What are the diagnostic criteria for type 2 diabetes mellitus and the recommended screening schedule for at‑risk adults?

Diagnosis of Type 2 Diabetes

Diagnose type 2 diabetes when fasting plasma glucose is ≥126 mg/dL (7.0 mmol/L), HbA1c is ≥6.5%, 2-hour oral glucose tolerance test is ≥200 mg/dL (11.1 mmol/L), or random plasma glucose is ≥200 mg/dL with classic hyperglycemic symptoms. 1, 2

Diagnostic Thresholds

Any one of the following criteria establishes the diagnosis: 3, 1

• Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) after at least 8 hours of no caloric intake 3, 1
• HbA1c ≥6.5% (48 mmol/mol) measured in an NGSP-certified laboratory standardized to the DCCT assay 3, 1
• 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during a 75-gram oral glucose tolerance test 3, 1
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with classic symptoms of hyperglycemia (polyuria, polydipsia, unexplained weight loss) 3, 1

Confirmation Requirements

In the absence of unequivocal hyperglycemia (symptoms plus random glucose ≥200 mg/dL), all abnormal results must be confirmed with repeat testing on a separate day. 3, 1 This is critical because fasting glucose has 12-15% day-to-day variability, meaning the same person can fluctuate within this range without true metabolic change. 1, 4

Diabetes can be confirmed by: 1

• Repeating the same test on a different day with both results above threshold
• Using two different tests (e.g., HbA1c and fasting glucose) both above their respective thresholds

Preferred Screening Test

Fasting plasma glucose is the preferred screening test because it is the most practical, reproducible, cost-effective, and convenient for patients. 3, 5 HbA1c offers an alternative advantage of not requiring fasting and has superior preanalytical stability. 3, 1

The 2-hour OGTT is more sensitive but cumbersome, less reproducible, and less acceptable to patients, making it impractical for routine screening. 3, 5

Who Should Be Screened

Begin screening at age 35-45 years for all adults, or immediately at any age for those with BMI ≥25 kg/m² (≥23 kg/m² for Asian Americans) plus any additional risk factor: 5, 1

• First-degree relative with diabetes 5, 1
• High-risk ethnicity (African American, Latino, Native American, Asian American, Pacific Islander) 3, 5, 1
• Hypertension (≥140/90 mmHg or on treatment) 3, 5, 1
• HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL 3, 5, 1
• Physical inactivity 3, 5, 1
• History of gestational diabetes or delivery of baby >9 lbs 3, 5, 1
• Polycystic ovary syndrome 3, 5, 1
• Previous impaired fasting glucose or impaired glucose tolerance 3, 5, 1
• History of cardiovascular disease 3, 5, 1

Screen adults with hypertension or hyperlipidemia immediately regardless of age because detection markedly improves cardiovascular risk stratification and permits aggressive management that reduces cardiovascular events and mortality. 5

Screening Frequency

• Every 3 years if initial screening is normal 3, 5, 1
• Annually for patients with prediabetes (HbA1c 5.7-6.4%, fasting glucose 100-125 mg/dL, or 2-hour OGTT 140-199 mg/dL) 5, 1, 4
• More frequently (annually or more often) for patients with borderline results or multiple risk factors 5

Prediabetes Categories

Prediabetes is defined by any of the following: 5, 1, 4

• HbA1c 5.7-6.4% (39-47 mmol/mol) 5, 1, 4
• Impaired fasting glucose (IFG): 100-125 mg/dL (5.6-6.9 mmol/L) 5, 1, 4
• Impaired glucose tolerance (IGT): 140-199 mg/dL (7.8-11.0 mmol/L) on 2-hour OGTT 5, 1, 4

Prediabetes should be viewed as a risk factor for progression to diabetes and cardiovascular disease, not as a clinical entity itself. 1, 4

Critical Pitfalls to Avoid

Do not wait until age 45 for overweight/obese patients with risk factors—screen immediately. 5 The U.S. Preventive Services Task Force recommends screening adults 40-70 years of age who are overweight or obese. 6

Do not use HbA1c in conditions with increased red blood cell turnover: sickle-cell disease, second/third trimester pregnancy, hemodialysis, recent blood loss or transfusion, erythropoietin therapy, hemolysis, or hemoglobinopathies. 1, 4 In these conditions, rely solely on plasma glucose criteria. 1, 4

Do not rely on random capillary blood glucose for screening—sensitivity is only 39-75% even when age and time since last meal are considered. 5 Point-of-care HbA1c assays should not be used for diagnosis. 3

Avoid community screening outside healthcare settings—this ensures proper follow-up and appropriate targeting of high-risk populations. 5

Do not use HbA1c alone to rule out diabetes—values <6.5% do not exclude diabetes if fasting glucose or OGTT are elevated, as the concordance between tests is imperfect. 5, 1

Test-Specific Considerations

For the OGTT, ensure adequate carbohydrate intake (at least 150 g/day) for 3 days preceding the test. 1, 4 The test uses a 75-gram anhydrous glucose load dissolved in water. 3, 1

Plasma glucose has poor preanalytical stability—plasma should be separated immediately or samples kept on ice to prevent glycolysis. 1, 4

When test results are near diagnostic thresholds, monitor closely and repeat testing in 3-6 months to confirm diagnosis, given the inherent test variability. 1