Oral glucose tolerance test appears better than HbA1c for finding diabetes, prediabetes

The oral glucose tolerance test (OGTT) may outperform hemoglobin A1c (HbA1c) level in identifying patients with diabetes and prediabetes, a study reported this month.

Researchers recruited patients from nine diabetes clinics in Italy to assess insulin secretion and action as well as cardiovascular risk profile according to both HbA1c and the OGTT. Included patients, all of whom were considered at risk for type 2 diabetes, had OGTTs done after fasting overnight and after not smoking for at least 12 hours. While patients were fasting, plasma glucose, insulin, and C-peptide levels, lipid profile, and HbA1c were determined. Plasma glucose and C-peptide levels were then measured again 15, 30, 60, 90 and 120 minutes after patients ingested a 75-g glucose load over a five-minute period. The researchers compared patients' glucose regulation categories by the OGTT and HbA1c. The study results were published online Aug. 21 by Diabetes Care.

Eight hundred forty-four patients were included in the study. Forty-four percent were men, and the mean age was 49.5±12 years. Mean BMI was 29±5 kg/m². OGTT identified that 42% of patients had impaired glucose regulation and 15% had a new diagnosis of type 2 diabetes mellitus based on the American Diabetes Association's 2003 criteria. The HbA1c level found that 38% of patients had prediabetes (HbA1c 5.7%-6.4%) and 11% had type 2 diabetes (HbA1c ≥6.5%). The respective concordance rates were 54% and 44%.

Patients who had prediabetes on both tests had more insulin resistance, more impaired insulin secretion, and a worse cardiovascular risk profile than patients whose glucose tolerance was normal on both tests. The researchers performed logistic regression analyses adjusted for age, sex, and body mass index and found that patients with prediabetes or type 2 diabetes diagnosed with the OGTT versus HbA1c had a higher risk for insulin resistance and impaired insulin secretion.

The researchers pointed out that each diagnostic test was performed only once. In addition, they did not assess patients for factors that can affect HbA1c levels, such as anemia (which can falsely lower HbA1c), and the study participants did not represent the general population because they were recruited based on their increased risk for type 2 diabetes. The authors concluded that HbA1c level is a less sensitive test and identifies fewer patients with prediabetes and even fewer patients with type 2 diabetes when compared with the OGTT. Patients who were characterized as having prediabetes or type 2 diabetes on both tests had worse metabolic profiles. Insulin resistance, insulin secretion and cardiovascular risk profile did not differ between tests in patients identified as having prediabetes, the researchers noted.

"Our findings are in agreement with those observed in other Caucasian populations…and confirm that HbA1c is a specific but insensitive method for diagnosis of [type 2 diabetes mellitus] or prediabetes," they wrote.