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A1C versus OGTT

Diagnosis of prediabetes for prevention of diabetes and diabetes-related complications.

Diabetes is the 7th leading cause of death in United States with the prevalence of >29 million Americans with diabetes and 86 million Americans ≥20 years old with prediabetes. The latter is a condition where blood sugar levels are higher than normal but not high enough to be considered diabetes. However, a person is at high risk of developing type 2 diabetes (T2D) within a decade following the diagnosis of prediabetes. Fasting plasma glucose (FPG) and/or a 75-g oral glucose tolerance test (OGTT) are widely accepted diagnostic tests to measure blood glucose1. Prediabetes is determined by FBG levels between 100-125 mg/dL versus normal FBG levels of <100 mg/dL. Before 2010, most diabetes societies recommended blood glucose analysis as the exclusive diagnostic tool, however, over the past few years hemoglobin A1C has been popular to screen to diagnose diabetes4.

The purpose of the study was to evaluate the diagnostic performance of A1c comparing to OGTT in the minority-majority state of New Mexico. The study hypothesized a lack of concordance for the designation of prediabetes between A1c and OGTT amongst Hispanic and non-hispanic whites (NHW) in New Mexico. The study recruited 218 nondiabetic adults living in Albuquerque and New Mexico, ≥18 years old with at least

one of the following risk factors: a family history of T2D in a first-degree relative, a history of gestational diabetes, Hispanic ethnicity, non-Caucasian race, or obesity (BMI ≥30 kg/m2). All participants underwent medical and physical examinations and were instructed to eat a normal diet and have normal physical activity for at least 3 days prior to their appointment. OGTT and A1c were performed on the same morning and subjects were classified as not diabetic (A1c <5.7%), prediabetic (A1c 5.7 – 6.4%), and T2D (A1c >6.4%). Bowker’s test of symmetry was employed to determine agreement between A1c and the OGTT.

The study results showed poor agreement between the two tests. From the total 218 subjects, A1c classified 104 subjects as having prediabetes and OGTT designated only 45 subjects as having prediabetes. The study’s population had 45% Hispanics and showed significant difference in age between the Hispanic and NHW subjects (P <0.01) with a larger proportion of the study being female (P <0.01). Among Hispanic subjects, Bowker’s test of symmetry showed a P-value of 0.002 and McNemar’s chi-squared test comparing the OGTT and A1c without diabetes and with prediabetes showed a P-value of 0.0001, thus suggesting a significant difference between the tests for the diagnosis of prediabetes. Among NHW subjects, Bowker’s test of symmetry returned a P value of 0.003 and McNemar’s chi-squared test comparing A1c and OGTT between no diabetes and those with prediabetes revealed a P value of 0.003, demonstrating a significant difference between A1c and OGTT for the diagnosis of prediabetes. A1c had a sensitivity of 71% and a specificity of 55% with a 95% CI, compared to the OGTT tests among NHW participants.

Both A1c and OGTT are helpful in early diagnosis and prevention, but they have limitations as well. For instance, A1c is prognostic of microvascular complications of diabetes, but it is a surrogate marker for long-term glycemic control to measure protein glycation that can be affected by various blood pathologies. Similarly, OGTT may be better to diagnose postprandial pathology than A1c during the early stages, but it requires patient preparation prior to the test and the test does not work for everyone. Another study has proven that race and ethnic variation can impair glucose tolerance and generally A1c is higher in minorities with impaired GTT. However, this study found comparable lack of agreement between A1c and OGTT for the diagnosis of prediabetes. Also, the study indicated that usually the A1c test identified prediabetes among Hispanic subjects with the possibility of overdiagnosis, while OGTT identified the patients without diabetes. Overall, many studies suggest using both tests or a combination of tests (such as, A1c and FPG) for better diagnosis of diabetes and prediabetes than using one test alone. Lastly, the study stressed to advocate that one test is not better than the other.

In conclusion, further studies are required to determine the optimum diagnostic measure between people with prediabetes and diabetes. Although OGTT and A1c tests have limitations, greater emphasis was put on how A1c can be used and its effectiveness for the diagnosis of prediabetes due to its clear understanding of the results. Neither of them necessarily detects diabetes in the same individual, but they help in demonstrating the need for primary prevention of T2D.

Practice Pearls:

  • Diagnosis of prediabetes can have substantial clinical value for early implementation of important lifestyle changes.
  • Due to discordance between A1c and OGTT, using both tests yields the most accurate method for diagnosing diabetes and prediabetes.
  • Although A1c would result in overdiagnosis of prediabetes than OGTT, it will help in early detection, prevent overt diabetes, and reduce the microvascular and macrovascular risks associated with dysglycemia.

References:

  1.      American Diabetes Association. Diagnosis and classification of diabetes mellitus. Diabetes Care. 2014; 37 (Suppl.1): S81–S9. Web. 14 Feb. 2017.
  2.      Camacho JE, Shah V, Schrader R, Wong CS, and Burge MR. Performance of A1C versus OGTT for the diagnosis of prediabetes in a community-based screening. Endocr Pract. 2016; 22(11): 1288-1295.
  3.      Centers for Disease Control and Prevention. National Diabetes Fact Sheet: National Estimates and General Information on Diabetes and Prediabetes in the United States, 2014. Atlanta, GA: U.S. Department of Health and Human Services; 2014. Available at: http://www.cdc.gov/diabetes/pubs/statsreport14/nationaldiabetesreportweb.pdf. Accessed February 14, 2017.
  4.      Sacks DB. A1C Versus Glucose Testing: A Comparison. Diabetes Care. 2011 Feb; 34(2): 518-523.

Endocr Pract. 2016;22(11):1288-1295.

References: Centers for Disease Control and Prevention. National Diabetes Fact Sheet: National Estimates and General Information on Diabetes and Prediabetes in the United States, 2014. Atlanta, GA: U.S. Department of Health and Human Services; 2014. Available at: http://www.cdc.gov/diabetes/pubs/statsreport14/national-diabetes-report-web.pdf. Accessed April 30, 2016.  New Mexico Department of Health. New Mexico Prediabetes and Diabetes Fact Sheet. Available at: https://nmhealth.org/publication/view/general/108/. Accessed May 15, 2016.