Most physicians have embraced the hemoglobin A1c blood test to diagnose diabetes or track treatment success. It is also known as HbA1c (A1C for short), glycosylated hemoglobin or glycated hemoglobin. The A1C test is supposed to provide health professionals with a quick and easy assessment of average blood sugar levels over the past two to three months. All it requires is a blood draw and a lab analysis. The OGTT stands for oral glucose tolerance test. It provides an assessment of how the body responds to sugar over the course of three hours. Which is the best test to diagnose diabetes? Surprisingly, a new study suggests that the old-fashioned OGTT could be more accurate than the A1C (presentation at the annual Endocrine Society Meeting, March 25, 2019).
What’s the Difference Between the A1C and the OGTT?
The A1C blood test measures the amount of sugar attached to your blood hemoglobin. Your red blood cells contain hemoglobin, a protein that carries oxygen to cells throughout your body. Your hemoglobin proteins are also coated with sugar. The amount of sugar on your hemoglobin represents a kind of snapshot of your blood glucose over several weeks to three months.
By measuring the amount of sugar that is attached to hemoglobin the laboratory comes up with a percentage. A “normal” A1C level would be considered 5.7% or lower. If your percentage of sugar bound to hemoglobin is between 5.7% and 6.4% many doctors would consider your prediabetic. When the sugar-coated hemoglobin percentage goes over 6.5%, most health professionals would diagnose diabetes. Diabetologists encourage their patients to keep HbA1c levels at 7% or lower.
OGTT (Oral Glucose Tolerance Test)
This is a screening test to determine how your body reacts to a big dose of sugar. People are told to fast for eight hours before going to the lab. You will likely be told to avoid liquids as well. At the lab you will have your blood drawn to get a baseline blood glucose level. Then you will be given a gloppy drink containing a big slug of glucose (usually about 75 grams). A blood sample will be drawn every 30 to 60 minutes after you drink the syrup for up to three hours.
In a healthy person the glucose should be cleared by the body fairly quickly. Someone with prediabetes has “impaired glucose tolerance” if he has a value between 140 and 200 mg/dL after two hours. If at two hours a person has a glucose level of 200 mg/dL or above, the diagnosis will come back “diabetes.”
Why Has the A1C Test Become Popular?
There is a reason why clinicians frequently request an HbA1c blood test. It does not require that people fast or avoid liquids prior to the test. People do not have to drink a challenging sugary beverage and then hang around for hours while blood is drawn periodically. Instead, they give a little blood once and are done and gone. It’s fast and convenient. But is it as good as the OGTT test?
The New Study Suggests OGTT is the Best Test to Diagnose Diabetes:
A study presented at the annual meeting of The Endocrine Society (March 23-26, 2019 in New Orleans) has challenged standard medical practice when it comes to the diagnosis of diabetes. When researchers compared HbA1c values to the results of oral glucose tolerance testing, HbA1c was much less sensitive. In essence, the OGTT test was the best test to diagnose diabetes.
The data came from 9,000 adults who had not been diagnosed with diabetes. In a surprisingly large number of cases, people with normal HbA1c values reacted to sugar as people with diabetes do on the oral glucose tolerance test.
The lead author stated (March 23, 2019):
“Based on our findings, A1c should not be solely used to determine the prevalence of diabetes. It should be used in conjunction with the oral glucose test for increased accuracy.
“Our results indicated that the prevalence of diabetes and normal glucose tolerance defined solely by A1c is highly unreliable, with a significant tendency for underestimation of the prevalence of diabetes and overestimation of normal glucose tolerance.”
The researchers went so far as to suggest that the A1C test may miss up to three-quarters of people with diabetes. They maintain that the oral glucose tolerance test provides a more accurate reflection of how the body responds to a whopping dose of glucose. The OGTT is therefore more likely to detect an early problem with blood sugar control. Relying exclusively on A1C may delay diagnosis and early intervention. The authors caution their colleagues not to rely exclusively on the HbA1c test to diagnose diabetes.
Not the First Cautionary Tale About A1C Reliability:
A study published in the journal Family Practice (Oct. 14, 2015) compared the A1C test to the oral glucose tolerance test (OGTT). Over 1,100 people underwent both tests. The conclusion:
“HbA1c may have a role as the initial test to exclude DM [diabetes mellitus], but it may not have sufficient accuracy to be used as the sole diagnostic test for DM. Our results showed that the ADA [American Diabetes Association]-recommended HbA1c cut-off of ≥6.5% missed 66.8% of the diagnosis of DM…”
Missing two-thirds of the people with diabetes is not very reassuring. The results of this study mirror those of the study reported at the Endocrine Society Meetings in New Orleans this week.
A Finnish study published in Diabetes Medicine (Jan. 2011) came to a somewhat similar conclusion:
“Of those with diabetes diagnosis based on two oral glucose tolerance tests during the Diabetes Prevention Study follow-up, 60% would have remained undiagnosed if diagnosis had been based on HbA(1c) ≥ 6.5% (≥ 48 mmol/mol) criterion.”
So…What’s the Best Test to Diagnose Diabetes?
For our money it would be the oral glucose tolerance test (OGTT). Yes, it’s inconvenient. Yes, it’s time consuming. But from our reading of the research, it appears to be more accurate if you suspect you may be vulnerable to type 2 diabetes.
That doesn’t mean the A1C test should be thrown out with the bath water. It can be helpful in monitoring treatment progress. And if someone is borderline on the A1C test, it seems to us that a follow-up OGTT might be prudent.
And let’s not forget self-monitoring with a home blood glucose device. A finger-prick test will allow people with prediabetes or diabetes to monitor blood sugar periodically throughout the day. You will be able to determine the effects of exercise, diet and stress on your blood glucose and modify your life accordingly.
Share Your Own Diabetes Story!
How were you diagnosed with diabetes? Did you get the OGTT or the A1C test? How well are you controlling your blood sugar? Share your story in the comment section below.