HbA1c testing helps predict the likelihood that patients will develop diabetes in the future. Obesity was not as strongly predictive of diabetes as HbA1c.
There is controversy surrounding the issue of whether, and how, to screen adults for type 2 diabetes. The objective was to measure the incidence of new diabetes among outpatients enrolled in a health care system, and to determine whether hemoglobin A1c (HbA1c) values would allow risk stratification for patients’ likelihood of developing diabetes over 3 years.
A prospective cohort study was done with a 3-year follow-up at a single large, tertiary care, Department of Veterans Affairs Medical Center (VAMC). A convenience sample of 1,253 outpatients without diabetes, age 45 to 64, with a scheduled visit at the VAMC, were screened for diabetes using an initial HbA1c measurement. All subjects with HbA1c >/= 6.0% (normal, 4.0% to 6.0%) were invited for follow-up fasting plasma glucose (FPG). We then surveyed patients annually for 3 years to ascertain interval diagnosis of diabetes by a physician. The baseline screening process was repeated 3 years after initial screening. After the baseline screening, new cases of diabetes were defined as either the self-report of a physician’s diagnosis of diabetes, or by HbA1c >/= 7.0% or FPG >/= 7.0 mmol/L at 3-year follow-up. The incidence of diabetes was calculated as the number of new cases per person-year of follow-up.
The Results showed that one thousand two hundred fifty-three patients were screened initially, and 56 (4.5%) were found to have prevalent unrecognized diabetes at baseline. The 1,197 patients without diabetes at baseline accrued 3,257 person-years of follow-up. There were 73 new cases of diabetes over 3 years of follow-up, with an annual incidence of 2.2% (95% confidence interval [CI], 1.7% to 2.7%). In a multivariable logistic regression model, baseline HbA1c and baseline body mass index (BMI) were the only significant predictors of new onset diabetes, with HbA1c having a greater effect than BMI. The annual incidence of diabetes for patients with baseline HbA1c </= 5.5 was 0.8% (CI, 0.4% to 1.2%); for HbA1c 5.6 to 6.0, 2.5% (CI, 1.6% to 3.5%); and for HbA1c 6.1 to 6.9, 7.8% (CI, 5.2% to 10.4%). Obese patients with HbA1c 5.6 to 6.0 had an annual incidence of diabetes of 4.1% (CI, 2.2% to 6.0%).
At the baseline screening, we defined a case of diabetes as HbA1c >/= 7.0% or FPG >/= 7.0 mmol/L (126 mg/dl). These patients were then excluded from analysis of diabetes incidence. For the remaining patients, an incident case of diabetes was defined as self-report by patient at any of the 3 annual interviews, or by HbA1c >/= 7.0% or FPG >/= 7 mmol/L at 3-year rescreening. The use of self-reported diabetes as part of the definition is necessary because patients with diabetes may receive treatment that lowers their laboratory test values below the threshold for diagnosis; indeed, this is the goal of optimal diabetes treatment.
From the results it was concluded that HbA1c testing helps predict the likelihood that patients will develop diabetes in the future. Patients with normal HbA1c have a low incidence of diabetes and may not require rescreening in 3 years. However, patients with elevated HbA1c who do not have diabetes may need more careful follow-up and possibly aggressive treatment to reduce the risk of diabetes. Patients with high-normal HbA1c may require follow-up sooner than 3 years, especially if they are significantly overweight or obese. This predictive value suggests that HbA1c may be a useful test for periodic diabetes screening.
This study shows that in an outpatient population undergoing screening in a health care setting, HbA1c strongly predicts the development of diabetes. Obese patients were more likely to develop diabetes; however, obesity was not as strongly predictive of diabetes as HbA1c. We documented a very high incidence, 11% per year, of diabetes in obese patients with elevated HbA1c.
From these recent studies, we propose an opportunistic screening strategy for type 2 diabetes. First, a target population for baseline screening can be selected from patients in a medical setting based on risk factors such as hypertension, obesity, and family history of diabetes.21–23 Then, HbA1c results from the baseline screen can be used to suggest an interval until the next screening test, with follow-up testing more frequent for those patients with baseline HbA1c more than 2 standard deviations above mean and less frequent for those with HbA1c less than 1 standard deviation above mean.
Obesity also predicted new onset diabetes in our population. Patients with BMI over 27.5 and high-normal HbA1c had a modestly increased incidence of diabetes, and may also merit closer attention and more frequent periodic screening than patients of normal weight. Additionally, informing patients of the role of obesity in increasing their risk for developing diabetes, even if they are nondiabetic at their current weight, may help reinforce the message that weight loss is important.
J Gen Intern Med 19(12):1175-1180, 2004
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