How Does Your Doctor Diagnose Type 2 Diabetes?

Looking for Signs of Diabetes

By Debra Manzella, RN
Updated March 14, 2019

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How do you find out that you have type 2 diabetes? Often, because there may not be noticeable symptoms, the diagnosis is made during an annual physical or checkup. Your doctor may order a fasting blood sugar (FBS) test, or an oral glucose tolerance test (OGTT) to help determine whether you have diabetes. What do these tests mean?

Fasting Blood Sugar (FBS)

The FBS is a fasting test, meaning that you can't eat for 8 to 10 hours before you have your blood drawn. Most people like to go for the test first thing in the morning after fasting all night. A fasting blood glucose of 70 mg/dl to 99 mg/dl is normal. If your fasting blood glucose level comes back between 100 mg/dl and 125 mg/dl then you are considered to have impaired fasting glucose or pre-diabetes.

A fasting glucose higher than 125 mg/dl indicates that you have type 2 diabetes. Most doctors like to get a fasting blood sugar on two separate occasions to make sure of the diagnosis.

Oral Glucose Tolerance Test (OGTT)

The OGTT is a glucose challenge test. A fasting blood glucose is usually taken first to establish a baseline level. Then you are given a drink that contains 75 grams of glucose (sugar). Two hours later another blood sample is drawn to check your glucose level. If your blood glucose is under 140 mg/dl, then your glucose tolerance is considered normal. If it is 140 mg/dl to 200 mg/dl, then you have impaired glucose tolerance or prediabetes. If your glucose is over 200 mg/dl then a diagnosis of type 2 diabetes is made. Again, your doctor will usually perform this test on two different occasions before a definite diagnosis is made.

If You Are Pregnant

The OGTT is a little different if your doctor orders it when you are pregnant. The glucose drink is typically 50 grams of glucose instead of 75, and the blood glucose level is drawn after one hour instead of two. If your blood glucose level comes back less than 140 mg/dl then you have normal glucose tolerance. If it comes back over 140 mg/dl, then it is considered abnormal and you will need further testing.

Blood Glucose Unit Conversion

The unit of measurement for blood glucose is varies around the world. In the United States, the standard is milligrams per deciliter or mg/dl. In other countries, blood glucose may be measured in millimoles/liter or mmol/l. Here is a quick conversion chart courtesy of Usenet and FAQS.org

  • 4.0 mmol/l = 75 mg/dl
  • 5.5 mmol/l = 100 mg/dl
  • 5 - 6 mmol/l = 90-110 mg/dl
  • 8.0 mmol/l = 150 mg/dl
  • 10.0 mmol/l = 180 mg/dl
  • 11.0 mmol/l = 200 mg/dl

Type 2 Diabetes Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Man
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