Diagnosis of Type 2 Diabetes Mellitus

Diagnostic Criteria

The American Diabetes Association recommends diagnosing type 2 diabetes when a patient has a fasting plasma glucose ≥126 mg/dL, or a 2-hour plasma glucose ≥200 mg/dL during an oral glucose tolerance test, or an A1C ≥6.5%, with any abnormal test confirmed by repeat testing on a subsequent day 1, 2
Fasting plasma glucose (FPG) ≥126 mg/dL after at least an 8-hour fast 3
2-hour plasma glucose ≥200 mg/dL during a 75-g oral glucose tolerance test (OGTT) 3
A1C ≥6.5% using a method certified by the National Glycohemoglobin Standardization Program (NGSP) 3
Random plasma glucose ≥200 mg/dL in a patient with classic symptoms of hyperglycemia (polyuria, polydipsia, weight loss, blurred vision, fatigue) 4

In the absence of unequivocal hyperglycemia with acute metabolic decompensation, results should be confirmed by repeat testing on a subsequent day 1, 2
A single random plasma glucose level of ≥200 mg/dL with typical symptoms of hyperglycemia is sufficient for diagnosis without confirmation 4

The American Diabetes Association recommends screening for type 2 diabetes in adults aged 35 years and older 1, 2
Adults with BMI ≥25 kg/m² (or ≥23 kg/m² in Asian Americans) who have one or more risk factors should be screened for type 2 diabetes 5
Risk factors for type 2 diabetes include first-degree relative with diabetes, high-risk race/ethnicity, history of cardiovascular disease, hypertension, HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL, women with polycystic ovary syndrome, physical inactivity, and other conditions associated with insulin resistance 5

The ADA Risk Test can be used to identify individuals at high risk who should undergo diagnostic testing 1, 2
For asymptomatic individuals, screening should be repeated every 3 years if results are normal, or annually in those with prediabetes 5

Prediabetes is diagnosed when test results fall within these ranges: FPG 100-125 mg/dL, 2-h PG during OGTT 140-199 mg/dL, or A1C 5.7-6.4% 3

In patients without traditional risk factors for type 2 diabetes and/or of younger age, consider islet autoantibody testing to exclude type 1 diabetes 1, 2
Asian Americans have increased diabetes risk at lower BMI levels 1, 2
Women with history of gestational diabetes should have lifelong testing at least every 3 years 5

Relying on a single test without confirmation (except in cases with classic symptoms and random glucose ≥200 mg/dL) 4
Failing to recognize that type 2 diabetes frequently goes undiagnosed for many years because hyperglycemia develops gradually 1, 2
Not considering alternative diagnoses like type 1 diabetes, especially in younger patients or those without typical risk factors 1, 2
Overlooking the need for more frequent screening in high-risk individuals 5
Not accounting for conditions that may affect A1C accuracy (hemoglobinopathies, anemia, recent blood transfusion) 3

Assess risk factors and symptoms, then perform one of the diagnostic tests (FPG, OGTT, A1C, or random glucose if symptomatic) 2, 5
If results are abnormal, confirm with repeat testing on a different day (unless patient has classic symptoms and random glucose ≥200 mg/dL) 5
If confirmed, diagnose type 2 diabetes and begin appropriate management 1
If results indicate prediabetes, implement preventive measures and rescreen annually 3

The American Diabetes Association recommends screening for type 2 diabetes in adults with a history of cardiovascular disease 6
Hypertension (≥140/90 mmHg or on antihypertensive therapy) is a risk factor that triggers screening for type 2 diabetes 7
Physical inactivity is a risk factor that triggers screening for type 2 diabetes 7
Hemolytic anemias are conditions where A1C should NOT be used for diagnosis, and only plasma glucose criteria should be used 6
Pregnancy is a condition where A1C should NOT be used for diagnosis, and only plasma glucose criteria should be used 7

Type 2 diabetes frequently goes undiagnosed for many years because hyperglycemia develops gradually and may not be severe enough initially to cause classic symptoms 6, 7
The duration of glycemic burden is a strong predictor of adverse outcomes 6
Ignoring ethnic-specific BMI thresholds, such as Asian Americans having increased diabetes risk at lower BMI levels (≥23 kg/m² vs ≥25 kg/m²), can lead to delayed diagnosis 7