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HbA1c and OGTT: Understanding the Differences

Is A1c better than an Oral Glucose Tolerance Test in diabetes management?

The prevalence of diabetes keeps increasing in the United States and challenges often arise when managing patients with uncontrolled diabetes. Diagnosis and adequate glycemic control relies mostly on A1c levels. However, its diagnostic capability has been questioned to a certain extent in various clinical settings. For instance, debates have risen in terms of which A1c value better predicts outcomes in these patients. Previous studies have looked at the use of A1c and the oral glucose tolerance test (OGTT), and their threshold values. Some studies question the ability to predict cardiovascular complications, others question its diagnostic ability in certain patient populations. For instance, in a cohort study conducted in Denmark by Gokulan Pararajasingam and colleagues, the predictive value of A1c and OGTT and the risk of cardiovascular and all-cause mortality was examined. In this study, it was observed that OGTT and A1c did not provide any prognosis benefit in cardiovascular outcomes when they are analyzed separately. Nonetheless, it was seen that when both are taken into consideration, there is better long-term prognosis. Similar studies have looked into the relevance of both of these tests, independently and in conjunction.

In the effort to better understand the predictive value of OGTT and A1c, a recent study conducted by Jenny E. Camacho, MD and colleagues at the University of New Mexico looked into the reliability of OGTT and A1c values in predicting prediabetes. Camacho and colleagues hypothesized that there was going to be a lack of concordance between these two laboratory values in the diagnosis of prediabetes. Their cohort focus was Hispanics and Non-Hispanic Whites. Patients were eligible to participate in the study if they did not have a standing diagnosis of diabetes. Age for inclusion was >18 years with at least one of the following: family history of type 2 diabetes, either being Hispanic or non-Hispanic White, or obesity described as a BMI greater than 30 kg/m2. A total of 218 patients participated in this study and each were exposed to OGTT analysis via WHO criteria and A1c analysis per ADA recommendations during the same visit.  In order to standardize exposure to these subjects, each person was instructed to follow a normal diet.

After stratification based on race, 96 patients were Hispanic, of which 51 were classified as having no diabetes as determined by A1c alone, 18 subjects were classified as to having prediabetes through OGTT. On the other hand, 45 patients had prediabetes as indicated by A1c values (p=0.002). However, through a McNemar’s chi-squared test, they showed a significant difference between OGTT and A1c for diagnosis (p<0.0001). Furthermore, for those patients who were classified as non-white Hispanics, similar findings were observed as in the Hispanic group (p=0.003). Therefore, no major differences were observed in regards to race.

Based on these findings, it is postulated that each laboratory test can be used for predicting various outcomes in diabetes management. The oral glucose tolerance test provides a means for better understanding glycemic control in prediabetes patients. OGTT provides a more predictive value in post-prandial blood glucose levels than in A1c levels, which can aid the diagnosis of prediabetes or diabetes. Hence, why it is incorporated in current ADA treatment guidelines and WHO classification.

Nonetheless, the clinical utility of A1c levels relies on accurately identifying chronic elevations in blood sugar. A1c is thought to provide a better picture of macro and microvascular complications. However, based on findings from the study conducted by Pararajasingam and colleagues, this needs to be evaluated further, as A1c alone did not provide any information on the risk of other complications. Utilizing the clinical utility of both of these laboratory findings increases the ability to better predict outcomes and improve diagnostic sensitivity and specificity in patients who are at risk for diabetes. The use and validity of these tests is independent of race, as outlined in the study results.

Practice Pearls:

  • The Oral Glucose Tolerance Test (OGTT) provides more information behind the pathology of uncontrolled postprandial blood glucose.
  • HbA1c alone provides information in the setting of uncontrolled chronic hyperglycemia, specifically in impaired fasting blood glucose levels.
  • Utilizing both laboratory measures aids in providing early detection methods to prevent complications.

References:

Pararajasingam, Gokulan, Dan Eik Høfsten, Brian Bridal Løgstrup, Michael Egstrup, Finn Lund Henriksen, Jørgen Hangaard, and Kenneth Egstrup. “Newly Detected Abnormal Glucose Regulation and Long-term Prognosis after Acute Myocardial Infarction: Comparison of an Oral Glucose Tolerance Test and Glycosylated Haemoglobin A1c.” International Journal of Cardiology 214 (2016): 310-15. Web.

Camacho, Jenny E., Vallabh O. Shah, Ronald Schrader, Craig S. Wong, and Mark R. Burge. “Performance Of A1C Versus Ogtt For The Diagnosis Of Prediabetes In A Community-Based Screening.” Endocrine Practice Aop (2016): n. pag. Web.

 

Researched and prepared by Christian Gill, Pharm.D. Candidate, Class of 2017. Reviewed by Michelle Caetano, Pharm.D., BCPS, BCACP, CDOE, CVDOE