The diagnosis of diabetes mellitus is made through a blood sample. First, blood plasma is obtained — this is the liquid portion of the blood. The blood is drawn into a special tube that prevents clotting and contains substances that prevent blood glucose from breaking down.
A blood glucose value below 100 mg/dl (5.5 mmol/l) is normal. A person is considered diabetic if the fasting venous plasma glucose is above 126 mg/dl (7.0 mmol/l). Values between 100 and 126 mg/dl (5.5 and 7.0 mmol/l) indicate impaired fasting glucose, which suggests a prediabetic stage (prediabetes, impaired glucose tolerance).
A blood glucose level above 200 mg/dl (11.1 mmol/l) measured at any time of the day also indicates diabetes.
Oral Glucose Tolerance Test (OGTT) for Diabetes Detection
Another way to diagnose diabetes is the “oral glucose tolerance test” (OGTT) at the doctor’s office:
If the blood glucose level is below 140 mg/dl (7.7 mmol/l) two hours after glucose ingestion, diabetes is ruled out. A value over 200 mg/dl (11.1 mmol/l) after two hours confirms diabetes. Values between 140 mg/dl and 200 mg/dl (7.7 and 11.1 mmol/l) indicate prediabetes/impaired
Today, the OGTT is often skipped in favor of simply measuring the HbA1c value.
The HbA1c is a fraction of hemoglobin (the red blood pigment) that has bound to glucose — commonly called the “long-term blood sugar value.” It reflects the average blood glucose over the past eight weeks.
However, there are several factors that can affect HbA1c (severe liver or kidney failure, hemolytic anemia, etc.). A confirmed diagnosis therefore requires both an elevated HbA1c and a pathologically elevated plasma glucose ≥ 200 mg/dl (≥ 11.1 mmol/l) [1].
Diagnostic testing should be carried out if typical diabetes symptoms are present.
Common Symptoms of Diabetes
If you have one or more of these symptoms, you should definitely see your doctor to have your blood sugar checked. Diabetes is often detected by chance, as slightly elevated blood glucose levels may not cause symptoms. Many people are diabetic without knowing it.
For people at increased risk of diabetes, occasional blood sugar checks are advisable. Increased risk applies to people with diabetic relatives or those who are overweight.
Key ADA 2025 Statement
“Screening for presymptomatic type 1 diabetes should be accomplished with a panel of islet autoantibodies in those at increased risk.”
(ADA Standards of Care in Diabetes — 2025, Section 2, DOI: 10.2337/dc25-S002)
Who Should Be Screened & When?
Frequency of Screening
Rationale & Scope
A diagnosis of diabetes is made if any of the following is met (confirmed by repeat testing unless unequivocal hyperglycemia is present):
Test | Prediabetes Range |
FPG | 100–125 mg/dL (5.6–6.9 mmol/L) → impaired fasting glucose (IFG) |
2-hour OGTT | 140–199 mg/dL (7.8–11.0 mmol/L) → impaired glucose tolerance (IGT) |
HbA1c | 5.7–6.4% |
Summary Table
Criteria | Recommendation |
Age | All adults starting at age 35 |
Younger Adults | Any age with BMI ≥ 25 (≥ 23 for Asian Americans) + ≥1 risk factor |
If Initial Test Normal | Re-screen every 3 years |
If Prediabetes or High Risk | Re-screen annually |
References: American Diabetes Association Professional Practice Committee Diagnosis and Classification of Diabetes: Standards of Care in Diabetes—2025,
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