© 2005 by the American Diabetes Association, Inc.
Relationship Between HbA1c Level and Peripheral Arterial Disease
1 Department of Epidemiology, Tulane University School of Public Health and Tropical Medicine, New Orleans, Louisiana Address correspondence and reprint requests to Paul Muntner, Department of Epidemiology, Tulane University SPHTM, 1430 Tulane Ave., SL-18 New Orleans, LA 70112. E-mail: pmuntner{at}tulane.edu
OBJECTIVEHomeostatic glucose control may play an important role in the development of peripheral arterial disease among individuals without diabetes. We sought to evaluate the association of HbA1c (A1C) with peripheral arterial disease in a representative sample of the U.S. population with and without diabetes.
RESEARCH DESIGN AND METHODSA cross-sectional study was conducted among 4,526 National Health and Nutrition Examination Survey 19992002 participants
RESULTSAmong nondiabetic subjects, the age-standardized prevalence of peripheral arterial disease was 3.1, 4.8, 4.7, and 6.4% for participants with an A1C <5.3, 5.35.4, 5.55.6, and 5.76.0%, respectively (P trend <0.001). The prevalence of peripheral arterial disease was 7.5 and 8.8% for diabetic participants with A1C <7 and
CONCLUSIONSAn association exists between higher levels of A1C and peripheral arterial disease, even among patients without diabetes. Individuals with A1C levels
Abbreviations: ABI, ankle-brachial index NHANES, National Health and Nutrition Examination Survey
The risk for clinical cardiovascular disease among individuals with diabetes is two to four times higher compared with their counterparts without diabetes (14). Higher rates of subclinical cardiovascular disease among patients with diabetes have also been reported (57). Among the population of patients with diabetes, homeostatic glycemic control may be an important mediating factor in preventing cardiovascular disease (8).
More recently, some studies have noted elevated levels of glycemia to be associated with cardiovascular disease among the general population without diabetes (913). For example, after excluding individuals with diabetes (i.e., self-report of a previous diagnosis and/or fasting plasma glucose Given the strong association between glycemic control and the development of clinical and subclinical cardiovascular disease among diabetic patients and mounting evidence that mild abnormalities of glucose metabolism may be associated with cardiovascular disease in nondiabetic individuals, we hypothesized that a strong association may exist between homeostatic glycemia and subclinical cardiovascular disease among patients without diabetes. Therefore, we examined the association between A1C and peripheral arterial disease among a nationally representative sample of U.S. adults participating in the National Health and Nutrition Examination Survey (NHANES) 19992002.
NHANES 19992002 was a cross-sectional survey of the civilian noninstitutionalized population of the U.S. The procedures involved in NHANES 19992002 have been published in detail and are available online (14). In brief, this study included a stratified multistage probability sample based on selection of counties, blocks, households, and individuals within households.
Overall, 5,874 adults
NHANES consisted of an in-home interview followed by a medical evaluation and blood sample collection at a mobile examination center. Of relevance to the current analysis, variables collected during the in-home interview were age, race/ethnicity, sex, cigarette smoking, alcohol consumption, physical activity, and a history of diabetes. For the current analysis, race/ethnicity was defined as non-Hispanic white, non-Hispanic black, Mexican American, and other. Participants who reported having smoked The NHANES 19992002 examination procedures included measurements of waist circumference, blood pressure, and ABI. After asking the participant to lift up their shirt, waist circumference was measured at the iliac crest to the nearest 0.1 cm. Up to three blood pressure measurements were taken by a physician using the standard protocol of the American Heart Association during a single examination visit. Blood pressure was measured with the participant in a seated position after 5 min of quiet rest. Systolic blood pressure was calculated based on the average of all available blood pressure measurements.
For individuals with at least one arm and weighing
Detailed descriptions about blood collection and processing are provided in the NHANES Laboratory/Medical Technologists Procedures Manual. C-reactive protein was measured using latex-enhanced nephelometry, and total cholesterol was measured enzymatically. Serum creatinine was measured using the modified Jaffe reaction with glomerular filtration rate estimated by the abbreviated formula from the Modification of Diet in Renal Disease study after creatinine calibration (18,19). Chronic kidney disease was defined as an estimated glomerular filtration rate <60 ml · min1 · 1.73 m1 (2). Serum glucose was measured at the University of Missouri Diabetes Diagnostic Laboratory using a modified hexokinase enzymatic method. A1C was also measured at the University of Missouri using a boronate affinity high-performance liquid chromatography system. Diabetes was defined as a serum glucose
Statistical analysis The age-standardized prevalence of peripheral arterial disease was calculated for each A1C category for individuals with and without diabetes. The odds ratio (OR) of peripheral arterial disease was calculated for each A1C level, with the lowest quartile of A1C among individuals without diabetes as the reference, using multivariable logistic regression models. Initially, ORs were adjusted for age, and then they were adjusted for age, race/ethnicity, and sex, and finally they were adjusted for age, race/ethnicity, sex, systolic blood pressure, current and former cigarette smoking, alcohol consumption, being physical active, waist circumference, chronic kidney disease, total cholesterol, and C-reactive protein. Trends in the OR of peripheral arterial disease across A1C category were determined for the full population and limited to nondiabetic participants modeling A1C category as a continuous variable, using logistic regression. The age-adjusted; age-, race/ethnicity-, and sex-adjusted; and multivariable-adjusted OR of peripheral arterial disease associated with 1percentage pointhigher A1C was calculated, using logistic regression models. These ORs were calculated for the full study population; individuals with A1C <7 and <6.1%, separately; and, finally, for individuals without diabetes. The consistency of the association between 1percentage pointhigher A1C and peripheral arterial disease across important subgroups was determined by logistic regression models stratified by race/ethnicity, sex, and cigarette smoking status. Sample weights that account for the unequal probabilities of selection, oversampling, and nonresponse were applied for all analyses using SUDAAN (version 8.0; Research Triangle Institute, Research Triangle Park, NC). SEs were estimated using the Taylor series linearization method.
After age standardization, individuals with higher levels of A1C were, on average, older, less likely to be female, and more likely to be non-Hispanic black or Mexican American, versus non-Hispanic white (Table 1). Additionally, after age standardization, those with higher levels of A1C were more likely to be current smokers and had higher systolic blood pressure, larger waist circumference, and higher C-reactive protein. Additionally, age-standardized ABI was lower among individuals with higher levels of A1C.
Overall, 327 study participants (5.1%) in NHANES 19992002 had peripheral arterial disease. After age standardization, the prevalence of peripheral arterial disease was 3.1, 4.8, 4.7, and 6.4% among individuals without diabetes and with an A1C <5.3 (lowest quartile), 5.35.4, 5.55.6, and 5.76.0% (highest quartile), respectively (P trend <0.001) (Fig. 1). Additionally, the age-standardized prevalence of peripheral arterial disease was 7.5 and 8.8% among individuals with diabetes and an A1C <7 and 7%, respectively.
After age adjustment, a graded association was present between higher A1C levels and an increased OR of peripheral arterial disease (P trend <0.001) (Table 2). A graded association remained present after adjustment for age, race/ethnicity, and sex and in a multivariable model that included adjustment for age, race/ethnicity, sex, current and former cigarette smoking, alcohol consumption, being physically active, systolic blood pressure, waist circumference, total cholesterol, C-reactive protein, and chronic kidney disease (P < 0.001 and P = 0.008, respectively). A graded association of higher prevalence of peripheral arterial disease at higher quartile of A1C was also present when limiting the analysis to participants without diabetes (multivariable- adjusted P trend = 0.041).
Onepercentage pointhigher A1C was associated with a multivariable-adjusted OR of peripheral arterial disease of 1.24 (95% CI 1.051.47) (Table 3). When limiting the analysis to the population with A1C <7 and <6.1%, separately, 1percentage pointhigher A1C was associated with a multivariable-adjusted OR of peripheral arterial disease of 1.65 (1.072.55) and 1.87 (1.023.43), respectively. Finally, the multivariable-adjusted OR for 1percentage pointhigher A1C among nondiabetic subjects was 1.77 (0.973.21). Results were consistent across subgroups defined by race/ethnicity; sex; and current, former, and never smoking (each P interaction >0.10) (data not shown).
In the current study, higher A1C, across the entire range of concentrations, was a significant predictor of peripheral arterial disease. This association was present within the normal range and even after multivariable adjustment. For example, the OR of peripheral arterial disease associated with an A1C of 5.76.0%, compared with <5.3%, was 1.57 (95% CI 1.022.47) after adjustment for potential confounders and intermediate factors including total cholesterol, C-reactive protein, and chronic kidney disease. The definition of diabetes used in the current study reflects cut points from evidence-based guidelines (20). However, as noted in these guidelines, the risk of microvascular disease and target organ damage extends below these cut points (20). The results of the current study add to previously published evidence and suggest substantial benefits of tight glycemic control, even among patients without diabetes. A meta-analysis of observational studies has documented an increased risk of cardiovascular disease associated with higher levels of A1C among patients with diabetes (7). Specifically, the pooled relative risk (RR) of cardiovascular disease for each 1percentage pointhigher A1C was 1.18 (95% CI 1.101.26) from 10 studies that included 7,435 patients with type 2 diabetes. Additionally, this study also investigated the impact of A1C on peripheral arterial disease among patients with type 2 diabetes. Three studies of A1C and peripheral arterial disease among type 2 diabetic patients were identified (2123). Each of the identified studies reported A1C to be associated with a significant increased RR of peripheral arterial disease. After pooling together the results from the three studies, the RR of peripheral arterial disease associated with 1percentage pointhigher A1C was 1.28 (1.181.39) among patients with type 2 diabetes (7). Our study extends these findings to a large nationally representative population sample of nondiabetic patients. Among nondiabetic patients in the current study, each 1percentage pointhigher A1C was associated with an age-, race/ethnicity-, and sex-adjusted OR of 2.00 (1.133.54) and a multivariable-adjusted OR of 1.77 (0.973.21) for peripheral arterial disease.
Despite consistent results among diabetic patients, studies of glucose metabolism abnormalities and cardiovascular disease among nondiabetic patients have provided inconsistent results (10,12,24,25). In men without diagnosed diabetes participating in the European Prospective Investigation of Cancer and Nutrition (EPIC-Norfolk) study, the RR of all-cause mortality associated with A1C of 5.05.4, 5.56.9, and Studies of A1C with subclinical cardiovascular disease have produced conflicting evidence (2629). In a nested case-control study within the Atherosclerosis Risk in Communities (ARIC) study, the mean A1C level was 5.18 and 5.07%, respectively (P = 0.004), among case subjects (carotid artery far-wall thickness >2.5 mm or bilateral thickening exceeding the 90th percentile of the cohort distribution) and control subjects matched for age, race, sex, and Atherosclerosis Risk in Communities field center (far- and near-wall thickness lower than the 75th percentile of intima-media thickness on all carotid artery segments) (28). However, no association between A1C and intima media thickness was present after multivariable adjustment. Additionally, among 582 individuals at risk for diabetes, a significant age- and sex-adjusted association between A1C and intima-media thickness was no longer significant, with an OR of 1.24 (95% CI 0.821.89), after multivariable adjustment (35). Among older Caucasian adults without known diabetes in the Hoorn study population, each 1percentage pointhigher A1C was associated with a multivariable-adjusted OR of peripheral arterial disease of 1.35 (1.101.65) (26). Other studies have also found associations of higher rates of subclinical cardiovascular disease at higher levels of glycemia (29). These latter studies are consistent with the results from the current analysis; the multivariable-adjusted OR of peripheral arterial disease associated with an A1C of 5.76.0%, compared with an A1C <5.3%, was 1.57 (1.022.47). Additionally, previously published data from the Diabetes Control and Complications Trial have shown that the risk of retinopathy progression, microalbuminuria, and neuropathy increased continuously with higher levels of A1C over its entire range. Peripheral arterial disease is a particularly strong prognostic indicator of future clinical cardiovascular disease events, and the importance of its recognition has been highlighted in several guidelines (3033). Furthermore, it is easily diagnosed in clinical practice at minimal cost. It should be recognized that in the late stages of diabetes, calcification and increased arterial wall stiffness may lead to false elevations in ankle blood pressure. It is not clear whether such changes occur early in the disease process, but, if so, our findings may be more robust than indicated by the data. Despite the potential benefits of lowering A1C noted in the current study, to our knowledge there is no clinical trial data showing that the risk of cardiovascular and peripheral arterial diseases is reduced by the continued reduction of A1C below 6.1%. However, one of the primary aims of the Action to Control Cardiovascular Risk in Diabetes (ACCORD) trial is to assess whether a therapeutic strategy targeting an A1C of <6.0% will reduce the rate of cardiovascular disease events compared with a target of 7.07.9% (34). Recruitment for Action to Control Cardiovascular Risk in Diabetes began in 2003, with follow-up expected to last through 2009, and study results are anticipated to be available in 2010.
The results presented for the current study need to be interpreted in the context of the studys limitations. Most notably, the cross-sectional nature of the present study limits assessment of the causal nature of the association between A1C and peripheral arterial disease. Additionally, although the sample size was large, the power to detect associations and effect modification was limited because of the low prevalence of peripheral arterial disease. For example, although elevated ORs of peripheral arterial disease were present when comparing nondiabetic patients with an A1C of 5.35.4 and 5.55.6% vs. <5.3%, the 95% CIs were wide and included the null. Given the graded nature of the association detected, with a larger sample size, these ORs most likely would have been statistically significant. Additionally, although we analyzed patients with diabetes separately, NHANES 19992002 procedures did not include the oral glucose tolerance test. Therefore, some patients classified as not having diabetes in the current study, based on serum glucose <126 mg/dl (<200 for those not fasting for Despite these limitations, there are several strengths involved in the current study. This study involved a nationally representative sample of the U.S. population >40 years of age. Additionally, all data were collected following rigorous methodology, including a study protocol with quality control checks. In conclusion, higher levels of A1C among nondiabetic patients may be an indicator of increased risk of subclinical cardiovascular disease. A significant increased risk of peripheral arterial disease was noted at A1C levels between 5.7 and 6.0% among nondiabetic patients. Furthermore, nonsignificant increased risks of peripheral arterial disease were noted at A1C levels of 5.35.4 and 5.55.6%. Measurement of A1C or other indicators of mild glucose abnormalities in clinical practice may be valuable in identifying patients with an increased risk of subclinical cardiovascular disease.
A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances. Received for publication March 24, 2005. Accepted for publication May 8, 2005.
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