This prospective study involved 687 participants from the Rio de Janeiro Type 2 Diabetes (RIO-T2D) Cohort Study, who were enrolled between August 2004 and December 2008. These individuals were reexamined annually until December 2019 at the diabetes outpatient clinic of our tertiary-care university hospital. The study received approval from the Research Ethics Committee of the School of Medicine and University Hospital, Federal University of Rio de Janeiro (number 124/2004), and all participants provided written informed consent. Detailed descriptions of this cohort, the inception methods, and the diagnostic definitions have been previously published [17-20]. In summary, the inclusion criteria for the RIO-T2D cohort consisted of all adults with type 2 diabetes up to 80 years old who had either any microvascular (retinopathy, nephropathy, or neuropathy) or macrovascular (coronary, cerebrovascular, or peripheral artery disease) complication, or at least two other modifiable cardiovascular risk factors (hypertension, dyslipidemia, or smoking). The exclusion criteria included morbid obesity (body mass index [BMI] ≥40 kg/m²), advanced renal failure (serum creatinine >180 μmol/L or estimated glomerular filtration rate [eGFR] <30 mL/min/1.73 m²), or the presence of any serious concomitant disease limiting life expectancy.

All participants underwent a standard baseline protocol that included a thorough clinical laboratory evaluation. The diagnostic criteria for chronic complications of diabetes have been detailed in previous publications [17-20]. Briefly, coronary heart disease was diagnosed using clinical and electrocardiographic criteria, or by positive ischemic stress tests. Cerebrovascular disease was diagnosed based on history and physical examination, and peripheral arterial disease was identified with an anklebrachial index <0.9. The diagnosis of nephropathy required at least two instances of albuminuria exceeding 30 mg/day or a confirmed reduction in glomerular filtration rate (eGFR ≤60 mL/min/1.73 m², estimated by the Chronic Kidney Disease Epidemiology Colaboration [CKD-EPI] equation, or serum creatinine >130 μmol/L). Peripheral neuropathy was assessed through clinical examination (including knee and ankle reflex activities, feet sensation using the Semmes-Weinstein monofilament, vibration with a 128-Hz tuning fork, pinprick, and temperature sensations) and neuropathic symptoms were evaluated using a standard validated questionnaire [18]. Blood pressure (BP) in the clinic was measured three times using a digital oscillometric BP monitor (HEM-907XL, Omron Healthcare, Kyoto, Japan) with a suitably sized cuff on two separate occasions 2 weeks apart at the start of the study. The first measurement of each visit was discarded, and the BP considered was the average of the last two readings from each visit. Arterial hypertension was diagnosed if the mean systolic BP was ≥140 mm Hg or diastolic BP was ≥90 mm Hg, or if antihypertensive medications had been prescribed. Laboratory evaluations included FPG, HbA1c, serum creatinine, and lipids. Albuminuria was assessed in two non-consecutive sterile 24-hour urine collections.

HGI was calculated as previously proposed [1]. We conducted a linear regression analysis to examine the relationship between baseline HbA1c and FPG across the entire cohort. The resulting linear equation was HbA1c (%)=0.015×FPG (mg/dL)+5.6, with a correlation coefficient of 0.55. To estimate the predicted baseline HbA1c, we inserted the baseline FPG values into this equation and then calculated the baseline HGI by subtracting the predicted HbA1c from the observed HbA1c. Consequently, individuals with a high HGI exhibit a higher HbA1c than expected based on their FPG levels, whereas those with a low HGI show a lower HbA1c than predicted. Given that HGI may particularly change during the first year of follow-up due to medication adjustments and the initiation of insulin [12], we also evaluated the mean first-year HGI. This was done by calculating the HGI for each of the four HbA1c-FPG pairs measured during the first year and then averaging these values. The linear equation for the mean first-year HbA1c and FPG was HbA1c (%)=0.019×FPG (mg/dL)+5.0, with a correlation coefficient of 0.59. Additionally, we assessed the variability in HGI and HbA1c during the first year by calculating the standard deviation of these four measurements. Thus, we obtained measures of baseline and mean first-year HGI and HbA1c, along with their respective variabilities during the first year of follow-up.

All participants were followed up regularly, at least three to four times a year, until December 2019, receiving standardized treatment. The observation period was defined as the number of months from cohort entry to either the last clinical visit in 2019 or the occurrence of the first endpoint, whichever occurred first. The primary outcomes included the occurrence of any macrovascular or microvascular events and mortality. Macrovascular outcomes encompassed total cardiovascular events (CVEs; fatal or non-fatal myocardial infarctions [MIs], sudden cardiac deaths, new-onset heart failure, death from progressive heart failure, any myocardial revascularization procedure, fatal or non-fatal strokes, any aortic or lower limb revascularization procedure, any amputation above the ankle, and deaths from aortic or peripheral arterial disease), major adverse cardiovascular events (MACEs; non-fatal MIs and strokes plus cardiovascular deaths), and all-cause and cardiovascular mortality [17]. Cardiovascular and mortality outcomes were adjudicated from medical records (most non-fatal and fatal in-hospital events occurred at our hospital), death certificates, and interviews with attending physicians and patient families, using a standard questionnaire reviewed by two independent observers [17]. Microvascular outcomes included the development or worsening of retinopathy [19], renal outcomes [20] (new microalbuminuria development or progression to macroalbuminuria, and renal function deterioration defined as a doubling of serum creatinine or end-stage renal failure requiring dialysis or resulting in death from renal failure), and the development or worsening of peripheral neuropathy [18]. Microvascular outcomes were evaluated annually. Information on hypoglycemia occurrence during follow-up was available for 496 participants (72% of the entire cohort). Symptomatic moderate-severe hypoglycemia was defined as a symptomatic episode of confirmed hypoglycemia (blood glucose <50 mg/dL) where the individual could not self-treat.

In the initial exploratory analysis, participants were divided into tertiles based on their mean first-year HGI, and their baseline clinical-laboratory characteristics were compared using analysis of variance, the Kruskal-Wallis test, or the chi-square test as appropriate. We assessed differences in the cumulative incidences of adverse outcomes during follow-up among tertile subgroups of HGI, HbA1c, and their variabilities using Kaplan-Meier curves with the log-rank test. The adjusted risks associated with baseline and mean first-year HGI, HbA1c, and their variabilities were examined through multivariable Cox survival analyses for each outcome. Initially, each parameter was adjusted for age and sex (model 1); subsequently, adjustments were made for additional potential confounders (model 2: age, sex, BMI [body height in neuropathy analyses], physical activity, smoking status, diabetes duration, pre-existing macrovascular and microvascular complications, serum low-density and high-density lipoprotein cholesterol, the use of insulin, statins, and aspirin, and the number of antihypertensive drugs in use). Renal outcomes were further adjusted for baseline albuminuria and eGFR. Finally, HGI and HbA1c parameters were simultaneously adjusted (model 3). Both HGI and HbA1c parameters were assessed as continuous variables, with hazard ratios (HRs) estimated for standardized 1-standard deviation increments to allow comparisons between them; they were also categorized into tertiles, with HRs estimated for the upper tertile subgroup in relation to the lower tertile. Due to the high correlation between continuous HGI and HbA1c parameters (correlation coefficients between 0.84 to 0.93), to avoid collinearity in model 3, we uncorrelated these parameters by regressing HGI parameters on their corresponding HbA1c parameter and vice versa, using the residual of one parameter as the adjustment covariate for the other [21]. For the hypoglycemia outcome, logistic regressions were used with odds ratios adjusted for the same covariates as in the Cox analyses. In analyses of first-year mean HGI and HbA1c parameters, participants who reached any of the endpoints during the first year of follow-up were excluded. We assessed the predictive performance of each fully-adjusted model—whether with HGI, HbA1c, or both parameters—for each outcome by calculating the C-statistics of each model (analogous to the area under the curve [AUC] applied to time-to-event analysis) and compared them using the method proposed by DeLong [22]. Finally, we conducted interaction-sensitivity subgroup analyses for age (>/≤60 years), sex, diabetes duration (≥/<8 years), presence/absence of macro- and microvascular complications at baseline, and poor/good glycemic control during the first year of follow-up (mean HbA1c ≥/<7.5%), to determine whether there were any subgroups in which HGI parameters were better predictors of adverse outcomes than their corresponding HbA1c parameters. All statistics were performed using SPSS version 19.0 (IBM Corp., Armonk, NY, USA), and a 2-tailed P value <0.05 was considered significant.

Table 1 displays the primary baseline clinical-laboratory characteristics of all 687 individuals with type 2 diabetes who were evaluated, as well as those categorized by tertiles of mean HGI during the first year of follow-up. The individuals in the upper tertile HGI subgroup were younger, predominantly female, and had a longer duration of diabetes compared to those in the lower tertile subgroup. Additionally, they exhibited higher prevalence rates of retinopathy and nephropathy, primarily due to increased albuminuria, and were more likely to use insulin and less likely to use sulfonylureas. As anticipated, their glycemic control was poorer than that of individuals in the lower tertile subgroups.

Over a median follow-up period of 10.6 years (interquartile range, 6.3 to 13.2), corresponding to 6,596 person-years, there were 215 total CVEs, including 176 MACEs, and 269 all-cause deaths, 131 of which were due to cardiovascular diseases. Additionally, microalbuminuria developed in 126 participants, advanced renal failure occurred in 104, retinopathy developed or worsened in 161, and peripheral neuropathy developed or worsened in 177. The incidence rates of all adverse outcomes, except for the development of microalbuminuria, were higher among individuals in the upper tertile subgroup of the mean HGI than among those in the lower tertile subgroup (Table 1, bottom). This observation was supported by the Kaplan-Meier estimation of the cumulative incidence of endpoints during the follow-up, as illustrated in Figs. 1, 2. During the follow-up, 90 individuals experienced at least one episode of symptomatic moderate to severe hypoglycemia, which occurred marginally more frequently (23.9% vs. 13.3%, P=0.05) in the upper tertile HGI subgroup than in the lower tertile subgroups.

Figs. 1, 2 illustrate the cumulative incidences of adverse outcomes within tertile subgroups based on average HbA1c levels during the first year of follow-up. It is evident that there was no distinct advantage in using tertile subgroups to differentiate between higher incidences of outcomes when comparing mean first-year HGI and HbA1c, either for cardiovascular/mortality outcomes (Fig. 1) or for microvascular outcomes (Fig. 2). Similar observations were made for baseline HGI and HbA1c (Supplemental Figs. S1, S2), as well as for the variabilities of HGI and HbA1c during the first year of follow-up (Supplemental Figs. S3, S4).

Table 2 displays the adjusted risks linked with baseline and mean first-year HGI and HbA1c parameters for macrovascular and microvascular outcomes, mortality, and hypoglycemia. Generally, in the fully-adjusted analyses (model 2), both HGI and HbA1c parameters were significantly associated with increased risks for cardiovascular, mortality, and microvascular outcomes, with this association being particularly pronounced for mean first-year parameters. Conversely, the adjusted risks for HGI and their corresponding HbA1c parameters were roughly equivalent, showing no distinct advantage of one over the other for any of the outcomes. This observation was reinforced when both parameters were adjusted simultaneously in model 3. Neither HGI nor HbA1c parameters were linked to an increased likelihood of symptomatic hypoglycemia during follow-up. Table 3 presents the predictive performance of each fully-adjusted model for each outcome, whether including HGI, HbA1c, or both parameters, as evaluated by the C-statistic. The AUCs for each model showed no differences across the outcomes, further supporting the predictive equivalence of HGI and HbA1c.

Supplemental Tables S1-S3 present the results of the interaction/subgroup analyses. While there were some interactions noted (for example, the adjusted risks associated with HGI and HbA1c parameters for the retinopathy outcome were higher in younger individuals, in men, and in those without pre-existing macrovascular complications compared to their counterparts), none of the subgroups evaluated showed clear superiority of HGI parameters over their corresponding HbA1c parameters for any of the adverse outcomes.