Haemoglobin glycation index and risk for diabetes-related complications in the Action in Diabetes and Vascular Disease: Preterax and Diamicron Modified Release Controlled Evaluation (ADVANCE) trial

HbA_1c is an established means of monitoring average blood glucose levels and a surrogate marker of the effect of glucose-lowering interventions [1]. It is highly associated with the risk for diabetes-related complications, in particular those of microvascular origin [2‐5]. Although HbA_1c is almost universally accepted to guide and monitor diabetes treatment, its use in clinical practice has arguable limitations. There is a proposed inter-individual variation in the propensity for glycation, in both healthy individuals and those with diabetes [6‐13], limiting the use of HbA_1c as a one-size-fits-all measurement. Moreover, the value of HbA_1c as a surrogate endpoint was questioned by the results of the Action to Control Cardiovascular Risk in Diabetes (ACCORD) trial, where HbA_1c lowering may have had detrimental effects on the risk of premature mortality [14]. Therefore, ‘the lower the better’ may not universally hold for HbA_1c, and additional (bio)markers might be useful to individualise treatment targets and risk prediction [15].

The haemoglobin glycation index (HGI) quantifies the variation in the relation between HbA_1c and the plasma glucose concentration [16]. For any individual within a study population, HGI is defined as the difference between the observed HbA_1c and the fitted value from a simple linear model that predicts HbA_1c from the fasting plasma glucose (FPG) concentration, i.e. the residual from the fitted linear regression line. In previous studies, HGI was normally distributed, stable over time and consistent over a wide range of blood glucose concentrations [17‐20]. In an analysis in individuals with type 1 diabetes in the DCCT, a high HGI was associated with the risk for and progression of retino- and nephropathy [21]. In an analysis of the ACCORD trial, only participants in the highest HGI third were at higher risk for mortality and those with high HGI showed no benefit on cardiovascular outcomes after intensive glucose lowering, in contrast to participants with a low or intermediate HGI [16]. The use of HGI is not without controversy, as in the DCCT population it was shown that the effect of HGI on microvascular complications disappeared after adjustment for the effect of HbA_1c [22]. However, the use of HbA_1c in type 1 diabetes is undisputed, whereas in individuals with type 2 diabetes, HbA_1c seems to have shortcomings, as demonstrated by the ACCORD trial.

The aim of this study was to assess whether HGI is a predictor of adverse outcomes of intensive glucose-lowering therapy and a predictor of diabetes-related complications in the cohort of individuals with type 2 diabetes recruited for the Action in Diabetes and Vascular Disease: Preterax and Diamicron Modified Release Controlled Evaluation (ADVANCE) trial (ClinicalTrials.​gov registration no. NCT00145925) [23]. Additionally, we aimed to compare the predictive values of HGI and HbA_1c to assess the possible added value of HGI beyond HbA_1c.

In this population, HGI showed a normal distribution, ranging from −6.46 to 8.99, with a mean of 0 (SD 1.20). When dividing the study population into thirds, HGI cut-off points were ≤−0.53 for the low, −0.52 to 0.28 for the intermediate and ≥0.29 for the high HGI group. Mean baseline HGI was −1.14 (SD 0.57) in the low, −0.14 (SD 0.23) in the intermediate and 1.29 (SD 0.99) in the high HGI group (Table 1).

There were some significant differences in baseline characteristics between the HGI groups (Table 1). Individuals in the high HGI group had the longest duration of diabetes and a higher proportion used glucose-lowering medication. More of those in the high HGI group were smokers, but fewer consumed alcohol. The prevalence of previous microvascular events was highest in the high HGI group. Almost half of the individuals in the high HGI group were of Asian origin, in contrast to a third in the low and intermediate group. HbA_1c levels increased from low to high HGI group and FPG was lowest in the intermediate HGI group.

With this analysis of the ADVANCE trial we showed that HGI predicts diabetes-related complications, but no better than HbA_1c. Irrespective of treatment allocation, the high HGI group (i.e. individuals with higher HbA_1c levels than would be expected for their given fasting glucose levels) was at higher risk for macro- and microvascular complications and mortality compared with the low HGI group. Every SD increase in HGI gave a significant 14–17% risk increase for these three outcomes. Hypothetically, this could be explained by a higher propensity for glycation of membrane proteins and lipids other than haemoglobin, with these glycation products leading to microvascular complications and atherogenesis. This effect disappeared after additional adjustment for HbA_1c, which is in line with results on the effect of HGI in individuals with type 1 diabetes in the DCCT [22]. However, this might be considered over-adjustment, as HGI is so strongly related to HbA_1c (HGI is HbA_1c corrected for FPG). Therefore, we separately assessed the effect of HbA_1c on outcomes using the same model and found that HbA_1c was just as strong or even stronger for predicting complications in this cohort of individuals with type 2 diabetes. To our knowledge, this is the first time the predictive value of HGI has been compared with that of HbA_1c in this way. As HbA_1c does not need a population regression equation as does HGI, this makes HbA_1c more straightforward and convenient to use.

In our study, intensive treatment carried a lower risk for mortality in individuals with a high HGI, whereas it had no effect on mortality in individuals with a low or intermediate HGI. Thus, a high HGI identified a group of people who benefitted most from intensive HbA_1c-lowering treatment in terms of mortality. This finding remained after additional correction for baseline HbA_1c, but was not replicated by using HbA_1c as the predicting variable, as we found no interaction between treatment and HbA_1c groups. Overall, the estimates for the effect of intensive treatment as shown in Table 2 remained unchanged after additional adjustment for HbA_1c (ESM Table 1), which is not surprising given that HGI is just a linear function of HbA_1c and FPG (i.e. it is HbA_1c – (a + b × FPG), where a and b are regression coefficients). The above directly opposes the results of an analysis of the ACCORD trial, where intensive treatment was associated with a significantly higher, instead of lower, risk for mortality in participants with a high HGI [16]. Thus, the hypothesis put forward that a high HGI results in more complications due to more intensive treatment to lower HbA_1c than is necessary to lower plasma glucose is not supported by our study.

The inconsistency between the effect of HGI on outcomes in these two large outcome studies might be explained by important differences between ACCORD and ADVANCE. First, glucose-treatment strategies were different, although both took HbA_1c as predominant measure of glycaemia and both took glucose into account. Glycaemic treatment in ACCORD was intensified when HbA_1c level was ≥42 mmol/mol (≥6%) or when >50% of the self-monitored pre- or 2 h post-meal capillary glucose values were above a certain threshold [26].The treatment algorithm of the ADVANCE trial took discrepancies between HbA_1c levels and blood glucose values into account simultaneously [23]. When HbA_1c level was >47 mmol/mol (>6.5%) but fasting glucose was relatively low, mealtime interventions were optimised and the reliability of the tests was checked. Also ACCORD had participants who started with a higher HbA_1c and had a lower target HbA_1c in the intensive group. Further, 30% more individuals under intensive treatment received insulin in ACCORD compared with ADVANCE [5]. This is in agreement with the observation that FPG was treated more aggressively in ACCORD, with a decrease of 3.3 mmol/l from baseline to end of trial, compared with 1.9 mmol/l in ADVANCE. Moreover, in ADVANCE all participants received a sulfonylurea derivative at the start, while in ACCORD thiazolidinedione treatment was frequently used. The additional treatments differed between studies (i.e. use of aspirin and statins was substantially higher in ACCORD). Second, ACCORD was terminated prematurely, limiting the follow-up, and potentially misrepresenting estimates (no adjustment to standard errors was made for early stopping). Third, ADVANCE and ACCORD were discordant in the major findings, including mortality. In ACCORD, mortality rates were significantly higher in the intervention arm compared with the control arm [14], whereas in ADVANCE there were no significant differences in mortality between arms [23]. Post hoc, it was shown that intensively treated ACCORD participants with a high average on-treatment HbA_1c (>53 mmol/mol [>7%]) were at greater risk for mortality than intensively treated participants with average HbA_1c <53 mmol/mol (<7%) or standard-treated individuals with average HbA_1c >53 mmol/mol (>7%) [27]. The number of individuals experiencing severe hypoglycaemia was significantly higher with intensive treatment in both studies, but the event rates per person-year were higher in the ACCORD trial (3.5% per year with intensive treatment vs 1.0% per year in the control arm), whereas in ADVANCE rates were 0.7% per year in intensive treatment vs 0.4% per year in the control arm. We found no observable difference in the effect of HGI on severe hypoglycaemia due to intensive treatment, although the absolute rates and adjusted hazard ratios increased as HGI rose. With regard to HbA_1c, individuals with intermediate and high HbA_1c were at greater risk for severe hypoglycaemia when intensively treated compared with individuals with low HbA_1c, a finding consistent with previous literature [28].

Baseline characteristics of individuals with high HGI accorded with previous studies [16, 29]. The ethnic differences (i.e. more Asians in the high HGI group) are consistent with the observation that ethnicity influences the haemoglobin glycation, with, in general, relative higher HbA_1c levels in non-whites [17, 30‐33]. However, regression equations in Asian (HbA_1c = 4.6 + 0.373 × FPG, r² = 0.41) and non-Asian (HbA_1c = 4.5 + 0.340 × FPG, r² = 0.40) participants were very similar. The combination of a slightly younger age, longer duration of diabetes (on average 2 years), more use of glucose-lowering medication and higher rates of microvascular complications suggests that individuals with high HGI might have a form of diabetes that is more difficult to treat. This in itself can be the cause of diabetes-related complications, but there is also potential for confounding, as these characteristics could well be explained by the higher HbA_1c levels in individuals with high HGI [22]. The HGI concept is based on the proposed inter-individual variation in haemoglobin glycation, while an adequate method for measuring glycation rate is lacking. Erythrocyte lifespan is a major determinant of the variation in haemoglobin glycation and subtle natural variation in senescence of erythrocytes is complex to quantify [34, 35]. To our knowledge, there are no studies focusing on the pathophysiological mechanism explaining both the biological variation in haemoglobin glycation as well as the reason for the possible increased risk for complications associated with higher glycation rates. This study was limited by a single FPG measurement to determine the relationship with HbA_1c, so we could not take diurnal changes in plasma glucose into account. A measure of average glucose would have been preferred, but was not available in our data and might not be in clinical practice where individuals often use oral glucose-lowering medication only. The DCCT used seven-point glucose profiles to assess HGI [21], while ACCORD used FPG only [16].

In conclusion, we found that HGI predicted macro- and microvascular complications and mortality, but was no better than HbA_1c, which was a stronger predictor for these outcomes. Moreover, HbA_1c is simpler than HGI. High HGI does predict risk for mortality with intensive treatment, but results are the opposite of those from ACCORD. Bringing all this together, the evidence does not support the clinical relevance and usefulness of HGI above HbA_1c.