. This contrasts with the measurement of blood
glucose, which reveals the glucose concentration at the instant blood is sampled and
which is acutely altered by multiple factors such as hormones, illness, food ingestion,
and exercise (2). While HbA1c is by far the most extensively used—and studied—glycated
protein (2–4), other glycated proteins that have been evaluated in clinical studies
include fructosamine, glycated albumin, and advanced glycation end products (AGEs).
Hemoglobin A1c
HbA1c is glycated hemoglobin in which glucose is attached to the N-terminal valine
residue of each β-chain of hemoglobin A (HbA). Glucose can also be attached at other
amino acids, predominantly lysine, in either the α- or β-chain of hemoglobin (5).
However, modern methods that measure HbA1c do not report these other glycated hemoglobin
species. The extent of hemoglobin glycation is influenced by the concentration of
glucose in the blood. Since the life span of erythrocytes is ∼120 days, HbA1c reflects
the average glucose concentration over the preceding 8–12 weeks (3).
HbA1c has been recommended by the American Diabetes Association since 1988 for routine
monitoring of patients with diabetes (6). Although the association of chronic hyperglycemia
with the risk of chronic complications of diabetes was suspected for many years, landmark
trials such as the Diabetes Control and Complications Trial (DCCT) in type 1 diabetes
(7) and the UK Prospective Diabetes Study (UKPDS) in type 2 diabetes (8) and their
follow-up studies (9,10) confirmed that lowering mean glucose, as measured by HbA1c,
significantly reduced the onset and progression of complications. This led to the
development of treatment goals for HbA1c and the use of HbA1c as a performance measure.
The increasing use of HbA1c in patient management is evident from the increase in
the number of clinical laboratories that are enrolled in proficiency testing surveys
conducted by the College of American Pathologists (Fig. 2). Note the large (more than
threefold) increase in participants during the 4 years after the publication of the
DCCT results in 1993.
Figure 2
Progressive increase in HbA1c testing over time. The number of clinical laboratories
enrolled in proficiency testing surveys from the College of American Pathologists
from 1993 to 2015 is depicted. Data used with permission from the College of American
Pathologists.
HbA1c was recently included as a diagnostic criterion for diabetes by the American
Diabetes Association (11), European Association for the Study of Diabetes, International
Diabetes Federation, and World Health Organization (12). This recommendation was motivated
by improvements in the measurement of HbA1c and by the certain advantages of its measurement
over that of glucose, such as the convenience of not requiring the patient to fast
and the reduced intraindividual variability compared with fasting or glucose measurements
after loading (11).
HbA1c can be measured by immunoassays, high-performance liquid chromatography (HPLC)
(the two most commonly used methods in the U.S. and many other developed countries),
affinity chromatography, capillary electrophoresis, and enzymatic assays (13). Standardization
of methods by the NGSP (formerly called the National Glycohemoglobin Standardization
Program) (14,15) and the International Federation of Clinical Chemistry and Laboratory
Medicine (16) has yielded highly consistent HbA1c results for a blood sample, regardless
of the method used (provided the method is certified by NGSP).
Interference
There are numerous published reports of conditions that change HbA1c independent of
glucose (reviewed in refs. 17 and 18). Based on the nature of the interference, these
can be conveniently divided into two groups: conditions that influence interpretation
(i.e., change HbA1c concentration in ways unrelated to changes in glucose) and conditions
that interfere with HbA1c measurement (i.e., analytic interferences) (Table 1).
Table 1
Nonglycemic factors that may influence HbA1c
Factors that may influence interpretation of HbA1c
1. Physiological (e.g., age, race)
2. Chronic renal failure
3. Iron-deficiency anemia
4. Erythrocyte life span
5. Glycation “phenotypes”
6. Drugs (e.g., dapsone, antiretroviral)
7. Other (e.g., vitamin C, vitamin E)
Factors that may interfere with HbA1c measurement
1. Uremia
2. Hemoglobin variants
3. Drugs (e.g., opiates)
4. Other (e.g., bilirubin, triglyceride, alcohol)
Factors That Influence HbA1c Interpretation
Physiological Factors.
HbA1c concentrations increase by ∼0.1% per decade after 30 years of age (19). It is
not known whether this gradual increase reflects an effect of age on the relationship
of mean glycemia to HbA1c or merely the higher prevalence of prediabetes and diabetes
with aging (a true increase in mean glycemia). There is contention surrounding the
influence of race on HbA1c concentrations. Herman (20) posits that African Americans
have higher HbA1c for any given level of mean glycemia, whereas Selvin (21) argues
that the increased mean HbA1c is a reflection of truly higher mean glycemia in African
Americans.
Chronic Renal Failure.
Chronic renal failure (CRF) is a common complication of diabetes, and diabetes is
the leading cause of end-stage renal disease (22). Red blood cell survival is reduced
in CRF, decreasing HbA1c. In addition, many patients with CRF are treated with erythropoietin
to stimulate erythropoiesis. The subsequent increase in the number of young erythrocytes
further reduces the HbA1c. Therefore the HbA1c concentration in patients with diabetes
and with CRF may not accurately indicate glycemic control.
Iron-Deficiency Anemia.
Iron deficiency and iron-deficiency anemia occur frequently. Some studies, generally
with small sample sizes, have reported increased HbA1c in individuals with iron deficiency.
Two recent systematic reviews reached opposite conclusions regarding the effects of
iron deficiency on HbA1c. The first, a meta-analysis and systematic review, concluded
that there was no statistically significant difference in HbA1c measured by HPLC in
the presence of iron deficiency or iron-deficiency anemia (23). By contrast, another
assessment determined that iron deficiency, with or without anemia, increased HbA1c
(24). This discrepancy is likely due to the differences in the studies selected and
the method of analysis. Several studies included in both meta-analyses of HbA1c in
iron deficiency were limited by their small sample sizes and the heterogeneity of
the methods. Two large investigations of the National Health and Nutrition Examination
Survey (NHANES) data have been conducted. Kim et al. (25) evaluated 6,666 female NHANES
participants without diabetes from 1999 to 2006 and concluded that iron deficiency
was associated with an increase in HbA1c from <5.5% to 5.5–6.0%; however, this association
was not apparent at higher HbA1c concentrations. A second investigation of NHANES
data from 1999 to 2002 included 8,296 patients with and without diabetes and found
an adjusted increase in HbA1c from 5.46 to 5.56% in the presence of iron deficiency
(26). Thus, while HbA1c seems to increase slightly with iron deficiency, the clinical
significance of this finding remains to be determined. We agree with Ford et al. (26)
that caution should be exercised in diagnosing prediabetes and diabetes when HbA1c
is near the decision threshold in patients with iron deficiency.
Erythrocyte Life Span.
A change in erythrocyte survival alters HbA1c. For example, assume HbA1c is 7.0% (53
mmol/mol), with a normal erythrocyte life span of 120 days. If the red blood cell
life span is 10 days shorter or longer, the corresponding HbA1c values would be 6.4%
(46 mmol/mol) and 7.6% (60 mmol/mol), respectively. HbA1c does not accurately reflect
average blood glucose concentration if erythrocyte survival is significantly altered,
as in, for example, hemolytic anemia or severe β-thalassemia. Since measurement of
red blood cell life span is extremely difficult, one cannot easily solve this problem
by, for example, applying a correction factor for erythrocyte age.
Variable Glycation.
Intraindividual variability of HbA1c is very low. Nevertheless, interindividual variation
occurs and has been ascribed by some to differences in glycation rates (27,28). This
postulate is contentious (29,30) because the data validating significantly different
rates of glycation are minimal and no mechanism for differences in this nonenzymatic
process has been documented. Moreover, a recent analysis, although indirect, reveals
that even the rate of glycation of hemoglobin variants S, C, D, E, J, and G is not
significantly different from that of HbA (31), undermining the premise of variable
rates of glycation of HbA. There has been speculation that the rate of deglycation
(i.e., the removal of glucose from HbA1c) might vary among individuals, resulting
in different HbA1c concentrations despite similar average glycemia. Although at least
three groups of deglycating enzymes have been identified, only one, fructosamine 3-kinase,
is found in humans. Importantly, fructosamine 3-kinase has no effect on valine-1 of
the β-chain of hemoglobin (32), the residue where glucose is attached in HbA1c, and
it cannot deglycate HbA1c. Thus the concept of variable glycation remains to be validated.
Factors That Interfere With Measurement
Numerous publications have described interferences in HbA1c measurement, but many
reports had small numbers of subjects and described changes that were small and unlikely
to have clinical significance (33–35). Furthermore, improvements in analytic methods
have eliminated interferences from some factors (e.g., aspirin, bilirubin, and triglycerides)
that affected older methods. While the possible interference of all substances in
each modern method has not been rigorously investigated, it is likely that few drugs
or other factors interfere significantly in current HbA1c assays.
Uremia.
Isocyanic acid, derived from urea, is covalently attached to proteins. The nonenzymatic
process, termed carbamylation, increases when blood urea concentrations are high,
yielding increased carbamylation of circulating proteins, including on lysine or arginine
residues of the N-terminus of hemoglobin. Carbamylated hemoglobin altered HbA1c values
in some early methods (36), but uremia has no significant effect on HbA1c analysis
with most contemporary methods (23,37,38).
Hemoglobin Variants.
Over 1,200 hemoglobin variants have been identified; the β gene is involved in ∼70%
of these (39). While the vast majority are uncommon or rare, certain hemoglobin variants,
particularly HbAS, HbAC, HbAD, and HbAE, occur at relatively high frequencies in some
populations. One cannot measure HbA1c in individuals who are homozygous for these
common variants or who have HbSC disease (36) because they have no HbA. While total
glycated hemoglobin can be determined using borate affinity methods in patients with
these homozygous hemoglobin variants, there is no convincing clinical evidence that
these values can reliably be used to monitor glycemia and predict complications, particularly
since some patients may have reduced erythrocyte life span because of hemolytic anemia.
Most interferences are method-specific (36). Manufacturers of HbA1c methods have considerably
reduced analytic interference from variant hemoglobin. Therefore HbA1c can be measured
accurately in the presence of the overwhelming majority of variant hemoglobins, provided
a suitable assay is used (40). Since common heterozygous variants rarely alter erythrocyte
life span, accurate and reliable HbA1c values can be obtained in heterozygous individuals.
Glycated Serum Proteins
Glucose attaches nonenzymatically to amino groups of proteins other than hemoglobin
to form ketoamines (Fig. 1). Measures of several glycated serum proteins, including
fructosamine and glycated albumin, have been proposed as markers of glycemia that
might complement or replace HbA1c in select patient populations. Serum proteins turn
over more rapidly than erythrocytes; for example, albumin (the protein found in the
highest concentration in serum) has a circulating half-life of about 14–20 days. Therefore
the concentration of fructosamine or glycated serum albumin reflects mean glucose
over a period of 2–3 weeks. Additionally, glycated serum proteins are not influenced
by changes in erythrocyte life span or hemoglobin variants such as homozygous HbS.
Glycated serum proteins have therefore been proposed as measures of more rapid changes
in glycemia and to monitor glycemic control in patients with conditions that alter
the normal relationship of HbA1c to mean glucose (e.g., hemolysis, blood transfusion).
Fructosamine
Fructosamine is the common name for 1-amino-1-deoxy fructose and the generic name
for plasma protein ketoamines (41,42). All glycated serum proteins are fructosamines,
and since albumin is the most abundant serum protein, measurement of fructosamine
is thought to largely reflect the concentration of glycated albumin, though this has
been questioned (43). The fructosamine assay is readily automated and is less expensive
than measurement of HbA1c. There is disagreement as to whether fructosamine results
are independent of serum protein concentrations (absent significant alterations in
the latter) or whether fructosamine values need to be corrected for the concentration
of serum proteins (44). Most agree, however, that fructosamine is not valid when serum
albumin is <30 g/L.
The first commercial method to measure fructosamine suffered from several problems,
particularly a lack of specificity and interference by other reducing substances in
the serum, such as urates (43,45). Thus many early studies of fructosamine generated
confusion regarding its clinical value, with reviews (covering many of the same studies)
leading to conflicting conclusions as to whether fructosamine is a reliable test for
routine clinical use (41,46). The assay was extensively modified in 1991, which markedly
improved the specificity of fructosamine (47). Strong correlations with HbA1c, prognostic
value for the development of diabetes and microvascular complications, and good precision
have been demonstrated for fructosamine using modern assays on automated platforms
(48,49).
There is interest in the role of fructosamine in special populations for whom HbA1c
may not provide an accurate assessment of glycemic status. One such potential use
of fructosamine is the diagnosis of gestational diabetes mellitus (GDM). Hyperglycemia
develops relatively quickly with the onset of GDM, and red cell turnover may be altered
in pregnancy, precluding the use of HbA1c to diagnose this form of diabetes. Studies
evaluating this use of fructosamine (50) were generally small and used various fructosamine
thresholds and diagnostic criteria for GDM. Measurement of fructosamine is not currently
recommended to screen for GDM (50).
Other conditions for which fructosamine has shown a potential role in monitoring glycemic
status include end-stage renal disease, certain types of anemia, and transfusion (49).
Combining HbA1c with fructosamine has been used as a screening strategy to identify
patients with prediabetes; however, the combination was not statistically significantly
better than the use of HbA1c alone (51). A major limitation of the fructosamine assay
is the lack of an evidence base linking the test to long-term complications of diabetes.
Hence, unlike HbA1c, there are no generally accepted treatment targets for fructosamine.
Glycated Albumin
Albumin comprises almost two-thirds of total serum protein and accounts for over 80%
of total glycated serum proteins (52). HPLC tandem mass spectrometry of human plasma
using [13C6]glucose labeling has identified 35 glycation sites on albumin (53). Analogous
to HbA1c, which is most commonly reported as a percentage of total hemoglobin, glycated
albumin is usually expressed as a percentage of total albumin in the blood. A number
of glycated albumin assays are commercially available, but these lack standardization
and values vary widely among methods (54). Specifically, the reference intervals have
considerable variation depending on the method and range from 0.8–1.4% to 18–22% (52,54).
A U.S. Food and Drug Administration–approved method for glycated albumin measurement
manufactured by Diazyme Laboratories (Poway, CA) is commercially available (55). A
glycated albumin assay developed by Asahi Kasei in Japan (56) is the method most widely
used globally and most extensively evaluated in clinical studies.
Values of glycated albumin in blacks are significantly higher than in whites, for
reasons that are unclear (54). Factors that influence albumin metabolism may alter
glycated albumin independent of glycemia. These factors include the nephrotic syndrome,
cirrhosis, thyroid disease, hyperuricemia, hypertriglyceridemia, and smoking (57).
As with fructosamine, glycated albumin concentrations can be affected by altered protein
levels that occur with liver, thyroid, and renal disease (58). The clinical use of
glycated albumin is limited by the same caveats that apply to fructosamine—namely,
a paucity of evidence relating it to clinical outcomes, specifically the chronic complications
of diabetes. As is the case with fructosamine, further studies are required to determine
its clinical utility in the management of diabetes (48,59).
A recent investigation by Sumner et al. (51) identified a potential role for glycated
albumin in the diagnosis of prediabetes in African immigrants to the U.S. Using the
oral glucose tolerance test as the gold standard, the combination of HbA1c with glycated
albumin detected 78% of African immigrants with prediabetes, compared with only 50%
detected with HbA1c alone and 72% with HbA1c paired with fructosamine (51). An investigation
of 302 adults in Japan found that HbA1c or glycated albumin could diagnose patients
at risk for developing diabetes; fructosamine was considered unsuitable as a screening
test (60). Glycated albumin is used extensively as a screening test for diabetes among
blood donors in Japan, identifying patients who are at risk for the disease (56).
Intriguing data are emerging suggesting that glycated albumin may be a better test
than HbA1c for diabetes screening in nonobese patients. Koga et al. (61) found a negative
correlation between glycated albumin and BMI in Japanese individuals; this finding
has been replicated in other Asian populations (62,63). Similarly, in the study by
Sumner et al. (51), the African immigrants whose prediabetes was identified by glycated
albumin, but not HbA1c, were more likely to have a lower BMI. The converse implication
of these data is the potential for glycated albumin to underestimate glycemic status
in the obese.
AGEs
Glycation of tissue proteins may contribute to the link between hyperglycemia and
the chronic complications of diabetes. Nonenzymatic attachment of glucose to proteins,
lipids, or nucleic acids produces stable Amadori products, which can undergo further
modifications to form AGEs (64,65). Irreversible rearrangements of Amadori products
occur via both oxidative and nonoxidative pathways, or via condensation of the side
chains of lysine, arginine, or cysteine, forming reactive dicarbonyl compounds such
as glyoxal, methylglyoxal, and deoxyglucosones that ultimately form irreversible AGEs
by forming cross-links between many proteins, altering their structure and function
(65). For example, glyoxal can form N-(carboxymethyl)lysine (CML), glyoxal-derived
lysyl dimer, or N-(carboxymethyl)arginine, whereas methylglyoxal may induce the formation
of methylglyoxal-derived lysyl dimer, argpyrimidine, N-(carboxyethyl)lysine (CEL),
and others (65). The most common cross-linked AGE is glucosepane, formed by a mechanism
of action that has not yet been fully elucidated (65). More than 20 AGEs have been
identified (66,67). These products do not return to normal, even when hyperglycemia
is corrected, so they accumulate continuously over the life span of the protein; AGEs
also accumulate as an individual ages. Hyperglycemia accelerates the formation of
protein-bound AGEs, and patients with diabetes have more AGEs than age-matched subjects
without diabetes. There is evidence that AGEs in the diet contribute to AGE accumulation
in tissues (68).
Through their heterogeneous effects on the functions of proteins and extracellular
matrix, AGEs may contribute to the chronic microvascular and cardiovascular complications
of diabetes (69,70). Plasma concentrations of CEL, CML, and pentosidine were correlated
with incident, but not prior, cardiovascular outcomes in patients with type 2 diabetes
(71,72). AGEs have also been linked to other diabetic complications including nephropathy,
retinopathy, and neuropathy (73–78). There is significant heterogeneity among these
studies in the specific AGEs evaluated and the method of AGE measurement. Of potential
interest, certain publications reported no correlation between serum AGE concentrations
and HbA1c (71–73). Levels of AGEs in the skin biopsies of patients from the DCCT were
found to be a better predictor of retinopathy and nephropathy progression than HbA1c
(74,77). Collectively, these results raise the possibility that AGEs may provide additional
independent information to predict microvascular diabetic complications. Thus AGE
burden may explain why only a subset of patients with poor glycemic control develop
complications and why some patients with good glycemic control also develop certain
diabetic complications.
Several methods have been proposed to measure AGEs. Some AGE products fluoresce, which
has led to the development of noninvasive measurement of skin autofluorescence to
estimate the burden of AGEs in tissues. A meta-analysis of seven studies showed that
skin autofluorescence was positively associated with mortality, neuropathy, nephropathy,
and cardiovascular events (79). Certain studies found that skin autofluorescence predicted
microvascular and macrovascular complications of diabetes independent of HbA1c (80,81),
whereas others found that adjustment for HbA1c rendered these associations nonsignificant
(82). These discrepant findings are possibly accounted for by differences in the patient
population and statistical methods. The utility of skin autofluorescence measurements
is limited by several factors. First, most AGEs are not fluorescent, specifically
CML and CEL, which have been shown to be important in predicting cardiovascular outcomes
(67). Second, skin fluorescence is not specific; numerous skin proteins fluoresce
with spectra that overlap the spectra of AGEs (83). Furthermore, skin autofluorescence
does not correlate directly with AGE burden.
There is considerable interest in the measurement of AGEs in the circulation as a
biomarker to monitor the risk of diabetes complications, given the numerous studies
correlating AGEs with various diabetic complications (66). Assays to determine total
AGE fluorescence have been used in selected studies, but these methods have limitations
similar to those of skin autofluorescence, namely, the most important AGEs are not
fluorescent and many other serum proteins interfere. Methods for measuring specific
AGEs have been developed, many of which use immunoassays. However, heterogeneity of
the structures (ranging from single molecules to complex cross-linked compounds) and
composition of AGEs have resulted in assay variability. Questions have been raised
regarding antibody specificity (AGEs such as CML and CEL share certain common epitopes),
the use of excess blocking proteins that have oxidized and glycated fragments, and
the high temperature and pH of the assay (67). Furthermore, the lack of immunoassay
standardization has yielded variable results (84).
Isotope dilution analysis liquid chromatography–tandem mass spectrometry (LC-MSMS),
with careful preanalytic sample preparation, is a promising technique for circumventing
the problems of immunoassays and fluorescence-based methods (67). Analytes are first
separated by HPLC from related compounds that have not been oxidized or glycated;
then they are detected based on a specific chromatographic retention time, molecular
ion mass-to-charge ratio, and fragment ion mass-to-charge ratio, rendering this technique
highly specific for the desired analyte. LC-MSMS quantitatively analyzes the modification
of proteins by glycation, nitration, and oxidation, as well as free adducts, using
a small sample volume. This technique has been applied to a number of clinically important
AGEs including pentosidine, CML, CEL, 3-deoxyglucosone, and methylglyoxal hydroimidazolones,
and it has aided in the discovery of new candidate AGE products (67,85,86). A limitation
of LC-MSMS is the need for specialized (and expensive) equipment and highly trained
personnel. Furthermore, isotope-labeled standards are not commercially available for
the full range of analytes (67,86), preventing assay standardization.
Certain AGEs activate the receptor for AGE (RAGE), inducing intracellular signaling
that results in the production of proinflammatory cytokines and increased oxidative
stress (66,69). RAGE is expressed on the surface of several cells, including endothelial
and renal cells, raising intriguing hypotheses about the role of RAGE in the pathophysiology
of specific diabetes complications. Nevertheless, some studies cast doubt on whether
AGE-modified proteins activate RAGE (87). Proteolysis of RAGE leads to a truncated
soluble form of RAGE (sRAGE) (66), which is found in serum and can be measured by
a commercially available ELISA. There is evidence of clinical value of sRAGE. In a
case-cohort study of 3,763 patients with type 2 diabetes, both AGE and sRAGE plasma
values predicted decreasing renal function and all-cause mortality, but hazard ratios
were only 1.1 to 1.2 (88). There is, however, controversy over the associations between
sRAGE concentrations and diabetes complications; some studies show a positive association
(89) and others an inverse one (90). The associations between sRAGE and health outcomes
remain unresolved. Differences in studied populations and genetic mechanisms have
been suggested as a cause of the discrepancies (91,92).
Inhibitors of AGE formation, such as aminoguanidine, prevented signs of microvascular
complications of diabetes in animal models, although initial clinical trials in humans
failed to show a significant benefit (66). Nonetheless, anti-AGE therapy remains an
area of active research. Of interest, patients with type 2 diabetes taking metformin
had lower AGE levels than those not receiving metformin (93). Promising studies of
the use of recombinant sRAGE in animals suggest the potential of future therapies
in humans to reduce the risk of diabetes complications. The recent total synthesis
of the lysyl-arginine cross-link glucosepane (94), the main in vivo cross-link in
AGEs (95), is likely to permit the generation of relevant reagents (e.g., specific
antibodies) to enhance our comprehension of the role of AGEs in disease.
Future Directions
The development and standardization of HbA1c measurement have revolutionized research
and clinical care in the field of diabetes (2–4). The role of HbA1c in diabetes has
been extensively studied in large, prospective trials with long-term follow-up (9,10),
which has extensively validated the value of HbA1c in predicting many diabetic complications.
Additionally, diagnostic thresholds for using HbA1c to diagnose prediabetes and diabetes
have been established (11,96). Yet despite the documented utility of HbA1c in diabetes
research and care, controversies remain. As argued from opposing perspectives by Herman
(20) and Selvin (21) in this issue, whether there are clinically significant differences
in the relationship between HbA1c and average glucose in different racial groups remains
contested, and similar questions exist about age groups. If there are differences
in what HbA1c “means” in different groups, what are the implications for the diagnosis
and management of diabetes? Considerable progress has been made in reducing interference
from hemoglobin variants and other factors in HbA1c assays and in achieving high levels
of standardization of the assay in developed countries. We need to continue to overcome
the barriers to worldwide standardization of HbA1c assays, particularly in developing
countries.
Since the discovery of HbA1c, other potentially useful additional or adjunct measures
of protein glycation, glycated serum proteins, and AGEs have emerged. It is unlikely,
however, that the newer measures of glycated proteins will be studied as markers of
diabetic complications in the same thorough manner as HbA1c because of limited funding
for long-term clinical trials with large numbers of patients. We need to develop innovative
strategies to establish the evidence base for the link between other glycated proteins
and clinical outcomes, so that treatment targets or diagnostic thresholds can be developed.
Furthermore, glycated albumin and AGE assays need to undergo standardization, as has
been done for HbA1c, to enable comparison among studies and decrease imprecision (15).
AGEs have the potential to identify—independent of HbA1c—a subset of patients who
develop cardiovascular and microvascular complications of diabetes. It is important
to determine whether AGEs are a cause or consequence of the pathophysiology of diabetes.
Because the term comprises a large group of diverse compounds, future studies of AGEs
will require detailed knowledge of the specific compound(s) being studied. Although
AGEs may go beyond simple biomarkers into pathophysiology, substantial research needs
to be done to use AGE-related measures to improve the prediction of risk for diabetes
complications or to ultimately develop risk-reduction therapies based on these pathways.